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 measurements, and 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 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 m) 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 Anglophone Canadians in daily conversation. Ireland switched to metric beginning in the 1970s and is now fully metric in official context, though the imperial system still survives to some extent in colloquial usage. In the rest of the Anglosphere (Australia, Guyana, New Zealand or South Africa), imperial units won't get you anywhere. Singapore is mostly fully metricated, though an exception is property sizes, which are still advertised in square feet. 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.
Since use of the metric system is taught in schools in science classes, younger generations in most of the world exclusively know metric units, and historic units gradually die out. 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. The most notable exception to this rule is the United States, 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. Exceptions exist in fields where particular units are well established. Jewelers typically measure the weight of diamonds in carat (which is 0.2 g), 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. An example from everyday life is the energy contained in food, which is traditionally measured in calories. Food items will often list the value in kcal (kilocalories) and metric unit kJ (kilojoules) side by side.
There are so many historic units that listing them all on this page would be nearly impossible, and of limited use to the traveler. The goal here is to list the most commonly used historic units and their conversions to metric equivalents to give the traveler a rough idea of quantity. Not all the conversions below are exact, even though the "=" is used.
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 an a very large or a very small number to make it meaningful, then a prefix is added to indicate a multiple or fraction of a 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.
The most common units of measure (without prefixes) that the traveller will come across are:
Quantity | Unit | Symbol |
---|---|---|
length | metre | m |
volume | litre | L |
mass (weight) | gram | g |
power | watt | W |
potential<br/> difference | volt | V |
electric <br/> 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) |
The SI for length is the metre (m) (spelled as "meter" in the U.S. and its former colonies).
The SI unit for mass (weight) is the kilogram (kg).
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 jīn, 37.5 g.
In Germany and France the pound (Pfund or livre) likewise nowadays refers to a weight of exactly 500 g.
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 (about 907 kg), while the imperial ton is 160 stone (2240 pounds, about 1016 kg). The metric ton (UK: "tonne") is 1000 kg.
The SI unit for area is the square metre (m<sup>2</sup>). 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 (km<sup>2</sup>).
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 m<sup>2</sup> 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.
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 (m<sup>3</sup>), 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 liters 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 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 (approx 1233 m<sup>3</sup>).
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.
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.
<gallery heights="50" widths="50"> File:Vienna_Convention_road_sign_C14-V1-20.svg File:Vienna_Convention_road_sign_C14-V1-30.svg File:Vienna_Convention_road_sign_C14-V1-50.svg File:Vienna_Convention_road_sign_C14-V1-80.svg File:Vienna_Convention_road_sign_C14-V1-130.svg </gallery>This sequence means that
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.
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/cm<sup>2</sup> = 14.7 psi
This table gives common tyre pressures for motor cars.
kPa | bar | kgf/cm<sup>2</sup> | 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 |
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
mpg<sub>UK</sub> = 1.2 mpg<sub>US</sub>.
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 mpg<sub>US</sub> = 2.82 mpg<sub>UK</sub>
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/mpg<sub>US</sub> = 282/mpg<sub>UK</sub>
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 mpg<sub>US</sub> 16.79 L/100 km = 16.79 mpg<sub>UK</sub>
| | | | This is a two-way table based on the first (bold) column.<br>The data in the first row tells us:<br/>4 L/100km = 70.5 mpg<sub>UK</sub> = 58.5 mpg<sub>US</sub> = 25 km/L<br/>4 mpg<sub>UK</sub> = 70.5 L/100 km<br/>4 mpg<sub>US</sub> = 58.8 L/100 km<br/>4 km/L = 25 k/L | | | | | ! L/100 km | mpg<sub>UK</sub> | mpg<sub>US</sub> | km/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 |
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.
See also: Cold weather
| | | | °C | style="padding-right:0.8em;" | °F | | | 40 | style="padding-right:0.8em;" | 104 | style="text-align:left; border-left:solid red 5px; padding-left:0.8em;" | sweltering | | 35 | style="padding-right:0.8em;" | 95 | style="text-align:left; border-left:solid orange 5px; padding-left:0.8em;" |very hot | | 30 | style="padding-right:0.8em;" | 86 | style="text-align:left; border-left:solid orange 5px; padding-left:0.8em;" |very hot | | 28 | style="padding-right:0.8em;" | 83 | style="text-align:left; border-left:solid yellow 5px; padding-left:0.8em;" |hot | | 25 | style="padding-right:0.8em;" | 77 | style="text-align:left; border-left:solid yellow 5px; padding-left:0.8em;" |hot | | 23 | style="padding-right:0.8em;" | 73 | style="text-align:left; border-left:solid yellowgreen 5px; padding-left:0.8em;" |warm | | 20 | style="padding-right:0.8em;" | 68 | style="text-align:left; border-left:solid yellowgreen 5px; padding-left:0.8em;" |warm | | 18 | style="padding-right:0.8em;" | 64 | style="text-align:left; border-left:solid lightgreen 5px; padding-left:0.8em;" |mild | | 15 | style="padding-right:0.8em;" | 59 | style="text-align:left; border-left:solid lightgreen 5px; padding-left:0.8em;" |mild | | 10 | style="padding-right:0.8em;" | 50 | style="text-align:left; border-left:solid lightblue 5px; padding-left:0.8em;" |cool | | 5 | style="padding-right:0.8em;" | 41 | style="text-align:left; border-left:solid lightblue 5px; padding-left:0.8em;" |cool | | 0 | style="padding-right:0.8em;" | 32 | style="text-align:left; border-left:dotted blue 5px; padding-left:0.8em;" |cold | | −5 | style="padding-right:0.8em;" | 23 | style="text-align:left; border-left:dotted blue 5px; padding-left:0.8em;" |cold | | −10 | style="padding-right:0.8em;" | 14 | style="text-align:left; border-left:solid blue 5px; padding-left:0.8em;" |very cold | | −18 | style="padding-right:0.8em;" | 0 | style="text-align:left; border-left:solid blue 5px; padding-left:0.8em;" |very cold | | −25 | style="padding-right:0.8em;" | −13 | style="text-align:left; border-left:solid grey 5px; padding-left:0.8em;" |frigid | | −40 | style="padding-right:0.8em;" | −40 | style="text-align:left; border-left:solid grey 5px; padding-left:0.8em;" |frigid |
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:
<math>Fahrenheit = {(} Celsius \times {1.8} {)} + {32}</math> or <math>Fahrenheit = {(} Celsius \times {9 \over 5} {)} + {32}</math> And to go the other direction: <math>Celsius = { {(} Fahrenheit - {32} {)} \over {1.8} }</math> or <math>Celsius = { {(} Fahrenheit - {32} {)} } \times { 5 \over {9} }</math><br clear=all>
A Celsius poem
Zero is freezing<br> 10 is not<br> 20 is warm<br> and 30 is hot
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 |
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.
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 | small craft advisory | |
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 | gale warning | 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 | storm warning | 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 scale. The scale below only applies to hurricanes originating in the Atlantic Ocean and northeastern 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+ |
mm | cm | inches |
---|---|---|
1 | 0.1 | 0.04 |
2 | 0.2 | 0.08 |
5 | 0.5 | 0.2 |
10 | 1 | 0.4 |
20 | 2 | 0.8 |
50 | 5 | 2 |
100 | 10 | 4 |
200 | 20 | 8 |
500 | 50 | 20 |
1000 | 100 | 40 |
2000 | 200 | 80 |
Rainfall is often measured in inches in the United States, but in most other countries in the world it is measured in millimetes or in litres per square metre. One inch of rain is equal to 25.4 mm rain, while millimtres 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 on the right 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.
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.