How to convert cups, milliliters, and gallons
Volume is where unit conversion gets genuinely treacherous, because the same words mean different sizes on different sides of the Atlantic. This converter uses US customary volumes throughout. A US cup is 236.588 mL, a US fluid ounce is 29.574 mL, a US tablespoon is 14.787 mL, and a US teaspoon is 4.929 mL. Everything normalizes through milliliters, so the metric units are exact and the customary ones carry their full defined precision.
The relationships among the US units are clean once you know them: 3 teaspoons make a tablespoon, 2 tablespoons make a fluid ounce, 8 fluid ounces make a cup, 2 cups make a pint, 2 pints make a quart, and 4 quarts make a gallon. So a US gallon is 128 fluid ounces, which works out to 3,785.41 mL — just under 3.8 liters. Memorizing that ladder lets you sanity-check almost any kitchen conversion without a calculator.
The single biggest gotcha is the gallon. A US gallon is 3.785 liters, but an imperial (UK) gallon is 4.546 liters — about 20% larger. The imperial pint and fluid ounce differ too. This is why a car's fuel economy in "miles per gallon" isn't comparable between the US and UK without knowing which gallon, and why a British recipe's "pint" of liquid is bigger than an American one. This converter is US throughout, so feed it US measures.
A second trap is confusing volume with weight. "One cup of flour" and "one cup of water" occupy the same volume but weigh very different amounts, because density differs. This converter handles volume-to-volume only — milliliters to cups, cups to liters. If you need cups of flour as grams, that's a density conversion, not a unit one, and it belongs in a baking calculator rather than here.
Worked example
A US recipe calls for 2 cups of stock, but your measuring jug is marked only in milliliters.
- 1 US cup = 236.588 mL
- 2 × 236.588 = 473.18 mL
→ 2 US cups ≈ 473 mL — almost exactly a US pint, which makes sense, since 2 cups is 1 pint.