Units in recipes and travel, without the surprises

In the kitchen: cups and spoons

American recipes measure by volume. The US customary cup is 236.6 millilitres, the tablespoon 14.8 mL and the teaspoon 4.9 mL, with 3 teaspoons to a tablespoon and 16 tablespoons to a cup. Nutrition labels in the United States round the cup to 240 mL, so you will see both figures.

Other countries use different cups under the same name. Australia, New Zealand and Canada use a 250 mL metric cup, and Australia also uses a 20 mL tablespoon rather than 15 mL. For most dishes the difference of about 6% is harmless. For baking, where the ratio of flour to liquid matters, it is not: if a recipe comes from an Australian site, a US cup measure will give you slightly less of everything, and an Australian tablespoon of baking powder is a third more than an American one.

Ounces: weight or volume?

An ounce (oz) is a unit of weight, 28.35 grams, so 8 oz of butter is 226.8 g. A fluid ounce (fl oz) is a unit of volume, 29.6 mL in the US. Recipes often write just "oz" for both. A useful rule: if the ingredient is a liquid and is measured in a jug, it is fluid ounces; if it is weighed on a scale, it is weight.

Converting between weight and volume — cups of flour to grams, for example — needs the density of the ingredient, and it varies with how the ingredient is packed. A cup of flour can weigh anywhere from about 120 to 150 grams depending on whether it was spooned in or scooped. A unit converter cannot do this conversion, and one that claims to is using an assumed density. For baking, a recipe that gives grams is more reliable than any conversion; if it gives only cups, use the recipe author's own gram equivalents when they publish them.

Oven temperatures

Fahrenheit and Celsius do not convert by simple multiplication, because their zero points differ: °F = °C × 9/5 + 32. The unit converter handles this correctly, but recipes rarely use exact conversions. 180 °C is exactly 356 °F, and recipes round it to 350 °F; 350 °F is really 176.7 °C, and recipes round it to 180 °C. Oven thermostats are seldom accurate to within ten degrees anyway, so the rounded pairs are fine. What matters more is whether the recipe assumes a fan (convection) oven, which is usually set about 20 °C lower.

Older British recipes sometimes give a gas mark instead of a temperature. Each step from Gas Mark 1 to 9 adds about 25 °F: Gas Mark 1 is 275 °F, Gas Mark 4 is 350 °F (about 180 °C) and Gas Mark 9 is 475 °F.

On the road: speed and distance

The United States and the United Kingdom sign speeds and distances in miles. One mile is 1.609 km, so 100 km/h is 62.1 mph and a 70 mph limit is 112.7 km/h. A marathon, 42.195 km, is 26.2 miles. A quick mental shortcut is that 5 miles is about 8 km. Nautical miles (1,852 m) and knots are used at sea and in aviation, and appear on ferry and flight information.

Fuel is where travellers get caught out. A US gallon is 3.785 litres; an imperial (UK) gallon is 4.546 litres, about 20% more. The UK sells fuel by the litre but still quotes fuel economy in miles per imperial gallon. Because the UK gallon is bigger, the same car gets a higher mpg figure there: 30 US mpg is 7.84 litres per 100 km, while 30 UK mpg is 9.42 L/100 km — a thirstier car. When comparing cars from different markets, convert both to litres per 100 km. The unit converter does not convert fuel economy directly, but the arithmetic is short: litres per 100 km = 235.2 ÷ US mpg, or 282.5 ÷ UK mpg.

Weather, body temperature and height

Forecasts in the United States are in Fahrenheit. A few anchors make them readable without a converter: 0 °C is 32 °F, 10 °C is 50 °F, 20 °C is 68 °F, 30 °C is 86 °F, and −10 °C is 14 °F. Normal body temperature, 37 °C, is 98.6 °F. A rough mental conversion — double the Celsius figure and add 30 — is close enough for deciding on a coat, but it drifts badly at the extremes, so use the converter for anything medical.

Heights are given in feet and inches: 5 ft 10 in is 177.8 cm and 6 ft is 182.88 cm. Body weight in the UK and Ireland is often given in stone, where one stone is 14 pounds or 6.35 kg, so 11 stone is 69.9 kg.

Luggage, shopping and housing

Airline baggage allowances are usually set in kilograms with a rounded pound equivalent: a 23 kg checked bag is quoted as 50 lb, though 23 kg is actually 50.7 lb and 50 lb is 22.7 kg. If your bag is close to the limit, the stricter of the two numbers is the one the check-in scale will be set to.

Food in American shops is priced by the pound (0.454 kg). In China, markets price by the jin, which is exactly 500 g, so a pound is a little less than a jin — 0.907 jin. Property listings in the US and UK use square feet: one square metre is 10.76 square feet, so an 80 m² flat is about 861 ft². In China, land is still measured in mu, where one mu is 666.7 m² and one acre is 6.07 mu.

Tips, taxes and discounts

Travel also brings percentage arithmetic that is easy to get wrong in your head. The percentage calculator handles each of these in one step:

• Tips. A 15% tip on a bill of 86 is 12.90; 20% is 17.20. In the US, tips are usually worked out on the amount before sales tax.
• Prices that include tax. A price of 120 including 20% VAT is 100 before tax — not 96, which is what subtracting 20% of 120 gives. This matters when claiming a VAT refund on departure.
• Stacked discounts. "20% off, plus an extra 10% off" is 28% off, not 30%: the second discount applies to the already-reduced price.
• Up and down. A price that rises 50% and then falls 50% ends 25% lower than it started.

Before you convert

1. Find out which country the recipe, sign or listing comes from. That settles which cup, gallon or ton is meant.
2. Check whether "oz" is a weight or a volume.
3. Convert, then round to the precision the task needs: a whole gram for baking, a whole kilometre per hour for driving.
4. For temperatures, prefer the recipe's own rounded pair over an exact conversion.

Limits

• Weight-to-volume conversions are not supported, because they depend on the ingredient. Use a recipe's own gram values.
• Regional units are listed by name. Check which cup, tablespoon, gallon or ton your source means before converting.
• Results show up to ten significant figures. Round them to a sensible precision for the task — no kitchen scale needs 226.796185 g.

Sources and further reading

• NIST: Unit conversion (US customary and SI)
• NIST Special Publication 811: Guide for the Use of the SI
• 21 CFR 101.9: US nutrition labelling (1 cup = 240 mL)