Almost all countries in the world use the metric system, officially known as the International System of Units (SI). Before the introduction of the metric system, there was no worldwide standard in units of measurement — every geographic region or country used their own system. Some of these historic systems of units are still in use, for example variations of the old English system (or its descendants U.S. customary and the imperial system) of weights and measures survive in various forms in the United States, United Kingdom, Bahamas, Ireland, Canada, Liberia and Hong Kong, while Myanmar still plows its own furrow. In all these countries except for the U.S., the metric system is widely understood. In the U.S., you'll find the metric system only used in scientific, military, and most medical contexts, while in the UK and Canada, usage is more mixed. In scientific usage, the metric system is used exclusively worldwide.

## Understand

In most of the world the metric system is widely understood, the most prominent exception being the U.S.A., where metric units are virtually unknown to the average person in the street, and would likely only be understood if you are lucky enough to meet a doctor or a scientist. In the United Kingdom, Bahamas, Liberia and Hong Kong usage is mixed. Ireland and Canada have officially fully switched to metric usage, but imperial units continue to be widely used in daily conversation in Anglophone Canada, and to some extent in Ireland. Singapore is mostly fully metricated, though an exception is property sizes, which are still advertised in square feet. In the rest of the Anglosphere (Australia, Guyana, New Zealand or South Africa), imperial units won't get you anywhere.

Some countries that are officially metric use non-standard units colloquially. While most of them are "metricated" – e.g. a German Pfund ("pound") being exactly 500 grams or a Dutch ons ("ounce") being exactly 100 grams, some are not, and the vague definitions of what exactly is meant by a "pound" give you a sense of the confusion that led to the introduction of the metric system in the first place.

Since use of the metric system is taught in schools in science classes, younger generations in most of the world learn the metric units, and historic units gradually die out where they aren't in common use. When traveling in the Netherlands for example, the elderly may still use ons (ounce) in spoken language meaning 100 grams, but teenagers won't have any idea what they are talking about.

Some special uses still apply (standardised) non-metric units almost globally, such as inches for bicycles and television sets. Jewellers typically measure the weight of diamonds in carat, so entering a shop and asking for a diamond ring of 2.4 g will definitely raise some eyebrows. Seafarers and aviators use nautical miles and knots, and feet in some contexts. The energy contained in food is traditionally measured in calories and food items will often list the value in kcal (kilocalories) and metric kJ (kilojoules) side by side.

There are so many historic units that listing them all would be impossible, and of limited use to the traveller. The goal here is to list some non-metric units that are still commonly used, and their conversions to metric equivalents, to give the traveller a rough idea of quantity. Not all the conversions below are exact, even though the "=" is used.

## Metric units

Unlike the imperial or customary systems of units, most metric units have two parts: the unit name and a prefix. The unit name is unique for any particular quantity, for example "metre" for length, "gram" for mass (weight), "litre" for volume and so on. If the resultant quantity requires a very large or a very small number to make it meaningful, then a prefix is added to indicate a multiple or fraction of the unit concerned. For example, the prefix "kilo-" means "one thousand", thus one kilometre is one thousand metres, one kilogram is one thousand grams and so on. Likewise the prefix "centi-" means one hundredth part of", thus one centimetre is one hundredth part of a metre.

The most common units of measure (without prefixes) that the traveller will come across are:

Quantity Unit Symbol
length metre m
volume litre L or l
mass (weight) gram g
power watt W
potential
difference
volt V
electric
current
ampere A

The most common prefixes that will be encountered by a traveller are:

Prefix Multiplier
kilo- (k) 1,000 (thousand)
hecto- (h) 100 (hundred)
deca- (da) 10 (ten)
deci- (d) 0.1 (tenth)
centi- (c) 0.01 (hundredth)
milli- (m) 0.001 (thousandth)

Some metric units do not follow this scheme. Thus, you won't hear about megagrams (1,000 kg); instead the equivalent tonnes are used. Also, land area is often measured in hectares (1 ha = 100 m × 100 m = 0.01 km2).

### Length and distance

The SI for length is the metre (m) (spelled as "meter" in the U.S. and its former colonies).

