How to use it
Type an amount, choose the unit you have and the unit you want. Fractions like 3/4 or 1 1/2 are accepted, and the answer changes as you type. The graduated jug beside the result fills to your amount and marks one cup in each regional standard, so you can see at a glance how a US cup compares with a Japan cup or a Taiwan rice cup. The “Standard used” box explains each unit in your conversion: its size in millilitres, whether that size is exact or conventional, and a short note on how it is defined.
How the conversion works
The converter stores every unit as a number of millilitres and divides one by the other. Nothing else is involved, so no ingredient data or rounding enters the calculation. The US units are derived from the US gallon of 231 cubic inches with the inch fixed at 2.54 cm, as set out in NIST Special Publication 1038: a US fluid ounce is 1/128 of that gallon, a cup is 8 fluid ounces, a tablespoon half an ounce and a teaspoon one sixth. That is why a US tablespoon is 14.787 mL rather than a round 15.
The UK fluid ounce comes from the Weights and Measures Act 1985, which defines the imperial pint as 568.26125 mL and the fluid ounce as one twentieth of it. The metric cup (250 mL), Japan cup (200 mL), metric spoons (15 mL and 5 mL) and the Australian tablespoon (20 mL) are round values by design. The Taiwan rice cup and the UK cup are marked conventional because no law fixes them.
When to double-check
The UK cup is the weakest entry. British law never defined a cup, and older British recipes used a breakfast cup of about 7 to 8 imperial fluid ounces. This site uses 8, which is 227.3 mL, but some converters use half a pint, 284 mL. If an old British recipe fails, try weighing instead of trusting either figure. Rice cooker cups also vary a little between brands, so the one in your drawer may not hold exactly 180 mL.
Fluid ounces are another trap. A recipe that just says “fl oz” could be US or UK, and a plain “oz” usually means weight, not volume. For water and milk the gap is small, but for spices, leavening and salt measured by the spoon, check whether the recipe uses a 15 mL or a 20 mL tablespoon before converting.