The two cups, side by side
The US customary cup is defined as 8 US fluid ounces, or 236.588 ml. The metric cup is 250 ml, chosen because it is a round number in the metric system. A recipe that calls for two cups of milk means 473 ml in the US and 500 ml in Australia — a difference of 27 ml, or about one and a half tablespoons.
The discrepancy compounds with dry ingredients because their weight per cup also depends on the measuring convention. Two US cups of all-purpose flour (spooned) weigh about 240 g; two metric cups of the same flour weigh about 255 g. That 15-gram difference is roughly three tablespoons of flour and can be the difference between a moist crumb and a dry one.
How to tell which cup a recipe uses
Start with the source. American websites and cookbooks generally use US cups; Australian, New Zealand, and many European sites use 250 ml cups; UK recipes often use metric weights entirely. If the recipe lists grams alongside cups, that is a strong signal the author was working in metric and the cup is likely 250 ml.
Look for other clues: temperatures in Celsius, oven fan settings, and ingredients sold by weight all point to a metric-origin recipe. A recipe that lists “1 cup (250 ml)” or “1 cup (8 fl oz)” settles the question directly. When nothing is stated, assume the author's local convention rather than converting blindly.
Converting recipes between the two systems
For liquids, multiply US cups by 236.588 to get milliliters, or by 250 to go from metric cups. The difference is about 13.4 ml per cup. For a single cup in a sauce, adjust to taste; for a cake or bread, use the exact volume the author intended.
For dry ingredients, convert through weight rather than volume. Look up the ingredient's grams per US cup, multiply by the number of cups, then divide by the metric cup weight if you only have a 250 ml cup. Better still, use a scale: 240 g of flour is 240 g regardless of which cup the recipe had in mind.
Spoons differ too
Australia uses a 20 ml tablespoon, while the US uses 14.7868 ml and the UK standard is 15 ml. A recipe calling for three Australian tablespoons adds 60 ml, not the 44.4 ml a US cook would measure. Teaspoons are 5 ml in both the US and metric systems, so small spoon measures are less affected.
This is another reason to prefer weights for anything sensitive. Spoon conventions vary by country in ways that are rarely stated, while grams do not.
A practical rule set
Use the recipe's own cup when the author states it. When they do not, check the site's country and the units elsewhere in the recipe. For baking, convert everything to grams and note your numbers on the recipe. For cooking, a 5 percent difference in liquid volume almost never matters.
If you regularly cook from international sources, keep both a US cup set and a 250 ml metric cup in the drawer. Labeling them once saves a conversion on every recipe.
A quick conversion table
The values below convert the same nominal quantity between the two cup systems. They are useful when you are reading a recipe from another country and only have one set of cups in the drawer.
- 1 US cup = 236.6 ml · 1 metric cup = 250 ml
- 2 US cups = 473.2 ml · 2 metric cups = 500 ml
- 3 US cups = 709.8 ml · 3 metric cups = 750 ml
- 4 US cups (1 US quart) = 946.4 ml · 4 metric cups = 1,000 ml
- 1 US tablespoon = 14.79 ml · 1 Australian tablespoon = 20 ml