Unit Conversion Done Right: Exact Factors, Mental Shortcuts, and the Traps

In 1999 NASA lost a $125 million Mars orbiter because one team worked in pound-force seconds and another in newton-seconds. Most unit mistakes are cheaper than that, but they follow the same pattern: two numbers that look compatible, one hidden assumption, and nobody checking. This guide covers the conversions people actually need, the ones that are traps (there are three kinds of ton, two kinds of gallon, and a “mile” that isn’t a mile), and how to convert without introducing rounding errors.

The conversions worth memorising

From	To	Multiply by	Rough mental version
Inches	Centimetres	2.54 (exact)	×2.5
Feet	Metres	0.3048 (exact)	×0.3
Miles	Kilometres	1.609344 (exact)	×1.6, or ×8÷5
Pounds	Kilograms	0.45359237 (exact)	÷2.2
Ounces	Grams	28.3495	×28
US gallons	Litres	3.78541	×3.8
UK gallons	Litres	4.54609	×4.5
°F	°C	(°F − 32) × 5⁄9	Subtract 30, halve
°C	°F	°C × 9⁄5 + 32	Double, add 30

Four of those are exact by international agreement (inch, foot, mile and pound were all redefined in terms of metric units in 1959), which is why the Unit Converter can give you as many decimal places as you like for those without accumulating error.

The traps

Three tons

A UK (long) ton is 2,240 lb, a US (short) ton is 2,000 lb, and a metric tonne is 1,000 kg, which is 2,204.6 lb. Shipping quotes, vehicle weights and commodity prices all use different ones. If a document says “ton” without qualification, ask.

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Two gallons, two pints, two fluid ounces

The US gallon is about 83% of the UK (imperial) gallon. A US pint is 16 US fl oz; an imperial pint is 20 imperial fl oz, and the fluid ounces themselves differ by about 4%. Recipes and fuel economy figures are where this bites: 30 miles per US gallon is 36 miles per imperial gallon for the same car.

The nautical mile

1,852 metres, about 15% longer than a statute mile, used at sea and in the air. A knot is one nautical mile per hour. The Speed Conversion Calculator handles knots alongside km/h and mph.

Cups, tablespoons and teaspoons

A US cup is 240 ml (legally, for nutrition labels) or 236.6 ml (customary); a metric cup is 250 ml; an Australian tablespoon is 20 ml against 15 ml almost everywhere else. Baking recipes fail on exactly these differences. Weigh ingredients when it matters.

Temperature differences vs. temperatures

A temperature of 20 °C is 68 °F. A temperature rise of 20 °C is a rise of 36 °F, because the +32 offset only applies to absolute readings. Weather reports get this wrong regularly.

Rounding without lying

A conversion cannot add precision. If a spec says “about 5 feet”, writing “1.524 m” implies millimetre accuracy nobody measured. Match the number of significant figures to the input: 5 ft → 1.5 m. Conversely, when a limit is stated in one system (a 20 kg baggage allowance), convert generously in the safe direction (44 lb, not 44.1) so rounding never pushes you over.

Chain conversions carefully. Converting inches to feet to metres, rounding at each step, drifts; convert directly in one step and round once at the end. The converter does this internally, which is the main reason to use one rather than a chain of mental approximations.

Where each system still rules

• Metric everywhere except the US, Liberia and Myanmar for daily life; science and medicine worldwide.
• US customary in the US for road distances, body weight, cooking, construction and fuel.
• Imperial remnants in the UK: miles on roads, pints in pubs, stones and pounds for body weight, but Celsius for weather and litres for fuel.
• Aviation uses feet for altitude and knots for speed almost universally, even in metric countries.
• Screens and print use inches for diagonal size and points (1⁄72 inch) for type, everywhere.

Frequently asked questions

How many centimetres in an inch, exactly?

2.54, by definition since 1959. It is one of the few conversions with no rounding.

Why are US and UK gallons different?

The UK redefined the gallon in 1824 based on the volume of 10 lb of water; the US kept the older “wine gallon” of 231 cubic inches. Both survived.

What is the quickest way to convert Celsius to Fahrenheit in my head?

Double it and add 30. It is within 2–3 degrees between 0 and 40 °C, which is fine for weather. Use the exact formula (×1.8 + 32) for cooking or medicine.

Is a metric tonne the same as a ton?

No. A tonne is 1,000 kg; a US ton is 907 kg; a UK ton is 1,016 kg.