• 1 inch (1" (U.S.), 1" or 1 in (UK and Ireland)) = 2.54 cm (exact). 1" ≈ ¼ dm (approx.).
• 1 foot (1' (U.S.), 1' or 1 ft (UK and Ireland)) = 12 inches = 30 cm
• 1 yard (1 yd) = 3 feet = 90 cm
• Yards are not used as frequently as miles, feet, and inches in the U.S., though they are regularly seen on road signs in the UK (note that for road sign purposes, 1 yard = 1 metre). Americans mostly know that a meter is "about a yard", and that yards are used in some sports.
• 1 mile (1 mi) = 1,760 yards = 1.6 km, or 1 km = 0.6 miles
• For mental arithmetic, this gives several useful conversions that are fairly accurate:
• 3 miles = 5 km (off by 5%)
• 5 miles = 8 km
• 6 miles = 10 km (off by 5%)
• 10 miles = 16 km
• 1 nautical mile = 1.852 km (not used in normal conversation, but standard in air and sea navigation; the "knot", used for speeds in those contexts, is 1 nautical mile per hour). Rounding the nautical mile to 2 km is surprisingly accurate (off by 7.4%).
• 1 furlong = 1/8 miles = 220 yards = 201.168 m (rarely used in daily life outside of Myanmar, but often used in horse racing)
• 1 chain = 1/80 miles. Used only in context of railways in the UK.
• There are also other miles, such as the Scandinavian "mil", which is 10 km. German explorers deliberately misleading local leaders on Prussian miles versus English miles (with the former being much larger and unknown to the locals) also played some role in the early colonial history of Namibia

#### Comparisons

• A credit card is about 0.75 mm thick
• 1 cm (centimetre) is the width of an average fingernail.
• 1 foot is, unsurprisingly, the length of an average man's foot in shoes. 1 foot is slightly more than the long side of a sheet of paper (U.S. letter or A4).
• Most adults are between 1.5 and 2 meters tall.
• The average person walks 5 km/h or 3 miles/hour (without heavy luggage). At that speed, 1 km takes 12 minutes, and 1 mile takes 20 minutes.
• Ten yards are a little less than a yard shy of ten meters. An American football field is exactly 100 yards from end zone to end zone, with each end zone being 10 yards deep. In Canadian football, the field is exactly 110 yards between end zones, and the end zones are 20 yards deep.
• Denver (Colorado), Johannesburg (South Africa) and Volcán Concepción on Ometepe are roughly at one mile (5280 ft or 1600 m) altitude. Chamonix town and Katoomba are both about a kilometre (3300 ft or 1000 m) above sea level. Everest is about 8,848 m (29,031 ft) above sea level while most long-haul passenger airliners fly at an altitude of 9,000 m (30,000 ft) to 10,500 m (35,000 ft) above sea level.
• Human heights in the U.S., UK, Canada, and Ireland are commonly given in feet and inches, stylized as 6'3" (pronounced as "six three", or "six foot three" when the context is unclear), which would be 190.5 centimetres. The same height could also be stylized "6 ft 3 in".
• A marathon is run over 42.195 kilometres, or 26 miles and 385 yards.
• The maximum speed limit for legacy rail lines in Germany that the signalling technology allows is 160 km/h or roughly 100 mph
• 100 km/h is around 62 mph — in many parts of the world this is the speed limit on highways
• Despite what you may have heard, the average length of a step is not a meter or a yard, an error that likely originates from counting both feet. Step lengths of adults usually fall in the range of 60 to 80 centimetres — on the parade ground, a soldier's pace is 75 cm (30 in). When running, the step grows to be above a meter.

### Mass (weight)

The SI unit for mass (weight) is the kilogram (kg).

• 1 kilogram (1 kg) ≈ 2.2 pounds
• 1 ounce (1 oz) ≈ 28.35 grams
• 1 pound (1 lb) = 16 ounces ≈ 454 grams
• 1 stone (1 st) = 14 pounds ≈ 6.35 kg (used in the UK and Ireland)

Some food in some countries is sold per 100 g (hectogram), i.e. about ¼ lb or 4 oz.

A Troy ounce, customarily used only for precious metals and gemstones, is approximately 31.1 grams.

