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" or pound being exactly 500 grams or half a kilogram or a Dutch ons being exactly 100 gram) some are not and the vague definitions of what exactly is meant by a "pound" or a "vara" (a Latin American unit of distance, somewhere between 0.5 and 1 meter, and sometimes weight) 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 or feet in aviation, although these are probably the only ones of interest to the average traveler.

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.

Temperature

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

Comparisons 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

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

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

Length and distance

• 1 inch (1") = 2.54cm
• 1 foot (1') = 30.48cm
• 1 yard = 91.44 cm (not used as frequently as mile foot and inch), when asked about the length of a meter Americans will likely reply it to be "about a yard"
• 1 mile = 1.609344 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)

Comparisons

• A credit card is about 0.75mm (3/4 of a millimeter) thick
• 1cm (centimeter) is the width of an average fingernail.
• Most adults are between 1.5 and 2 meters tall.
• 1 km (kilometer) takes about 15 minutes to walk (without heavy luggage).
• 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 the Volcano Concepcion 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" or 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

Weight

• 1kg = 2.2 pounds
• 1 ounce = 28.35 grams
• 1 pound = 454 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

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.

Comparisons

• 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.

Volume

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 is 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. A US quart is 32 fluid ounces while Imperial is 40; a liter is in between at 35. A US gallon is 128 ounces or 3.78 liters, while an Imperial gallon is 160 ounces or 4.54 liters.

For car and motorcycle engines, displacement might be given in cc or liters or cubic inches. 1000cc or one liter is 61 cubic inches.

Comparisons

• 1L of water weighs 1kg. 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.

Metric units

1mm = 1/1000th meter

1mg = 1/1000th g

1mL = 1/1000th liter

1cm = 1/100th meter

1km = 1000 meters

1kg = 1000 grams