Article — Oz to mL Converter
Oz to mL: how to convert fluid ounces to milliliters
What is a fluid ounce?
A fluid ounce measures volume, not weight. One US fluid ounce is 29.5735 mL. One UK (Imperial) fluid ounce is 28.4131 mL. If you just need a ballpark, multiply ounces by 30 and you will be off by less than 1.5%.
The fluid ounce (abbreviated fl oz) shows up on beverage labels, in cooking recipes, and in pharmaceutical dosing. In the US it is defined as exactly 1/128 of a US gallon. In the UK, the imperial fluid ounce is slightly smaller because it comes from a different gallon definition entirely.
The gap between the two is roughly 4%. Small enough to ignore when you are making soup. Not small enough for a pharmacist filling a prescription.
A standard US soda can holds exactly 12 fl oz (354.88 mL), while the European standard is 330 mL (11.16 US fl oz). The US can holds about 7.5% more liquid. You can actually see the difference when you put them side by side.
History
The word "ounce" traces to the Latin uncia, meaning one-twelfth. Originally it was one-twelfth of a Roman pound. The original idea tied volume to weight: one fluid ounce was supposed to be the volume occupied by one ounce of water, by weight.
The US adopted its gallon from the English wine gallon of 231 cubic inches, codified by Queen Anne in 1707. Britain went a different direction in 1824, defining the Imperial gallon as the volume of 10 pounds of water at 62 degrees F. That single fork cascaded through every smaller unit, including the fluid ounce.
In 1793, France sent scientist Joseph Dombey to the United States with official metric standards: a copper kilogram and a meter rod. Pirates intercepted his ship in the Caribbean. Dombey died in captivity. The standards never arrived. If they had, Americans might have been using milliliters for 230 years and nobody would need this converter.
Why two systems?
The US inherited the English wine gallon before Britain's 1824 Imperial reform. When Britain redefined its gallon, the US kept the older standard. Two centuries later, we still have two parallel systems.
The Imperial gallon is about 20% larger, but it splits into 160 fluid ounces instead of 128. So the UK fluid ounce ends up smaller than the US one, even though the UK gallon is bigger. That's the counterintuitive part.
A UK recipe calling for "4 fl oz of cream" means 113.65 mL (Imperial), not 118.29 mL (US). The 4% difference can affect delicate sauces and baked goods. Always check the recipe's origin country.
Where fluid ounces are used
Fluid ounces remain standard in a handful of countries. Most of the world uses metric exclusively.
Only three countries have never officially adopted the metric system: the US, Liberia, and Myanmar. Even the UK, which metricated in the 1960s, still sells beer and milk in pints and measures road distances in miles.
What is a milliliter?
A milliliter (mL) equals one-thousandth of a liter, or one cubic centimeter (cm³). It belongs to the SI system and is the global standard for measuring small volumes of liquid.
The appeal is decimal simplicity: 1,000 mL = 1 liter, 1,000 liters = 1 cubic meter. No memorizing that 16 ounces make a pint, 2 pints make a quart, 4 quarts make a gallon. Just powers of ten.
In medical contexts, "cc" (cubic centimeter) means the same thing as mL. A syringe labeled "5 cc" holds exactly 5 mL. The terms are physically identical.
When do you need to convert?
The oz-to-mL conversion comes up more often than you would think:
Never round fluid ounces to "30 mL" for pharmaceutical dosing. The precise value is 29.5735 mL. A 1.4% error can be clinically significant, especially for pediatric patients. Always use a calibrated dosing device.
Quick mental math
You will not always have a calculator. These approximations get you close enough for everyday use:
1 oz ≈ 30 mL3 oz ≈ 90 mL (small cup)8 oz ≈ 240 mL (1 cup)12 oz ≈ 355 mL (soda can)16 oz ≈ 475 mL (US pint)33.8 oz ≈ 1,000 mL (1 liter)Shortcut oz × 30 ≈ mLThe "multiply by 30" shortcut overestimates by about 1.4% for US ounces. Close enough for cooking. For UK ounces, multiplying by 28 gets you closer.
The FDA puts "1 fl oz (30 mL)" on food labels. That is not a mistake. It is the official FDA rounding, and for nutritional information it works fine.
Quick halving trick: 1 liter is about 34 oz. Half a liter (500 mL) is about 17 oz. A quarter liter (250 mL) is about 8.5 oz. Handy round numbers for shopping.
Common mistakes
An ounce of weight (oz) measures mass; a fluid ounce (fl oz) measures volume. They match only for water at standard conditions. One fl oz of honey weighs about 1.5 oz. One fl oz of olive oil weighs about 0.95 oz.
A UK recipe calling for "8 fluid ounces" means 227.3 mL, not the US 236.6 mL. Over a full recipe with multiple liquid ingredients, this 4% error compounds and can ruin results.
Rounding 1 fl oz to "30 mL" works for cooking, but for medication the precise value of 29.5735 mL matters. A 1.4% error on a critical drug dose can be clinically significant.
A US cup is 8 US fl oz (236.6 mL). An Australian cup is 250 mL. A Japanese cup is 200 mL. The old UK cup (now obsolete) was 10 Imperial fl oz (284.1 mL). Always verify which "cup" a recipe uses.