Mug capacity is the most common source of disappointment in drinkware, because “large” is not a measurement and photographs have no scale. Here is what the numbers correspond to in real drinks.
2-5 oz – espresso and demitasse
A single espresso is about 1 oz, a double about 2 oz. Demitasse cups are sized so a double looks right rather than lost. Anything over 5 oz stops being an espresso cup and starts being a small mug.
6-8 oz – cappuccino and tea
The traditional cappuccino cup is 6 oz. Most bone china teacups sit here too. If you drink a proper cappuccino or take tea in the afternoon, this is the size, and it is smaller than most people expect.
10-12 oz – the standard mug
The default. A 12 oz mug filled to a sensible level holds about 10 oz of coffee, which is a normal cup with room not to spill. If you are buying one size for a household, buy this one.
14-16 oz – the everyday large
What most people mean by “a big mug”. Suits filter coffee, a large latte, or anyone who refills less often. Note that a 16 oz mug of coffee is heavy – around 1.2 lb filled – which matters if you have wrist trouble.
18-20 oz – jumbo and soup mugs
Genuinely large. Good for soup, hot chocolate and slow weekend mornings. They cool fast because of the surface area, so they work better in stoneware than in porcelain.
Travel mugs are measured differently
A travel mug’s stated capacity is usually to the brim, with no allowance for the lid. A “16 oz” travel mug typically holds 14 oz once the lid is on without pushing coffee out of the drink spout. Assume roughly 2 oz less than the label.
The rule of thumb
Buy one size up from the drink, never the exact size. A 12 oz drink in a 12 oz mug is a mug you cannot carry across a room.
Every listing here states the capacity the manufacturer publishes, so you are comparing numbers rather than photographs.