US Unit Conversion: Imperial and Metric Guide
The United States is one of only three countries in the world that has not fully adopted the metric system for everyday use. Most of the world uses kilometres, kilograms, Celsius and litres. Americans use miles, pounds, Fahrenheit and gallons.
This creates regular conversion needs for Americans traveling abroad, using international products, reading foreign recipes, working in science or medicine or communicating with colleagues in other countries. This guide covers every major US to metric conversion with quick reference tables and links to the free tools on CalConvs. Browse all converters at the Unit Converters hub.
Distance: Miles to Kilometres
When driving in Canada, Mexico or Europe, or when using international navigation apps, you will need to convert miles to kilometres.
| Miles | Kilometres |
|---|---|
| 1 mile | 1.609 km |
| 5 miles | 8.047 km |
| 10 miles | 16.09 km |
| 25 miles | 40.23 km |
| 50 miles | 80.47 km |
| 100 miles | 160.93 km |
| 1 km | 0.621 miles |
| 100 km | 62.14 miles |
Use the Length Converter and the Speed Converter for all distance and speed conversions including km/h to mph.
Weight: Pounds to Kilograms
International shopping, travel weight limits, medical records abroad and nutrition science all use kilograms. The quick mental rule is to divide pounds by 2.2.
| US Pounds | Kilograms | UK Stone (approx) |
|---|---|---|
| 100 lbs | 45.4 kg | 7 st 2 lbs |
| 120 lbs | 54.4 kg | 8 st 8 lbs |
| 140 lbs | 63.5 kg | 10 st 0 lbs |
| 160 lbs | 72.6 kg | 11 st 6 lbs |
| 180 lbs | 81.6 kg | 12 st 12 lbs |
| 200 lbs | 90.7 kg | 14 st 4 lbs |
| 250 lbs | 113.4 kg | 17 st 12 lbs |
Use the Weight and Mass Converter for all weight conversions. For health calculations, use the BMI Calculator which accepts both pounds and kilograms.
Temperature: Fahrenheit to Celsius
When traveling abroad, checking an international weather forecast, cooking from a foreign recipe or working in a laboratory, you will need to convert Fahrenheit to Celsius.
Fahrenheit to Celsius Formula
Exact: Celsius = (Fahrenheit − 32) ÷ 1.8
Quick estimate: subtract 32, then divide by 2 (slightly low but close enough for weather)
Example: 77 F → (77 − 32) ÷ 1.8 = 45 ÷ 1.8 = 25 C
| Fahrenheit | Celsius | Context |
|---|---|---|
| 32 F | 0 C | Freezing point of water |
| 50 F | 10 C | Cold day |
| 68 F | 20 C | Comfortable room temperature |
| 77 F | 25 C | Warm spring day in Europe |
| 86 F | 30 C | Hot summer day in southern Europe |
| 98.6 F | 37 C | Normal body temperature |
| 212 F | 100 C | Boiling point of water |
| 350 F | 177 C | Moderate oven |
| 400 F | 204 C | Hot oven |
Use the Temperature Converter for any Fahrenheit to Celsius or Kelvin conversion.
Volume: Gallons, Cups and Fluid Ounces to Litres
American recipes use cups, tablespoons and fluid ounces. International recipes use millilitres and grams. Fuel abroad is priced per litre rather than per gallon.
| US Unit | Metric Equivalent |
|---|---|
| 1 US fluid ounce | 29.57 ml |
| 1 US cup (8 fl oz) | 236.6 ml |
| 1 US pint (16 fl oz) | 473.2 ml |
| 1 US quart (32 fl oz) | 946.4 ml |
| 1 US gallon (128 fl oz) | 3,785 ml (3.785 litres) |
| 1 litre | 33.81 US fl oz or 4.23 US cups |
| 1 litre | 0.264 US gallons |
Use the Volume Converter for all liquid volume conversions.
Area: Square Feet to Square Metres
When buying property, renting apartments or comparing real estate internationally, you will often need to convert square footage to square metres.
| Square Feet | Square Metres |
|---|---|
| 500 sq ft | 46.5 m² |
| 750 sq ft | 69.7 m² |
| 1,000 sq ft | 92.9 m² |
| 1,500 sq ft | 139.4 m² |
| 2,000 sq ft | 185.8 m² |
| 3,000 sq ft | 278.7 m² |
Use the Area Converter for property and land area conversions including acres, hectares and square miles.
Fuel Economy: MPG to L/100km
When renting a car abroad or comparing an imported vehicle, fuel economy is expressed differently. The US uses miles per gallon (MPG). Most other countries use litres per 100 kilometres (L/100km).
| US MPG | Litres per 100km |
|---|---|
| 25 MPG | 9.41 L/100km |
| 30 MPG | 7.84 L/100km |
| 35 MPG | 6.72 L/100km |
| 40 MPG | 5.88 L/100km |
| 50 MPG | 4.70 L/100km |
| 60 MPG (hybrid) | 3.92 L/100km |
Use the Fuel Consumption Converter for MPG to L/100km and all other fuel economy conversions.
Related Tools for US Users
- Length Converter: miles, feet and inches to metric
- Weight Converter: pounds and ounces to kilograms and grams
- Temperature Converter: Fahrenheit to Celsius and Kelvin
- Volume Converter: US cups, gallons and fluid ounces to metric
- Area Converter: square feet and acres to metric
- Fuel Consumption Converter: MPG to L/100km
- All Unit Converters: browse all converters on CalConvs
Frequently Asked Questions
Why does the US still use imperial units?
The US has attempted metrication several times, most notably with the Metric Conversion Act of 1975, but adoption remained voluntary and largely failed in public life. Science, medicine, the military and the pharmaceutical industry in the US already use metric units. Consumer and everyday use of imperial measurements is deeply embedded in culture, infrastructure and legislation, making a full transition politically and practically difficult.
What is the quickest way to convert miles to kilometres mentally?
Multiply miles by 1.6 for a quick approximation. So 50 miles is approximately 80 km. For a more precise figure, multiply by 1.609. If you prefer, use the Fibonacci sequence trick: each Fibonacci number in miles is approximately the next Fibonacci number in kilometres (5 miles ≈ 8 km, 8 miles ≈ 13 km, 13 miles ≈ 21 km).
Why does the US use different sized cups and pints than the UK?
US customary units and UK imperial units diverged historically. The US gallon (3.785 litres) is smaller than the UK imperial gallon (4.546 litres). As a result, the US pint (473 ml) is smaller than the UK pint (568 ml) and the US cup (236.6 ml) differs from the Australian metric cup (250 ml). Always specify which country's cup or pint is intended when following international recipes.
Do US doctors use metric or imperial units?
US medicine uses metric units almost exclusively: weight in kilograms (though patients are often weighed in pounds and converted), height in centimetres, temperature in Celsius in clinical contexts, drug dosages in milligrams and millilitres, and laboratory values in international SI units. If you are preparing for a medical appointment in the US, having your weight in kilograms and height in centimetres is helpful.