Fluid Ounces to Milliliters Calculator

Background.

A fluid-ounces-to-milliliters calculator matters most when the number on the label has to match the number on the dosing device, and getting it wrong is not a rounding curiosity - it is a dosing error. US over-the-counter liquid medicine bottles print the dose in fluid ounces or teaspoons, but the syringe, dropper, or dosing cup that ships in the box is almost always graduated in milliliters, because that is the international pharmaceutical standard. A parent measuring out a 4 fl oz bottle of children's fever reducer needs to know that 4 fl oz is 118.29411825 mL - not an approximation, but the number the syringe markings are actually calibrated against.

This calculator uses the exact US fluid ounce, defined as 1/128 of a US gallon and therefore exactly 29.5735295625 milliliters per NIST Special Publication 811 - not the commonly rounded 29.57 mL figure that loses precision at larger volumes. The same factor applies whether you are converting a cough-syrup dose, a recipe's liquid measurement from an American cookbook, or a beverage bottle's fill volume for a nutrition label. Because the factor is exact by definition, the result is exact too, to as many decimal places as the context requires.

What is fluid ounces to milliliters calculator?

A US fluid ounce is a unit of volume equal to 1/128 of a US liquid gallon, used throughout US retail packaging, cooking, and over-the-counter medicine labeling. A milliliter is 1/1000 of a liter, the standard volume unit used on pharmaceutical dosing devices and in most of the world's cooking measurements. The two units are related by an exact factor: 1 US fl oz = 29.5735295625 mL, derived directly from the US gallon's definition of 3.785411784 liters divided by 128. Note that this is the US fluid ounce; the UK imperial fluid ounce is a different size (about 28.4131 mL), so a UK-labeled bottle and a US-labeled bottle of the same nominal ounce count do not hold quite the same volume. This calculator converts using the US fluid ounce, which is the standard on US-market packaging and US recipes.

How to use this calculator.

1. 01Enter the volume in US fluid ounces - for example, a medicine bottle dose, a recipe measurement, or a drink size.
2. 02Read the converted value in milliliters, shown to full precision.
3. 03Check the liters field if you are working with a larger container, such as a multi-dose bottle or a bulk liquid.
4. 04For a dosing device graduated in mL, use the milliliters result directly - do not round the ounces first and then convert.

The formula.

The conversion is purely multiplicative because volume has no offset - zero fluid ounces is zero milliliters. The US fluid ounce is defined as exactly 1/128 of a US liquid gallon; since the US gallon is exactly 3.785411784 liters (NIST Special Publication 811), dividing by 128 gives 29.5735295625 milliliters per fluid ounce, with no rounding anywhere in the derivation. In the worked example, a 4 fl oz bottle multiplied by 29.5735295625 gives 118.29411825 mL. For a 12 fl oz can, the same factor gives 354.88235475 mL. All arithmetic runs through Decimal.js at full precision, so a syringe marked in tenths of a milliliter, or a lab pipette marked in hundredths, gets a number that matches the definition exactly rather than one that has drifted from repeated floating-point rounding.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Institute of Standards and Technology. NIST Special Publication 811, 2008 Edition - Guide for the Use of the International System of Units (SI), Appendix B. Source for the US gallon (3.785411784 L) from which the exact US fluid ounce factor is derived.
2. [2]National Institute of Standards and Technology. NIST Handbook 44, Appendix C - General Tables of Units of Measurement. US legal-metrology reference for the fluid ounce and other US customary volume units.