The jīn (斤) or catty, a Chinese unit of weight, was traditionally about 605 g, though in mainland China it is now exactly 500 g. The jīn was traditionally divided into 16 liǎng (兩) or taels, though in mainland China it is now 10 liǎng. The traditional jīn and liǎng continue to be used in Hong Kong and Macau, particularly in wet markets, and public weighing scales in Hong Kong wet markets are required to display the traditional Chinese, British Imperial and metric units side by side. Taiwan's system of units is based on the Japanese system of units but gives the units Chinese names, meaning that the Taiwanese jīn is 600 g, equivalent to the Japanese kin, and the Taiwanese liǎng is 116 jīn, 37.5 g.

In Germany, France and the Netherlands the pound (Pfund, livre or pond) refers to a weight of exactly 500 g, a practice dating from the nineteenth century.

Both the U.S. and the UK have their own version of the hundredweight (cwt) and the ton (ton). The imperial hundredweight is 8 stone (112 pounds), while the U.S. hundredweight is 100 pounds, and a ton is 20 hundredweights in both systems. This means that the U.S. ton is 2000 pounds (about 907 kg), while the imperial ton is 160 stone (2240 pounds, about 1016 kg). The metric ton (UK: "tonne") is 1000 kg and is therefore about 0.98 imperial tons

The tonne (symbol "t") is widely used outside the United States on motor vehicle weight prohibition signs.

### Surface area

The SI unit for area is the square metre (m2). The floor area of apartments, houses and the size of building plots are often quoted in square metres while larger areas such as parks are quoted in hectares (ha) or square kilometres (km2).

• 1 square inch (in2) ≈ 6.5 square centimetres (cm2).
• 1 square meter (m2) ≈ 11 square feet
• 1 hectare (ha) = 10,000 m2 ≈ 2.5 acres
• 1 square kilometre (km2) = 100 ha ≈ 0.4 square miles.

In Greece, 1000m² is called stremma.

Taiwan continues to use both the traditional Japanese and Dutch systems of measurements when measuring area, albeit with the units now having been given Chinese names. Smaller property sizes, such as those of apartments and offices are usually measured in 坪 (píng in Mandarin, pêⁿ in Taiwanese), approximately 3.306 m2 and equivalent to the Japanese tsubo (坪). Larger sizes, such as those of farmland, are measured in 甲 (jiǎ in Mandarin, kah in Taiwanese), equivalent to the Dutch morgen, and approximately equivalent to the hectare. Across the Taiwan Strait, large areas like farmland in China are usually measured in mu (亩), which was metricated and equal to 666.67m².

#### Comparisons

• The area inside the track on an athletic field is about one hectare.
• In the U.S., two of that country's states are often used for nation-sized objects, depending on the area being compared:
• Rhode Island, the smallest state by area, has a surface area of 1,214 square miles or 3,140 square kilometres.
• Texas, the largest by area of the contiguous states, has a surface area of 270,000 square miles or 700,000 square kilometres.
• Wales, commonly used for such comparisons in the UK, has an area of 8,000 square miles or 20,000 km2.

### Volume

The everyday units for volume among metric users are the litre (spelled so except in the U.S.A.) and its derivatives, despite the SI unit for volume being the cubic metre (m3), 1,000 litres.

For many things, the derivatives are used: mL (millilitres) or equivalently in cc (cubic centimetres), cL (centilitres) and dL (decilitres). Roughly, a teaspoon is 5mL and a fluid ounce (depending on the system) is a little under 30mL.

In both the U.S. and imperial systems, 4 quarts = 1 gallon and 2 pints = 1 quart but the U.S. pint is divided into 16 fluid ounces while the imperial pint is divided into 20 fluid ounces. An imperial gallon is 4.55 litres and a U.S. gallon is 3.79 litres while an imperial fluid ounce is 28.125 mL and a U.S. fluid ounce is 29.5 mL.

For car and motorcycle engines, displacement is usually given in cc or in litres. American cars used to have their displacement measured in cubic inches, but car manufacturers switched to litres in the early 1980s. 1,000 cc, one litre, is 61 cubic inches.

