Most tablespoons hold 15 mL and most teaspoons 5 mL. The US spoons are a hair smaller, about 14.8 mL and 4.9 mL, a difference too small to notice in cooking. The one spoon that really differs is the Australian tablespoon: 20 mL, a third larger than the others, as the table above shows.
Where each size comes from
The US tablespoon and teaspoon are fractions of the US fluid ounce. NIST Special Publication 1038 lists the tablespoon as half a fluid ounce and the teaspoon as one sixth, and since the fluid ounce is about 29.57 mL, that gives 14.787 mL and 4.929 mL. They are exact by definition, just not round. For nutrition labels, the FDA rule at 21 CFR 101.9 rounds them to 15 mL and 5 mL, and NIST gives the same round figures as the practical metric equivalents for cooking. In practice, US and metric spoons can be used interchangeably.
Japan settled on the same round sizes. Kagawa Nutrition University, which introduced the 200 mL measuring cup for Japanese home cooking, uses a 15 mL 大さじ and a 5 mL 小さじ, and the university also sells a tiny 1 mL spoon for seasonings. Taiwan recipes use the same pair, called 大匙 and 小匙. Australia went a different way. Its measuring spoons follow an Australian Standard with a 20 mL tablespoon equal to 4 teaspoons, as listed in the taste.com.au measurement charts. We could not access the standard itself, so the 20 mL figure here rests on that published chart.
How to tell which one a recipe means
The source is the best clue. Recipes from Australian magazines, supermarkets and food sites use 20 mL tablespoons, and some say so on the page. American, Japanese and Taiwan recipes can all be read as 15 mL. If a recipe gives both “tbsp” and a millilitre figure, the millilitres settle it: 15 mL or 20 mL.
Look at your own spoons as well. A set bought in Australia usually has a 20 mL tablespoon, and using it on an American recipe adds a third to every tablespoon. Watch the abbreviations too: a capital “T” or “tbsp” is a tablespoon, a lower-case “t” or “tsp” is a teaspoon, and confusing them triples the amount.
When the difference matters
Salt shows the problem clearly. King Arthur Baking and USDA put a US tablespoon of fine table salt at about 18 g. An Australian 20 mL tablespoon of the same salt is about 25 g. If you make an American bread with an Australian spoon, the dough gets about 7 g of extra salt per tablespoon, which you will taste in the bread. Going the other way, an Australian recipe measured with a 15 mL spoon ends up a quarter short on every tablespoon of baking powder or spice. The fix is easy: count an Australian tablespoon as 4 teaspoons.
Salt also shows that the spoon is not the only variable. King Arthur Baking notes that a tablespoon of kosher salt weighs roughly 8 to 16 g depending on the brand, because the flakes are larger and hollower. For large amounts of salt, sugar or flour, grams beat spoons every time. For a teaspoon of vanilla or soy sauce, the 1.4% gap between US and metric spoons will never show.