Temperature: the one that confuses people most
Temperature is where visitors to the US get confused first, and where Americans traveling elsewhere get confused equally fast. A weather forecast showing 32°C and one showing 90°F are describing the same very hot day.
The exact formula: Fahrenheit to Celsius — subtract 32, multiply by 5, divide by 9. Celsius to Fahrenheit — multiply by 9, divide by 5, add 32.
In practice, mental math shortcuts work better for quick estimates while you're traveling:
For Celsius to Fahrenheit: double the Celsius temperature and add 30. It's not perfectly accurate (the real formula adds 32 and uses a 9/5 multiplier), but "20°C → double to 40, plus 30 = 70°F" gives you a useful ballpark. The actual answer is 68°F, which is close enough for deciding what to wear.
For Fahrenheit to Celsius: subtract 30 and halve it. 80°F → minus 30 = 50, halved = 25°C. The actual answer is 26.7°C. Again, good enough for practical purposes.
A few landmarks worth memorizing: 0°C/32°F is freezing. 20°C/68°F is a comfortable room. 37°C/98.6°F is normal body temperature. 100°C/212°F is boiling water.
Distance: speed limits, road signs, and walking
European road signs show kilometers per hour. US signs show miles per hour. A speed limit of 100 km/h is roughly 62 mph. A speed limit of 65 mph is roughly 105 km/h.
For rough mental conversion: 1 mile is roughly 1.6 km. So to convert miles to kilometers, multiply by 1.6 (or: add 60% of the value). To convert km to miles, divide by 1.6 (or: multiply by 0.6).
Walking and running distances: a 5K run is 3.1 miles. A 10K is 6.2 miles. A half marathon (21.1 km) is 13.1 miles. A marathon (42.2 km) is 26.2 miles.
In cities, a common question is how far something is. One kilometer of easy walking takes about 10–12 minutes. One mile takes about 15–20 minutes. "It's about a kilometer away" and "it's about a half mile away" are roughly the same distance.
Weight: luggage limits and personal measurements
Airlines post luggage limits in either kilograms or pounds depending on the route and airline. European carriers typically use kilograms; US domestic flights typically use pounds.
1 kilogram is approximately 2.2 pounds. So a 23 kg luggage limit (common for economy class international flights) is about 50 pounds. A 32 kg limit (business class or one-bag allowances) is about 70 pounds.
If you're weighing yourself or discussing your weight between countries: 1 kg = 2.2 lbs. A person who weighs 70 kg weighs roughly 154 pounds. Someone who weighs 150 lbs weighs about 68 kg.
For food and recipes: 1 pound is 454 grams. American recipes often call for ingredients in cups, which can cause confusion. 1 cup of water or milk is 240 ml. 1 cup of flour is around 120–130 grams depending on how tightly it's packed.
Volume: fuel, drinks, and medicine
Gas or petrol is sold by liters in most of the world and by gallons in the United States. 1 US gallon equals 3.785 liters. If you're renting a car and trying to budget for fuel, 1 liter is roughly worth a quarter gallon, meaning filling a tank that takes 50 liters is roughly equivalent to filling a 13-gallon tank.
Drinks: European beer is often described in centiliters on menus (33 cl, 50 cl), which some visitors find confusing. 33 cl is 330 ml, the standard can size. 50 cl is 500 ml, half a liter, a standard pint-ish serving.
Medicine doses are always in milliliters or milligrams internationally. If an American over-the-counter medicine lists a dose in teaspoons or tablespoons, 1 teaspoon is 5 ml and 1 tablespoon is 15 ml. Converting back to metric before using a measuring syringe avoids dosing errors.
Height: you, your children, doorframes
American heights are described in feet and inches. Most other countries use centimeters or meters. 1 inch is 2.54 centimeters. 1 foot is 12 inches, which is 30.48 centimeters.
Common heights converted: 5 feet = 152 cm. 5'6" = 168 cm. 5'10" = 178 cm. 6 feet = 183 cm. 6'2" = 188 cm.
For room heights and doorframes: standard residential ceiling height in the US is 8 feet (2.44 m). European apartments often have higher ceilings, 2.6–3 m, though older buildings vary widely.
Quick reference: conversions worth memorizing
These are the ones that come up most often, prioritized by how frequently travelers actually need them:
1 mile = 1.6 km. 1 km = 0.62 miles. 1 kg = 2.2 lbs. 1 lb = 0.45 kg. 1 liter = 0.26 US gallons. 1 US gallon = 3.79 liters. 1 inch = 2.54 cm. 1 foot = 30.5 cm. 1 meter = 3.28 feet. Celsius to Fahrenheit: (C × 1.8) + 32. Fahrenheit to Celsius: (F − 32) ÷ 1.8.
If you're in a country where you don't recognize the units and need to do a quick conversion, our online unit converter handles length, weight, and common measurements without requiring an account or app installation.
A word about clothing sizes
Clothing sizes deserve a brief mention because they cause genuine problems, and they don't follow a clean conversion formula—each brand calibrates differently. European sizes for clothes generally run about 30 higher than US sizes for women (a US size 6 is roughly a European 36, though this varies significantly by brand and country), and a full size larger for men's shoes (US 10 is roughly EU 44).
The only reliable approach with clothing is to look at the garment's actual measurements in centimeters and compare to your own measurements. A size chart gives approximations; the measurements give you something concrete to work with.