Large amounts of liquids are sometimes measured in hectolitres equivalent to hundred litres or in cubic meters equivalent to a thousand litres. A small brewery might e.g. state their beer production to be below 100 hectolitres while a municipal water service will likely bill you in cubic meters. For very large volumes, the U.S. also uses acre-feet (approx 1233 m3).

#### Comparisons

• 1 L of water weighs 1 kg at 4°C. Since many liquids (milk, orange juice) are sold in litre containers, it is easy to judge 1 L or 1 kg.
• 1 L is equivalent to a cube 10 cm x 10 cm x 10 cm.
• One U.S. pint of water weighs one pound
• One Imperial gallon of water weighs ten pounds
• One fluid ounce of water weighs one ounce
• 1 cubic meter (1 m3) = 1000 litres. 1 m3 of water weighs 1,000 kg = 1 tonne.
• In Europe and many parts of Canada, wine is usually sold in 750 mL (0.75 L) bottles (occasionally 700 mL or 1 L). A glass of wine is usually 12 or 16 cL (120 or 160 mL), so 4–5 glasses to a bottle.
• 12 fl. oz. (common size for beer bottles in the Americas) is roughly equivalent to 355 millilitres, more or less the same as a "small" European beer at 333 mL (a third of a litre)
• One imperial pint (a common serving size for draught beer in the UK, Ireland and Canada) is 568.26125 millilitres (exactly) or roughly 10% more than a big (or regular depending on whom you ask) "continental" European beer at 500 mL.
• The fuel tank on a small family car (e.g. Volkswagen Golf or Toyota Corolla) typically has a capacity of 50 L or 13 US gallons.
• One acre-foot (in the U.S. commonly cited as a rough estimate for a family's yearly water needs) is equivalent to roughly 1233 m³

## Driving

Outside the United States, the United Kingdom, Belize and many small territories that are within the Anglo-American sphere of influence, most measurements related to driving are in metric units.

### Driving speed

The speed limits in most countries are quoted in kilometres per hour (km/h). The principal exceptions are the United States, Liberia, the United Kingdom and many Caribbean countries, which all use miles per hour (mph). Americans who take their cars to Canada or Mexico and Britons who take their cars to Ireland or the European continent often need to convert between mph and km/h while driving. Many (but not all) modern cars in these countries have both mph and km/h scales on their speedometers. Some cars that use an LED display for a speedometer can change between mph and km/h by changing a setting. If not, an easy conversion is to remember the sequence.

This sequence means that

• 20 mph ≈ 30 km/h
• 30 mph ≈ 50 km/h
• 50 mph ≈ 80 km/h
• 80 mph ≈ 130 km/h

The symbol for "kilometres per hour" is "km/h", used in most countries regardless of how it is written in full in the local language – for example the Italians use "km/h" even though the Italian for "kilometres per hour" is "chilometri all'ora". The few exceptions are in places such as Malaysia which use "km/j" (with the j that stands for jam, the Malay word for hour) although it is unlikely to cause much confusion.

### Tyre pressure

Historically, in the English-speaking world, tyre pressure was measured in psi (pounds per square inch), while elsewhere it was measured in kilograms [weight] per square centimetre or in atmospheres (bars). With the designation of the Pascal (Pa) as the SI unit for pressure in 1960, the kilopascal (kPa) has increasingly been used for tyre pressure.

The relationship between these various units is

1 bar = 100 kPa ≈ 98 kgf/cm2 ≈ 14.7 psi

This table gives common tyre pressures for motor cars.

Pressure equivalents
kPa bar kgf/cm2 psi
140 1.4 1.43 20.6
160 1.6 1.63 23.5
180 1.8 1.83 26.5
200 2 2.04 29.4
220 2.2 2.24 32.3
240 2.4 2.45 35.3
260 2.6 2.65 38.2

### Fuel consumption

Note: All conversions in this section are approximate

Traditionally fuel consumption in the United Kingdom and the United States was quoted in miles per gallon, but since the two countries had different gallons, the figures are not comparable. The conversion between the two is

mpgUK ≈ 1.2 mpgUS.

