Mug and Bowl Size Converter: Capacity in ml, Ounces and Cups

How the mug and bowl size converter works

The mug and bowl size converter treats the inside of your piece as a cone with its tip cut off, which geometry calls a frustum. A straight-sided mug is the simple case where the top and bottom are the same width. The volume is pi times the depth divided by three, times the sum of the top radius squared, the top radius times the bottom radius, and the bottom radius squared. One cubic centimetre is one millilitre, so the answer comes out ready to use.

For a straight mug 7 cm wide inside and 9 cm deep, that’s pi times 3.5 squared times 9, about 346 ml. For a tapered bowl 14 cm across the rim, 6 cm across the floor and 6 cm deep, it’s about 496 ml. If you fill to less than the brim, the tool shortens the depth and works out the narrower width at that fill line too, which matters for tapered bowls.

The unit conversions use the standard definitions: 29.5735 ml to the US fluid ounce, 28.4131 ml to the imperial fluid ounce, 250 ml to the metric cup used in Australia and 236.588 ml to the US cup. Tick the wet sizes box and the capacity is multiplied by one minus the shrinkage, cubed, because a pot shrinks in every direction at once.

How many ml or ounces is a standard mug?

A standard mug usually holds 250 to 355 ml, or about 8.5 to 12 US fluid ounces, brim full. Australian makers list mugs right across that range, from 250 ml to 355 ml, and a 12 ounce mug of about 355 ml is a common size in the US.

Large mugs start at around 400 ml, and a 15 ounce mug is about 444 ml. Espresso cups are tiny by comparison, 50 to 100 ml, while cereal bowls about 15 cm across tend to hold 350 to 385 ml and wide soup or pasta bowls a good deal more. The table above lists these with their sources so you can see where the figures come from.

Remember that listed capacities are almost always brim full. Nobody drinks from a mug filled to the rim, so a 350 ml mug really serves about 300 ml of tea. If you’re making mugs for a coffee shop or to match a set, ask whether the buyer means brim full or a working fill, and use the fill box to show both.

How do you measure a mug or bowl for capacity?

Measure the inside, not the outside. Use a ruler or calipers across the rim from inside wall to inside wall, then measure the depth by laying a straight edge across the rim and measuring down to the lowest point of the floor. For tapered shapes, measure the inside width across the flat of the floor too.

Real pots curve, so the shape model is an approximation. A mug that bellies out in the middle holds more than a straight cylinder of the same rim, and a bowl with a rounded floor holds a little less than a flat-floored one. When the number really matters, check it the old-fashioned way: fill the piece with water from a measuring jug, or weigh it empty and full on a kitchen scale, since a gram of water is a millilitre.

That water check is worth doing on your first few pieces in any new form. Once you know how your mugs compare with the model, you can trust the converter for planning and keep the jug for the final check.

Why you need to throw bigger: shrinkage and mug capacity

Capacity shrinks much faster than size, because clay shrinks in width, depth and height at the same time. A clay with 12% linear shrinkage leaves 0.88 times 0.88 times 0.88 of the volume, about 68%, so a mug that holds 346 ml wet holds only about 236 ml once fired.

That’s why potters who make to a size throw noticeably bigger than they expect. To hit a fired 350 ml mug in a 12% stoneware, the wet mug needs to hold about 514 ml. Tick the wet sizes box in the tool, enter your clay’s shrinkage, and adjust the width and depth until the fired capacity reads the number you want. Our clay shrinkage calculator works out the matching wet dimensions for any single measurement.

The shrinkage figure on the clay bag is usually the total from wet to fully fired, which is the one to use here. If you only know the drying shrinkage, the answer will come out too big. A test bar of your own clay gives the most reliable number.

US, imperial and metric: which ounce and cup is which?

The US fluid ounce is 29.57 ml and the imperial or UK fluid ounce is 28.41 ml, about 4% smaller. Cups differ too: the Australian metric cup is 250 ml, while the US cup is 236.6 ml. Mixing them up won’t spoil a mug, but it can confuse a customer or a recipe.

Most mug sizes quoted online in ounces are US ounces, since that’s where the convention comes from. A 12 ounce mug in the US is about 355 ml, but 12 imperial ounces is about 341 ml. For Australian buyers, millilitres are the clearest way to label a mug, and the converter shows every unit side by side so you can quote whichever your customer uses.

If you sell interstate or overseas, it’s worth putting both millilitres and US ounces on a product listing. It answers the most common question before anyone has to ask it.

Common mistakes when sizing mugs and bowls

The most common mistake is measuring the outside. On a mug with 6 mm walls, the outside width is 1.2 cm wider than the inside, which can add 30% or more to the capacity estimate. Always measure inside.

The second is forgetting shrinkage when planning, then being disappointed by a mug that holds a third less than the one on the wheel. The third is quoting a brim-full capacity as if it were a serving size. And finally, mixing up US and imperial ounces in a listing. None of these are hard to avoid once you know about them, and the converter handles the sums. If you’re working out what to charge for a set once it’s done, the clay weight calculator helps you plan how much clay a run of mugs will take.