Quick answer: work out the volume in cubic inches, then divide — ÷ 1.8047 for fluid ounces, ÷ 14.438 for cups, ÷ 231 for US gallons. In metric it is easier still: cubic centimeters are milliliters, so a container measured in cm needs no conversion at all.
The US liquid measurement chain
Every US liquid unit is a simple multiple of the one below it, which makes the whole system easy to reconstruct from memory if you know the four links:
16 tablespoons = 1 cup
2 cups = 1 pint · 2 pints = 1 quart · 4 quarts = 1 gallon
| Unit | Fl oz | Cups | Milliliters |
|---|---|---|---|
| 1 teaspoon | 0.167 | 0.021 | 4.93 |
| 1 tablespoon | 0.5 | 0.0625 | 14.79 |
| 1 fluid ounce | 1 | 0.125 | 29.57 |
| 1 cup | 8 | 1 | 236.6 |
| 1 pint | 16 | 2 | 473.2 |
| 1 quart | 32 | 4 | 946.4 |
| 1 US gallon | 128 | 16 | 3,785 |
| 1 liter | 33.81 | 4.227 | 1,000 |
| 1 imperial gallon | 153.7 | 19.2 | 4,546 |
| 1 oil barrel | 5,376 | 672 | 158,987 |
The cup problem
"One cup" means different things in different kitchens, and the spread is wide enough to affect a recipe:
| Cup | Milliliters | Used in |
|---|---|---|
| US customary cup | 236.6 | United States |
| US legal cup (nutrition labels) | 240 | US food labelling |
| Metric cup | 250 | Australia, NZ, Canada, much of Europe |
| Imperial cup | 284.1 | Older UK recipes |
| Japanese cup (gō) | 180 | Japanese rice cookers |
An Australian cup is 5.6% bigger than an American one, and an old British cup is 20% bigger. For anything where ratios matter — bread, pastry, custards — weighing ingredients sidesteps the whole problem. This calculator uses the US customary cup.
Where this gets used
Sizing a container before you buy it
A cylindrical storage jar 5 inches across and 8 inches tall holds π × 6.25 × 8 = 157 in³ ≈ 87 fluid ounces, or about 2.6 quarts. That tells you whether a 2-quart batch of stock will fit before you carry it home.
Scaling a recipe to a pan
A 9 × 13 inch baking dish filled 2 inches deep holds 234 in³ ≈ 4 quarts. Knowing that stops the classic overflow when a recipe written for a larger dish gets poured into a smaller one.
Mixing solutions by ratio
Concentrates are dosed per gallon, but you often mix in a smaller vessel. A 1:128 dilution is exactly 1 fluid ounce per gallon — a ratio that exists precisely because 128 fluid ounces make a gallon. For laboratory dilutions, the dilution calculator handles C₁V₁ = C₂V₂ directly.
Common mistakes & pro tips
- Fluid ounces treated as weight ounces. They match only for water. Eight fluid ounces of honey weighs about 12 ounces.
- US and imperial pints. A US pint is 473 mL, a British pint 568 mL — a 20% difference that has surprised many a traveller.
- Measuring to the rim. Container capacity and usable capacity differ; leave headroom for anything you intend to carry or heat.
- Pro tip — the metric shortcut. Measure in centimetres and the cubic centimetres are already milliliters. No conversion, no error.
- Pro tip — verify by filling. Fill the container with a measuring cup and count. It takes a minute and settles any doubt about odd shapes.
Frequently asked questions
How many cups are in a gallon?
Sixteen US cups make a US gallon. The chain runs 3 teaspoons to a tablespoon, 16 tablespoons to a cup, 2 cups to a pint, 2 pints to a quart and 4 quarts to a gallon. Working through it: a gallon is 16 cups, 128 fluid ounces or 768 teaspoons.
Is a US cup the same as a metric cup?
No. A US cup is 236.6 mL, while the metric cup used in Australia and much of Europe is a round 250 mL — about 5.6% larger. A UK recipe may also use an imperial cup of 284 mL. For baking, where ratios matter, the difference is enough to notice.
How do I convert a container's dimensions into fluid ounces?
Compute the volume in cubic inches, then divide by 1.8047 — since one US fluid ounce is 1.8047 cubic inches. A cylindrical glass 3 inches across and 5 inches tall holds π × 2.25 × 5 = 35.3 in³, which is about 19.6 fluid ounces.
What is the difference between fluid ounces and ounces?
A fluid ounce measures volume; an ounce measures weight. They coincide only for water, where one fluid ounce weighs almost exactly one ounce. For honey, oil or flour the two diverge sharply — 8 fluid ounces of honey weighs about 12 ounces, which is why baking by weight is more reliable than by cup.
How many milliliters in a US fluid ounce?
29.5735 mL, usually rounded to 30 mL in cooking and medicine. The UK fluid ounce is slightly different at 28.413 mL, which is why a "pint" means 473 mL in the US but 568 mL in Britain.