One US tablespoon is 14.78676478125 millilitres. Almost every kitchen treats it as 15 ml, and for most cooking that rounding is harmless — but the tablespoon is the measure with the widest regional variation of any in the kitchen, and the differences are far larger than the rounding.
About the units
The US tablespoon is defined as half a US fluid ounce, which makes it exactly 3 US teaspoons and 1/16 of a US cup. Its metric value follows from the gallon chain: 3785.411784 ml per gallon divided by 256. The trouble starts outside the United States. The metric tablespoon used in most of Europe and in Sweden is exactly 15 ml. The imperial tablespoon in older British recipes is 17.758 ml. And the Australian tablespoon is 20 ml, four teaspoons rather than three — the only major tablespoon that breaks the three-teaspoon rule. An Australian recipe calling for two tablespoons of a strong ingredient converted at the American value delivers a third less than intended.
The exact factor
The exact value is 14.78676478125 ml, and the everyday approximation of 15 ml is high by 1.4 per cent. That error is invisible in a stew and meaningful in baking chemistry: three tablespoons of baking powder measured at 15 ml instead of 14.79 is an extra 0.6 ml of leavening. Against the Australian tablespoon the gap is not a rounding at all — 20 ml against 14.79 ml is 35 per cent more.
Where you meet this conversion
The tablespoon is the workhorse measure for oils, vinegars, sugar, flour thickeners and spices, which is exactly the range where proportion matters. Baking is the sensitive case, and salt is the notorious one: recipes that call for a tablespoon of salt assume a specific crystal size as well as a specific spoon volume, so the same measure delivers different amounts of sodium depending on whether the salt is fine or flaked. Medicine used to be dosed in tablespoons and no longer is, for the same reason teaspoons were abandoned — kitchen spoons are not measuring instruments.