The unit conversions that are quietly wrong

Most conversions are arithmetic. A handful are traps, because two different things share one name.

Gallons, pints and fluid ounces

The United States and the United Kingdom use different definitions, and the difference is not a rounding error.

• US gallon — 3.785 litres. Imperial gallon — 4.546. The imperial one is a fifth larger.
• US pint — 473 ml. Imperial pint — 568 ml.
• US fluid ounce — 29.57 ml. Imperial fluid ounce — 28.41 ml.

Note the reversal in that last line. The imperial gallon is bigger, but the imperial fluid ounce is smaller — because an imperial gallon is divided into 160 fluid ounces and a US gallon into 128. Anyone who learned "imperial is bigger" as a rule gets this one backwards.

A converter offering a single "gallon" is wrong for half its readers and does not say so.

Two different tons

A short ton is 2,000 pounds and a long ton is 2,240 — twelve per cent apart. A metric tonne is 1,000 kg, close to a long ton but not equal to it. Freight quotes have gone wrong over this.

The temperature trap

This is the one nothing on the web handles. A temperature and a temperature difference convert differently.

10 °C is 50 °F. But a rise of 10 °C is a rise of 18 °F, not 50.

The reason is that the 32 belongs to where the Fahrenheit scale starts, and a difference has no starting point to shift — only the size of a degree matters, and a Fahrenheit degree is five ninths of a Celsius one.

So "the heating raised the room by 10 degrees, what is that in Fahrenheit?" gets answered as 50 by every converter that only does absolute values. It is 18.

Cups to grams depends on the ingredient

A US cup is a measure of volume. Grams are a measure of weight. Converting between them needs to know what is in the cup.

• A cup of flour is about 125 g.
• A cup of granulated sugar is about 200 g.
• A cup of honey is about 340 g.

Any page publishing one "cups to grams" number is wrong for nearly everything, which does not stop a great many of them doing it.

There are also three cups in current use: the US customary cup at 236.6 ml, the US legal cup at exactly 240 ml used on nutrition labels, and the metric cup at exactly 250 ml used in Australia and New Zealand.

Kilobytes and kibibytes

A kilobyte is 1,000 bytes. A kibibyte is 1,024. Drive makers use the decimal ones and operating systems mostly report the binary ones, which is the whole reason a "1 TB" drive shows up as 931 GB. Nothing is missing; the two are counting differently.

Frequently asked questions

Is a US gallon the same as a UK gallon?

No. A US gallon is 3.785 litres and an imperial gallon is 4.546 — the imperial one is a fifth larger. The same applies to pints and quarts derived from them.

Why is an imperial fluid ounce smaller when the imperial gallon is bigger?

Because an imperial gallon is divided into 160 fluid ounces and a US gallon into 128. The division differs as well as the total.

How do I convert a temperature difference?

Multiply by 9/5 with no offset. A rise of 10 °C is a rise of 18 °F. The 32 belongs to where the scale starts, and a difference has no starting point.

How many grams in a cup?

It depends entirely on what is in it — about 125 g of flour, about 200 g of sugar. A single figure cannot be right for both.

Why does my 1 TB drive show as 931 GB?

The maker counts a terabyte as a trillion bytes; the operating system reports tebibytes and gibibytes, which are powers of 1,024. Nothing is missing.