Some countries that use the metric system quote fuel consumption in km/L, a direct conversion from mpg. The conversion factors are

1 km/L ≈ 2.35 mpgUS ≈ 2.82 mpgUK

Most countries that use the metric system quote fuel consumption in L/100 km. Since this the inverse of mpg, a high figure quoted in L/100 km is equivalent to a low figure in mpg and vice-versa. The conversion relationship is:

1 L/100 km ≈ 235/mpgUS ≈ 282/mpgUK

Since there is an inverse relationship, there exists a value where the fuel consumption in L/100 km is numerically identical to the value in mpg. The values (different for US and UK gallons) are:

15.33 L/100 km ≈ 15.33 mpgUS
16.79 L/100 km ≈ 16.79 mpgUK
L/100 km mpgUK mpgUS km/L This is a two-way table based on the first (bold) column.The data in the first row tells us:4 L/100km ≈ 70.5 mpgUK ≈ 58.5 mpgUS = 25 km/L4 mpgUK ≈ 70.5 L/100 km4 mpgUS ≈ 58.8 L/100 km4 km/L = 25 k/L 4 70.5 58.8 25 5 56.4 47 20 6 47 39.2 16.6 7 40.3 33.6 14.3 8 35.3 29.4 12.5 10 28.2 23.5 10 12 23.5 19.6 8.3 14 20.1 16.8 7.1 16 17.6 14.7 6.2 18 15.7 13.1 5.6 20 14.1 11.8 5 22 12.8 10.7 4.5 24 11.8 9.8 4.2 26 10.8 9 3.8 28 10.1 8.4 3.6 30 9.4 7.8 3.3 35 8.1 6.7 2.9 40 7.1 5.9 2.5 45 6.3 5.2 2.2 50 5.6 4.7 2

### Power and energy

The SI unit for power is the watt (W), where 1 W = 1 V · 1 A. The SI unit for energy is joule (J), where 1 J = 1 W · 1 s. The alternative units are horsepower (hp) and calories (cal), both still used in some contexts even in countries where metric units dominate.

Often thousands of watts, joules and calories are used; particularly in a dietary context, "calories" is often used for what scientists would call "kilocalories". Electrical energy usage is sometimes measured in kilowatt-hours, where 1 kWh = 3.6 MJ.

Horsepower is an approximation of what power a horse can deliver on average in long-time work, such as pumping water out of a mine, as measured by James Watt. The horsepower has several definitions (depending on what units they are based on), the main ones being the mechanical or imperial horsepower (about 0.746 kW) and the metric horsepower (about 0.735 kW). For engines the figures vary also depending on what power is measured: to what extent friction in the motor and transmission as well as other factors are included.

One calorie is defined as the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. A human needs about 2,000 or 2,600 kcal i.e. 8,400 or 10,900 kJ a day (woman/man), with everyday activities and walking 2–5 km (1.5–3 mi). The figures vary significantly with age, weight and activity.

• 1 hp = 0.735 kW ≈ 3/4 kW (2% off)
or 0.746 kW ≈ 3/4 kW (0.5% off)
• 1 kW = 1.360 hp (metric)
or 1.341 hp (imperial)
• 1 kcal ≈ 4.2 kJ ≈ 4 kJ (<5% off)
• 1 kJ ≈ 0.24 kcal ≈ 1/4 kcal

## Weather and Climate

### Temperature

 °C °F 40 104 sweltering 35 95 very hot 30 86 28 83 hot 25 77 23 73 warm 20 68 18 64 mild 15 59 10 50 cool 5 41 0 32 cold −5 23 −10 14 very cold −18 0 −25 −13 frigid −40 −40

Temperature is among the most relevant of quantities for travellers. Thirty degrees in one scale (i.e. Fahrenheit, °F) is really cold whereas in another scale (i.e Celsius or Centigrade, °C), it can be very hot. Therefore, it is pretty useful to know the equivalents between the two scales (the SI unit Kelvin (K) is used only in scientific contexts), especially if you're from the US or one of the few other countries to use Fahrenheit. For everyday applications (e.g. weather), you can approximate with very simple math. To convert Celsius to Fahrenheit, double the number and add 30. To convert Fahrenheit to Celsius, subtract 30 and divide in half.

