Where unit conversions come up: travel, cooking, fitness and work

Why conversions are everywhere

Two measurement systems are in daily use. Most of the world uses metric (SI), while the United States still uses US customary units, and the UK mixes metric and imperial. Anyone who travels, cooks from foreign recipes, trains at a gym, buys online or works with international suppliers will convert units. This guide shows where conversions come up most often and which ones to learn by heart.

Travel

Travel is where most people first meet a conversion they cannot avoid. Road signs, airline baggage limits, hotel thermostats and fuel pumps all use whichever system the country prefers.

• Luggage. Airlines weigh bags in kilograms or pounds, and 50 lb is slightly less than 23 kg, so a bag at the limit in pounds is just under the limit in kilograms. See airline baggage limits in kg and lb.
• Driving. Speed limits in km/h look high to a US driver: 100 km/h is only about 62 mph. Distances in miles feel short to a European: 100 miles is 161 km.
• Weather. A forecast of 30°C is a hot 86°F, and 5°C is a chilly 41°F. Keep one anchor in mind: 20°C is 68°F.
• Fuel. Litres per 100 km and miles per gallon run in opposite directions, so a lower number is better in one and worse in the other. US mpg is 235.215 divided by L/100 km, and UK mpg is 282.481 divided by L/100 km.

Cooking

Recipes cross borders more easily than ingredients do. US recipes use cups, sticks of butter and Fahrenheit, while most others use grams, millilitres and Celsius.

• Oven temperatures. Gas marks, Celsius and Fahrenheit all appear. A fan oven runs hotter than a conventional one, so many recipes lower the setting by about 20°C.
• Cups versus grams. A cup is a volume and a gram is a weight, so the answer depends on the ingredient. See US cups to grams and liquid measuring units.
• Scaling. Doubling or halving a recipe is a conversion too. See how to scale a recipe.

Fitness

Gyms, running clubs and health apps mix units freely. Weights, heights and distances all have two common forms.

• Weights. Plates in the US are in pounds and in most other countries are in kilograms. A 45 lb plate is 20.4 kg, not 20 kg.
• Body measurements. UK body weight uses stone and pounds, and heights are in feet and inches. Health professionals often need both in metric.
• Running. Races are marked in kilometres or miles, and the paces differ by 61 percent. See running pace: min/km to min/mile.

Work and trades

Professionals meet conversions in every field, and the cost of getting one wrong is higher than in daily life.

• Construction and DIY. Lumber, fasteners and drill bits come in inches or millimetres, and a US 2 × 4 is not 2 by 4 inches. See metric and imperial lumber, fasteners and drill bits and how much paint for a room.
• Web and print design. Layouts move between pixels, points, rem and inches. See px, rem, em, points and DPI.
• Marine and aviation. Navigation uses nautical miles and knots. See nautical miles and knots.
• Precious metals. Gold and silver are priced per troy ounce, which is heavier than a normal ounce. See troy vs avoirdupois ounces.
• Healthcare. Many doses are calculated per kilogram of body weight, so patients' weights recorded in pounds must be converted before dosing.
• Science and engineering. SI units are the standard, so US data often has to be converted before use.

When a conversion goes wrong

Mixing units has caused serious accidents. In 1999 NASA lost the Mars Climate Orbiter after one team supplied thrust data in pound-force seconds while the navigation software expected newton-seconds, so the spacecraft approached Mars too low. In 1983 an Air Canada Boeing 767 ran out of fuel at altitude after the fuel load was calculated in pounds instead of kilograms. The pilots glided it to a safe landing at Gimli, Manitoba. Both cases came down to a missing or wrong conversion step, not to complicated physics.

How to keep conversions simple

1. Learn a few anchors. 1 kg is about 2.2 lb, 1 inch is 2.54 cm, 1 mile is 1.6 km, 20°C is 68°F, and 1 litre is a little over a quart.
2. Write the unit next to every number. If the units do not cancel, the factor is upside down.
3. Check whether the sources mean the same unit. A gallon, pint, ton or cup can be a different size from one country to another.
4. Convert once. Convert at the start of a project and write the result down, rather than converting again and again.
5. Sanity-check the answer. Know which way it should go. A mile is longer than a kilometre, so the number of miles is smaller.
6. Use the exact factor for precision. Use a rounded shortcut only for quick estimates.

Common mistakes

• Using the wrong unit system for the audience, such as giving a US reader only metric figures.
• Mixing US and UK gallons, pints and tons.
• Confusing weight and volume units that share a name, such as ounces and fluid ounces.
• Rounding too early and carrying the error through.
• Trusting a quick mental conversion for something that needs accuracy.

For the full list of formulas, see unit conversion formulas by category.

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