How this instrument works
Both units measure out the same American gallon, just in different-sized scoops. That gallon has held 231 cubic inches by law for centuries, and splitting it into 128 equal parts gives a fluid ounce while splitting it into 16 equal parts gives a cup. Divide one divisor by the other, 16 into 128, and 8 falls out — a whole number with no decimal tail, because both units were carved from the same container rather than measured independently.
That cleanliness is unusual in this corner of measurement. Most customary conversions carry ugly repeating decimals once metric units enter the picture, but ounces and cups never leave the American system, so 0.125 terminates exactly: 16 fl oz is precisely 2 cups, 4 fl oz is precisely half a cup, and no digit ever needs to be rounded away regardless of how many decimal places a recipe demands.
Recipes overwhelmingly print quantities in cups while store-bought liquids — broth, cream, canned tomatoes, vinegar — print their contents in fluid ounces on the label, which is exactly the gap this page closes. A can rated at 14.5 fl oz is not a clean fraction of a cup, which is why so many recipes call for oddly specific amounts like 1 3/4 cups rather than a whole number.
- Enter the amount into US fluid ounces (floz); it opens at 16, a full US pint's worth.
- Read the converted amount straight off US cups (cup), recalculated on every keystroke to six decimal places.
- Working from a recipe written in cups instead? Multiply the cup figure by 8 to get fluid ounces.
- Fractional cans and bottles convert cleanly too — try 14.5 for a standard can of crushed tomatoes.
Worked example — a 16 fl oz can of crushed tomatoes
A marinara recipe calls for 2 cups of crushed tomatoes, but the only can in the pantry is labelled 16 FL OZ with no cup markings printed anywhere on it. Enter 16 into US fluid ounces (floz) and US cups (cup) reads 2.0 exactly — the whole can goes straight into the pot with nothing left to measure out separately.
Scale that same can down for a half batch and 8 fl oz converts to exactly 1.0 cup, while a smaller 14.5 fl oz can — the common size for diced tomatoes — comes to 1.8125 cups, which is why so many recipes list that oddly precise fraction rather than a tidy round number.
Questions
Is a US cup always exactly 8 fluid ounces?
Yes, by definition rather than by measurement. The US customary cup is legally 1/16 of the 231-cubic-inch gallon and the fluid ounce is 1/128 of the same gallon, so 128 divided by 16 gives 8 with no remainder and no rounding at any step. Scale the relationship as far as you like — 3 cups is 24 fl oz, 0.375 cups is exactly 3 fl oz — and it stays a clean whole-number ratio throughout.
Why do canned goods list such odd fluid ounce amounts?
Can sizes were standardised around convenient production volumes decades ago, not around round numbers of cups. A 14.5 fl oz can of tomatoes, an 8 fl oz can of sauce, and a 28 fl oz can of whole tomatoes are industry-standard sizes that predate most recipes calling for them, which is why converting them into cups so often produces fractions like 1.8125 rather than a whole number.
Does this factor work for an imperial or metric cup too?
No — only for the American pair. An imperial fluid ounce is 28.4130625 ml against 29.5735295625 ml for the US version, and Australian, Canadian and New Zealand kitchens use a flat 250 ml metric cup rather than the US customary 236.5882365 ml one. Mixing an American fluid-ounce figure with a metric cup measure using this 0.125 factor will give a result that is close but not correct — confirm which country's units a recipe means first.
How do I convert cups back into fluid ounces?
Multiply by 8. Since 0.125 and 8 are exact reciprocals with no rounding in either direction, 1.5 cups converts to exactly 12 fl oz and 0.25 cups converts to exactly 2 fl oz. This reverse direction is just as clean as the forward one, which is unusual among unit pairs — most reciprocal factors in cooking measurement carry a repeating decimal somewhere.
Does a fluid ounce of liquid weigh the same as a cup measured by scale?
Not necessarily, because fluid ounces and cups both measure volume, not weight. A cup of water weighs close to 8.35 avoirdupois ounces, a cup of honey nearer 12 ounces, and a cup of flour typically 4 to 5 ounces depending on how it was scooped. Converting between fluid ounces and cups with the 0.125 factor is always safe; converting either one into a weight in ounces requires knowing the density of whatever is being measured.
Why does the calculator show 2.0 instead of just 2?
Trailing decimals are a formatting choice, not a sign of imprecision — 16 fl oz converts to exactly 2 cups, full stop, and the .0 simply keeps the display consistent for less tidy entries like 0.375 cups. Because 8 is a plain power of two, multiplying or dividing by it is one of the rare operations that ordinary decimal and binary arithmetic both handle with zero rounding error, so every digit shown here is exact.