Most countries in the world now use the metric system, more correctly known as the SI. The old British imperial system of weights and measures survives in various forms in the United States, United Kingdom, Canada and Liberia, while Myanmar still plows its own furrow, but 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, medical or military contexts, while in the UK and Canada, usage is more mixed. In scientific usage, the metric system is used exclusively in all countries of the world.
Some countries that are officially metric use non-standard units in everyday speech. 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" or a vara ("rod", a Latin American unit of distance, somewhere between 0.8 and 1.1 meters) give you a sense of the confusion that led to the introduction of the metric system in the first place. Canada is also officially fully metricated, though imperial units continue to be widely used by older Anglophone Canadians in daily conversation. Some special uses still apply non-metric units almost globally (such as inches for bicycles and television sets, feet in aviation, and knots and nautical miles in maritime contexts and aviation), although few are of interest to the average traveller.
To get by in countries that use a different system than you're used to, it's helpful to know some rough equivalents. We use "=" signs below, but some are approximations.
A Celsius poem
Zero is freezing
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:
And to go the other direction:
|55°C||131°C||Hot tap water|
|37°C||98.6°F||Human body temperature (+/− 0.5°C)|
|−40°C||−40°F|| Forty below zero!|
Mercury freezes and mercury thermometers stop working.
Length and distance
- 1 inch (1" (U.S.), 1 in (UK)) = 2.54 cm (exact)
- 1 foot (1' (U.S.), 1 ft (UK)) = 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. 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
- For mental arithmetic, this gives several useful conversions that are fairly accurate:
- 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 %).
- 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
- A credit card is about 0.75mm (3/4 of a millimeter) thick
- 1 cm (centimeter) 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 12-15 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) and Volcán Concepción on Ometepe are both roughly at one mile altitude. Chamonix town and Katoomba are both about a kilometer above sea level.
- Human heights in the U.S. 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 centimeters. The same height would be stylized "6 ft 3 in" in the UK.
- A marathon is run over 42.195 kilometers, or 26 miles and 385 yards.
- The maximum speed limit for legacy rail lines in Germany that the signaling technology allows is 160 km/h or roughly 100 mph
- 100 km/h is around 60 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, an error that likely originates from counting both feet. Step lengths of adults usually fall in the range of 60 to 80 centimeters (but running, the step grows to be above a meter).
- 1kg = 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 only in the UK)
A Troy ounce, customarily used only for precious metals and gemstones, is 31.1 grams.
Both the U.S. and the UK have their own version of the hundredweight (cwt) and the ton (t). 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 (907.18 kg), while the imperial ton is 160 stone (2240 pounds, 1016.05 kg). The metric ton (UK: "tonne") is 1000 kg.
- 1 square inch (in2) = 6.5 square centimeters (cm2).
- 1 square meter (m2) = 11 square feet
- 1 hectare (ha) = 10,000 m2 = 2.5 acres
- 1 square kilometer (km2) = 100 ha = 0.4 square miles.
In Greece, 1000m² is called stremma.
- An association football (soccer) pitch is roughly one hectare in area.
- In the U.S., two of that country's states are often used for nation-sized objects, depending on the area being compared:
- Wales, commonly used for such comparisons in the UK, has an area of 8,023 square miles or 20,779 km2.
The standard metric unit of volume is the liter.
Many things, however, are measured in mL (milliliters) or equivalently in cc (cubic centimeters). 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. However, the U.S. units are smaller than imperial counterparts.
An imperial gallon is 4.55 litres. A U.S. gallon is 3.79 liters.
The fluid ounce also differs; the smaller U.S. gallon is divided into 128 ounces (of 29.5 mL each) while the larger imperial gallon is divided into 160 ounces (of 28.125 mL each).
For car and motorcycle engines, displacement is usually given in cc or in liters. American cars used to have their displacement measured in cubic inches, but car manufacturers switched to liters in the early 1980s. 1000cc or one liter is 61 cubic inches.
Large amounts of liquids are sometimes measured in hectoliters equivalent to hundred liters or in cubic meters equivalent to a thousand liters. A small brewery might e.g. state their beer production to be below 100 hectoliters while a municipal water service will likely bill you in cubic meters. For very large volumes, the U.S. also uses acre-feet.
- 1L of water weighs 1kg at 4 °C. Since many liquids (milk, orange juice) are sold in liter containers, it is easy to judge 1L or 1kg.
- 1L is equivalent to a cube 10cm x 10cm x 10cm.
- 1 Cubic m (1 m3) = 1000 liters. 1 m3 of water weighs 1000 kg = 1 Tonne.
- In Europe and many parts of Canada, wine is usually sold in 750mL (0.75L) bottles (occasionally 700mL or 1L).
- 12 fl. oz. (common size for beer bottles in the Americas) is roughly equivalent to 355 milliliters, more or less the same as a "small" European beer at 333 milliliters (a third of a liter)
- One imperial pint (a common serving size for beer in the UK, Ireland and Canada) is 568.26125 milliliters (exactly) or roughly 10% more than a big (or regular depending on whom you ask) "continental" European beer at 500 milliliters.
- The fuel tank on a small family car (e.g. Volkswagen Golf or Toyota Corolla) typically has a capacity of 50 litres.
- 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
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). Knots and mph are roughly equivalent: slightly higher winds if a number is given in knots than when the same number is in 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. Hurricanes have their own scale. Here is a rough conversion table.
|0||calm||< 0.3||< 1||< 1||< 1|
|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|
|8||(fresh) gale||17–21||34–40||39–46||62–74||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||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+||Big objects in the air, windows crashed|
Power and energy
The SI unit for power is 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
- 1mm = 1/1000th meter
- 1mg = 1/1000th g
- 1mL = 1/1000th liter
- 1cm = 1/100th meter
- 1km = 1000 meters
- 1kg = 1000 grams