Recipes, cocktails and skincare instructions written in US fluid ounces need converting to milliliters for anyone using a metric measuring device — which is most of the world. This tool handles that conversion precisely.
Volume, not weight — a distinction worth keeping straight
The "oz" in a recipe for liquid ingredients refers to the fluid ounce, a volume measurement, not the everyday avoirdupois ounce of weight used for solids — the two happen to be roughly equivalent only for liquids close to water's density, which is a coincidence of history (both trace loosely to old English trade measures) rather than a designed relationship. This ambiguity is baked into the English language itself: "ounce" covers both without distinguishing, which is part of why cooks worldwide increasingly prefer weighing ingredients in grams over measuring them in ounces at all.
The precise conversion
1 US fluid ounce = 29.5735 milliliters exactly. The tool multiplies your fl oz input by this constant. Note that this is the US fluid ounce specifically — if a recipe or label is British or from another Imperial-measuring country, the correct multiplier is smaller, about 28.41 mL per fluid ounce.
Where this conversion is routinely needed
- Cooking from American recipes with a metric kitchen scale or measuring set — a very common scenario for home cooks outside the U.S. following recipes published by American food blogs and cookbooks.
- Bartending and cocktail recipes — professional bar recipes are often given in ounces in US-published guides but need mL conversion for jiggers and tools calibrated in metric, common outside North America.
- Dosing liquid medication — some medicine labels or dosing instructions use fl oz while dosing syringes and cups are marked in mL, requiring careful conversion to avoid dosing errors.
- Comparing product sizes while shopping internationally — a "16 fl oz" US bottle and its closest international counterpart don't automatically match a round mL number, and shoppers price-comparing per unit volume need an accurate conversion.
Frequently asked questions
Should I use the US or Imperial fluid ounce conversion? Use US (29.5735 mL) for anything sourced from the United States; use Imperial (28.4131 mL) for UK, older Canadian, Australian or other Commonwealth-sourced recipes and labels — mixing them up introduces roughly a 4% error.
Why do American recipes still use volume (cups, ounces) instead of weight (grams)? Culinary tradition — measuring cups became standard in U.S. home kitchens in the late 19th and early 20th centuries, well before kitchen scales were common household items, and the convention persisted even as professional and international kitchens shifted toward the greater precision of weight-based measurement.
Is a fluid ounce of water the same weight as a regular ounce? Approximately, yes, for pure water at standard conditions — this near-equivalence is precisely why the two units share a confusing name, even though they measure fundamentally different properties (volume versus mass) and diverge for any liquid denser or lighter than water.
Further reading
Wikipedia — Fluid ounce — Precise definitions of the US and Imperial fluid ounce and why they diverge.