Unit converter: the key conversion factors and classic mistakes

A recipe from an American site, height in feet in a profile, fuel economy in miles per gallon — the metric system constantly has to rub shoulders with the imperial one. Here's a cheat sheet for the main units and for the mistakes even experienced people make when converting.

Exact factors worth memorizing

• 1 inch = 2.54 cm (exactly, by international agreement);
• 1 mile = 1.609344 km; a nautical mile is different — 1.852 km;
• 1 pound = 453.59237 g — call it 454 g if the calculator scares you;
• 1 ounce (avoirdupois) = 28.35 g; don't confuse it with the fluid ounce — that's volume, 29.57 ml;
• 1 US gallon = 3.785 l, and the imperial gallon is bigger — 4.546 l.

Three classic mistakes

1. A fluid ounce and an ounce of weight are different units even though they look similar (fl oz vs oz). “8 oz of water” in a recipe is volume; “8 oz of flour” is weight.
2. A gallon without a country attached: US and imperial differ by about 20%. In movies and recipes, it's almost always the US gallon.
3. Feet and inches as one number: a height of “5 feet 7 inches” is not 5.7 feet but 5 + 7/12 feet — about 170 cm, not 173.

Converting without a calculator

For precise work there's the unit converter: pick a category (length, weight, volume, temperature), enter a value, get the result instantly. For mental estimates, two anchor points are enough: an inch is “about 2.5 cm” and a mile is “about 1.6 km”. Everything else reduces to multiplying by 4 or by 8.

Frequently asked questions

How do I convert centimeters to inches?

Divide the number of centimeters by 2.54: 30 cm is 11.81 inches. To go back, multiply inches by 2.54. The calculator above handles both conversions for length, weight and volume.

How many milliliters are in a tablespoon?

A metric tablespoon holds 15 ml and a teaspoon 5 ml. The US tablespoon is slightly smaller at 14.8 ml, but the difference is negligible in recipes.

Why do eyeballed unit conversions lead to mistakes?

Because systems disagree: a US mile is 1609 meters, a nautical mile is 1852, and a US gallon differs from an imperial one by almost a liter. One figure from a verified table beats a guess.