Metric vs. US Customary: Converting Recipes Between Systems

The US customary system defines its cooking volumes from the fluid ounce. A US legal teaspoon is one sixth of a US fluid ounce, which works out to exactly 4.92892159375 millilitres. A tablespoon is 3 teaspoons, or 14.79 mL. A cup is 48 teaspoons, or 236.6 mL.

The metric kitchen rounds everything to convenient numbers. A metric teaspoon is 5 mL, a metric tablespoon is 15 mL, a metric cup is 250 mL. These are definitions of convenience, not conversions of the US units, which is why they do not line up.

At teaspoon scale the discrepancy is negligible. 1.4% of a teaspoon of salt is 0.09 g — far below what any measuring spoon or human hand can resolve. Nobody has ever ruined a dish by using a metric teaspoon in an American recipe.

At cup scale it stops being negligible. A metric cup is 5.7% larger than a US cup. Three cups of flour measured metric instead of US is roughly 21 g of surplus flour, which is enough to change a cake.

Everything you need to reconcile the two systems in one list:

This is the deeper difference, and it matters more than the millilitres.

A metric recipe from Britain, Germany or France will typically say 250 g flour, 125 g butter, 8 g salt. A US recipe will say 2 cups flour, 1 stick butter, 1½ teaspoons salt. The first is a specification; the second is a procedure that depends on how you fill the cup.

That is why converting a US recipe to metric is harder than converting a metric recipe to US. Going metric-to-US means dividing a known weight by a density figure and accepting that the reader will introduce a few percent of error with their cup. Going US-to-metric means guessing which packing state the original author had in mind — and a cup of flour is 125 g spooned or around 150 g scooped, a choice the recipe rarely states.

Small quantities escape this problem, which is a large part of why teaspoons survive in metric kitchens. A teaspoon of baking powder is 4.0 g and a teaspoon of vanilla is 4.3 g regardless of which system the recipe was written in, because a teaspoon is too small to compress much and both systems agree on its size to within a percent and a half.

You do not need a table for most of a recipe. Six figures cover the overwhelming majority of cross-system conversions:

The most reliable way to move a recipe between systems is to stop moving volumes at all. Convert everything to grams once, cook in grams, and the question of whose cup or whose tablespoon disappears.

This works because a gram is a gram everywhere. There is no US gram and no metric gram, no packed or spooned gram, no brand variation. 202 g of sugar is the same quantity in Ohio and in Osaka, whereas "a cup of sugar" carries an implicit definition of the cup and an implicit assumption about how it was filled.

It also removes the compounding problem. Volume errors accumulate: a cup measured 5% heavy, three times over, is 15% heavy in total, and the recipe's ratios shift with it. Weights do not accumulate error at all, because each weighing is independent of the last.

Teaspoons remain the sensible exception at the small end. Below about 5 g, weighing on a typical 1 g kitchen scale is less accurate than a levelled spoon, because the scale's rounding is a large fraction of the reading. A teaspoon of yeast at 3.1 g or a quarter teaspoon of baking soda at 1.2 g is genuinely better measured with a spoon unless you own a 0.1 g scale.

That gives the hybrid format most professional recipe writing has settled on: grams for the bulk, teaspoons for the potent small quantities, and no cups at all. A recipe written that way needs no conversion when it crosses a border.

Volume is not the only thing that changes across the Atlantic, and the other two catch people out just as often.

Oven temperatures convert by the usual formula — subtract 32, multiply by 5/9 — but recipes almost never state the exact figure. A US recipe at 350°F becomes 177°C, which every European recipe writes as 180°C. At 375°F it is 191°C, written as 190°C. The rounding is harmless; the trap is gas marks and fan ovens. A fan oven runs roughly 20°C hotter than its dial suggests, so a conventional 180°C becomes 160°C fan.

Tin sizes are given in inches in the US and centimetres elsewhere. A 9-inch round tin is 23 cm and an 8-inch is 20 cm, and those two are not interchangeable: the 9-inch has about 27% more floor area, so the same batter bakes shallower and faster in it. If you swap tins, adjust the time before you adjust anything else.

Flour itself differs. US all-purpose flour sits at a higher protein content than British plain flour and lower than British strong bread flour. For cakes this rarely matters; for bread it changes hydration, and no volume conversion will tell you that.

Converting a metric recipe into US measures is mechanical. Take each weight, divide by the grams-per-teaspoon figure for that ingredient, and express the result in the largest sensible spoon or cup. 8 g of salt becomes 1.33 teaspoons; 20 g of sugar becomes 4.76.

Converting a US recipe into metric requires a judgement call on every volumetric ingredient, because you are reconstructing information the original author never recorded. The honest approach is to use spooned-and-levelled reference weights, note that you have done so, and be prepared to adjust hydration on the first bake.

If you are writing recipes rather than following them, give both: weights for anything above about 20 g, and spoons for anything below. That is the format that survives being read in either system, and it is why professional recipe books have quietly converged on it.