Measurements for Travelers: Luggage, Liquids, Speed Limits and Fuel Abroad
Travel is where unit conversion stops being an abstract exercise. A suitcase a few pounds over the limit can cost real money at the check-in desk, misreading a speed limit sign can cost a fine, and a fuel price that looks cheap per unit might be anything but. Most trips involve a switch between metric and US customary units at least once.
Here are the conversions worth knowing before you leave, with the rounding that airlines, road authorities and everyday life actually use.
Baggage allowances
Airlines usually state checked-baggage limits in both kilograms and pounds, and the two are rounded to convenient numbers rather than converted exactly. The common economy limit of 23 kg works out to about 50.7 lb, and US airlines typically quote it as 50 lb. The heavier 32 kg allowance used for many premium tickets is about 70.5 lb, usually quoted as 70 lb. Always check your own airline’s rules, because allowances vary by route and fare.
Since the pound figure is often rounded down, a bag that weighs exactly 51 lb on a home scale may still pass at an airline weighing in kilograms — but it is safest to stay under the lower number. A cheap handheld luggage scale pays for itself quickly.
| Kilograms | Pounds (exact) | Commonly quoted as |
|---|---|---|
| 7 kg | 15.4 lb | 15 lb (typical cabin-bag limit) |
| 10 kg | 22.0 lb | 22 lb |
| 20 kg | 44.1 lb | 44 lb |
| 23 kg | 50.7 lb | 50 lb |
| 32 kg | 70.5 lb | 70 lb |
The 100 mL liquids rule
Security screening at many airports limits liquids in carry-on bags to containers of 100 millilitres or less. In the US the TSA expresses the same rule as 3.4 ounces, since 100 mL is about 3.38 US fluid ounces. The containers must fit in a single clear quart-sized (roughly one-litre) bag, which is where the TSA’s “3-1-1” nickname comes from: 3.4 ounces, 1 quart-sized bag, 1 bag per passenger.
What matters is the size printed on the container, not how much is left inside. A 150 mL tube of toothpaste that is half empty will still be confiscated. Some airports with newer scanners have relaxed these limits, but rules change and differ between airports, so pack for the strictest rule on your route.
Speed limits and road distances
Almost every country outside the US and UK signs speed limits in kilometres per hour. To convert to miles per hour, multiply by about 0.62, or use the Fibonacci shortcut: 50 km/h is roughly 31 mph, 80 km/h roughly 50 mph, and 100 km/h roughly 62 mph. Many rental cars let you switch the speedometer’s main display between units — worth doing before you set off.
Distances on road signs follow the same pattern. A sign reading “Paris 120” in France means 120 kilometres, or about 75 miles.
| km/h | mph (approx.) | Typical setting |
|---|---|---|
| 30 | 19 | School zones, residential streets |
| 50 | 31 | Urban roads in much of Europe |
| 80 | 50 | Rural roads |
| 100 | 62 | Highways in many countries |
| 120 | 75 | Motorways in Spain, Italy and others |
| 130 | 81 | French motorways in dry conditions |
Fuel prices: per litre versus per gallon
Outside the US, fuel is priced per litre. To compare with American prices per gallon, multiply the per-litre price by 3.785 (the number of litres in a US gallon) and then convert the currency. A price of €1.80 per litre equals about €6.81 per US gallon before currency conversion. Fuel economy is commonly quoted as litres per 100 km in Europe; to convert, divide 235 by the L/100 km figure to get US miles per gallon (8 L/100 km ≈ 29 mpg).
Weather, hotel rooms and altitude
Weather forecasts abroad will usually be in Celsius. Remember a few anchors: 0°C is freezing, 20°C is mild, and 30°C is hot. Hotel listings outside the US commonly give room sizes in square metres; multiply by about 10.8 to get square feet, so a 25 m² room is roughly 270 sq ft.
Altitude is one place where feet are used internationally: aviation worldwide measures height in feet, which is why a pilot announces a cruising altitude of 35,000 feet (about 10,700 metres) even on a flight between metric countries. Mountain heights and ski resorts, however, are usually listed in metres outside the US.
Health: fever and medicine
If you need medical care abroad, body temperature will be measured in Celsius. A temperature of 38°C (100.4°F) is generally considered a fever. Medicine doses are almost always given in milligrams and millilitres, and doctors outside the US will record your weight in kilograms — it helps to know your own weight in kg before you travel.