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Metric and Imperial equivalents

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Countries by year of metrification, dark green countries use the imperial system

Most countries in the world now use the metric or SI system. The old British imperial system of weights and measures survives, in various forms, in the United States, United Kingdom, Canada, and Liberia, to greater or lesser extents and Myanmar still ploughs its own furrow, but in all these countries, except for the US, the metric system is widely understood. In the US, you'll find the metric system only used in scientific 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 "metricised" (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. Some special uses still apply non-metric units almost globally such as inches for bike-sizes or TV-sets, feet in aviation and knots and nautical miles in maritime contexts, although these are probably the only ones 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
10 is not
20 is warm
and 30 is hot

frigid freezing cold cool mild warm hot swelter cooked→
°C  -40 -18 -4 0 4 7 10 13 15 18 21 24 26 30 32 35 40
°F  -40 0 25 32 40 45 50 55 60 65 70 75 80 85 90 95 104


 100°C  =  212°F  =  Water boils
  58°C  =  136°F  =  Highest temperature recorded on Earth
  37°C  =   98.6°F =  Human body temperature
  20°C  =   68°F  =  Room temperature
   0°C  =   32°F  =  Water freezes
 -18°C  =    0°F  =  
 -40°C  =  -40°F  =  Forty below zero!
 -89°C  = -129°F  =  Lowest temperature recorded on earth
-273°C  = -459°F  =  Absolute zero

To convert Metric (Celsius) to Fahrenheit, double the number and add 30. To convert Fahrenheit to Celsius, subtract 30 and divide in half. Close enough for most everyday applications. For scientific use

Length and distance[edit]

A closeup of the National Prototype Meter Bar No. 27, given to the U.S. by the International Bureau of Weights and Measures in 1889
  • 1 inch (1") = 2.54 cm (exact)
  • 1 foot (1') = 30 cm
  • 1 yard (3 feet) = 90 cm
    • Yards are not used as frequently as miles, feet, and inches. Americans mostly know that a meter is "about a yard", and that yards are used in some sports.
  • 1 mile = 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 %).


  • 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 USA are commonly given in feet and inches stylized as, for example, 6'3" (pronounced as "six three", with "foot" added when the context is unclear), which would be 190.5 centimeters
  • The speed limit for legacy rail lines in Germany 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 = 28.35 grams
  • 1 pound = 454 grams

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

For China, the jin (斤) is almost exactly 500 grams. In Germany the pound (Pfund) nowadays refers to a weight of exactly 500g.

Surface area[edit]

For small things, one might use square inches or square centimeters. There are about 6.5cm2 in one in2.

For floor area of an apartment, there are about 11 square feet in one square meter.

For large areas, there are about 2.5 acres in one hectare, and 2.5 square kilometers in a square mile.


  • An association football (soccer) pitch is roughly one hectare in area.
  • US English often uses two of that country's states 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 kilometers.
    • Texas, the largest by area of the contiguous states, has a surface area of 268,601 square miles or 695,670 square kilometers.
  • Wales, commonly used for such comparisons in the UK, has an area of 8,023 square miles or 20,779 km2.


A full pint glass

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 US and Imperial systems, 4 quarts = 1 gallon and 2 pints = 1 quart. However, the US units are smaller than Imperial counterparts.

An Imperial gallon is 4.5 litres. A US gallon is 3.78 liters.

The fluid ounce also differs; the smaller US gallon is divided into 128 ounces (of 29.5 mL each) while the larger Imperial gallon is divided into 160 ounces (of 28.125mL each).

For car and motorcycle engines, displacement might be given in cc or liters or cubic inches. 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. The US 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, 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 and Ireland) 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.

Wind speed[edit]

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 Beafort scale was in wide use before, and winds are still often classified according to it. Hurricanes have their own scale. Here is a rough conversation table.

Wind speed
Bf name m/s knots mph km/h 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
7 near gale 14–17 28–33 32–38 50–61
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
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+

Metric units[edit]

1mm = 1/1000th meter

1mg = 1/1000th g

1mL = 1/1000th liter

1cm = 1/100th meter

1km = 1000 meters

1kg = 1000 grams


  • in Greece, 1000m² is called stremma

See also[edit]

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