The exact formulas are similar, but are harder to calculate in your head:

${\displaystyle Fahrenheit={(}Celsius\times {1.8}{)}+{32}}$

or

${\displaystyle Fahrenheit={(}Celsius\times {9 \over 5}{)}+{32}}$

And to go the other direction:

${\displaystyle Celsius={{(}Fahrenheit-{32}{)} \over {1.8}}}$

or

${\displaystyle Celsius={{(}Fahrenheit-{32}{)}}\times {5 \over {9}}}$
 A Celsius poem Zero is freezing 10 is not 20 is warm and 30 is hot
Comparisons
Celsius Fahrenheit
100°C 212°F Water boils
55°C 131°F Hot tap water
37°C 98.6°F Human body temperature (±0.5°C or ±0.9°F)
20°C 68°F Room temperature
4°C 40°F Refrigerator
0°C 32°F Water freezes
−18°C 0°F Freezer
−40°C −40°F Mercury freezes and mercury thermometers stop working.
The only temperature Fahrenheit and Celsius have in common.
−61°C −78°F Average January temperature in Oymyakon, Yakutia
−89 °C −129 °F Coldest natural temperature recorded on Earth, Vostok, East Antarctica
−273°C −459°F Absolute zero

### Precipitation

 mm 1 2 5 10 20 50 100 200 500 1000 2000 cm 0.1 0.2 0.5 1 2 5 10 20 50 100 200 inches 0.04 0.08 0.2 0.4 0.8 2 4 8 20 40 80

Rainfall is often measured in inches in the United States, but in most other countries in the world it is measured in millimetres or in litres per square metre. One inch of rain is equal to 25.4 mm rain, while millimetres of rain and litres of rain per square meter are numerically identical.

Snow depth is usually measured in feet and inches in the United States but in centimetres (or metres if it is deep) elsewhere in the world.

The table above shows an approximate conversion between inches and millimetres/centimetres. This conversion uses the approximation of one inch equals 25 mm which is usually sufficient for travellers.

### Wind speed

Wind speed is usually given in m/s, knots, km/h or mph, depending on country and context. 1 m/s = 2 knots (exactly in this context). 1 knot is 1 nautical mile per hour, which is about 1.15 mph. For km/h: divide by two to get knots (actually 1.852) or mph (actually 1.609), by four (3.6) to get m/s.

The Beaufort scale was in wide use before, and winds are still often classified according to it. Here is a rough conversion table.

Wind speed
Bf name m/s knots mph km/h Warning comments
0 calm < 0.3 < 1 < 1 < 1
1 light air 0.3–1.5 1–3 1–3 1–5
2 light breeze 1.6–3.3 4–6 4–7 6–11
3 gentle breeze 3.4–5 7–10 8–12 12–19
4 moderate breeze 6–8 11–16 13–18 20–28 Dust and snow is blown into the air
5 fresh breeze 8–11 17–21 19–24 29–38 Walking against the wind arduous
6 strong breeze 11–14 22–27 25–31 39–49 small craft advisory
7 near gale 14–17 28–33 32–38 50–61
8 (fresh) gale 17–21 34–40 39–46 62–74 gale warning Walking in open spaces awkward
9 strong gale 21–24 41–47 47–54 75–88 Roof tiles can be blown down
10 storm 25–28 48–55 55–63 89–102 storm warning Big trees derooted
11 violent storm 29–32 56–63 64–72 103–117 Large forest areas blown down
12 hurricane force 33+ 64+ 73+ 118+ hurricane force warning Big objects in the air, windows crashed

Hurricanes have their own scales. The scale below only applies to hurricanes originating in the Atlantic Ocean and north-eastern Pacific Ocean.

Saffir-Simpson scale m/s knots mph km/h Comment
category one 33–42 64–82 74–95 119–153
category two 43–49 83–95 96–110 154–177
category three 50–58 96–112 111–129 178–208
category four 58–70 113–136 130–156 209–251
category five 70+ 137+ 157+ 252+

## Miscellaneous

If you prefer not to do any maths at all, there are websites and apps that may be of use. For instance, major weather websites and apps have an option to allow you to display the relevant measures (e.g. temperature, wind speeds) in metric or imperial units. Smartphones have internal apps that help you convert between metric and imperial measurements.