One US fluid ounce equals 29.5735 milliliters. One UK imperial fluid ounce equals 28.4131 milliliters. If the measurement comes from a US bottle, tumbler, or recipe, use the US converter. Use the UK converter only when the source specifically refers to imperial fluid ounces.
The word “ounce” can also describe weight. This page converts fluid ounces, which measure liquid volume, to milliliters.
Ounces to Milliliters Converter
Enter a value below to see the conversion.
Based on US liquid measurements: 1 US fl oz = 29.5735 mL.
How to Convert Ounces to Milliliters
For US liquid measurements, multiply the number of fluid ounces by 29.5735:
Milliliters = US fluid ounces × 29.5735
For example, 8 US fluid ounces × 29.5735 = 236.588 milliliters. Rounded to two decimal places, this is 236.59 mL.
To convert milliliters back to US fluid ounces, divide by 29.5735:
US fluid ounces = milliliters ÷ 29.5735
For UK imperial measurements, use 28.4131 instead:
Milliliters = imperial fluid ounces × 28.4131
The conversion factors are supported by the NIST unit conversion tables, which list a US fluid ounce as 29.57353 mL, and the NIST Guide to the SI, which lists a UK imperial fluid ounce as 28.41306 mL.
Common Ounces-to-mL Conversions for Bottles
Bottle and tumbler capacities stated in ounces may be nominal sizes. The table below uses US fluid ounces and shows the calculated metric equivalent. Rounded values may appear on packaging or product pages, so check whether the supplier is stating nominal, usable, or brimful capacity when the exact volume matters.
| Bottle size | Exact US conversion | Useful rounded value |
|---|---|---|
| 1 fl oz | 29.5735 mL | 30 mL |
| 8 fl oz | 236.588 mL | 237 mL |
| 12 fl oz | 354.882 mL | 355 mL |
| 16 fl oz | 473.176 mL | 473 mL |
| 20 fl oz | 591.471 mL | 591 mL |
| 24 fl oz | 709.765 mL | 710 mL |
| 30 fl oz | 887.206 mL | 887 mL |
| 32 fl oz | 946.353 mL | 946 mL |
| 40 fl oz | 1,182.941 mL | 1,183 mL |
For practical examples, a 12oz insulated coffee cup converts to approximately 355 mL. A 20oz vacuum-insulated tumbler is approximately 591 mL, while a 30oz stainless steel tumbler is approximately 887 mL when the ounce values refer to US fluid ounces.
Is 1 oz the Same as 30 mL?
Not exactly. One US fluid ounce is 29.5735 mL, but it is often rounded to 30 mL for quick everyday estimates. The difference is only 0.4265 mL for one fluid ounce, but it becomes larger as the volume increases. For example, estimating 20 fluid ounces as 600 mL gives a different result from the more precise conversion of 591.47 mL.
Rounding to 30 mL is usually sufficient when you only need a quick estimate. Use 29.5735 when checking a bottle specification, packaging statement, measuring scale, or another application where the stated capacity needs to be consistent.
Why Are US and UK Fluid Ounces Different?
The US customary and British imperial systems define their fluid ounces differently. A US fluid ounce is approximately 29.5735 mL, while an imperial fluid ounce is approximately 28.4131 mL.
The difference is small for one ounce but more noticeable for larger containers. Twenty US fluid ounces equal about 591.47 mL. Twenty imperial fluid ounces equal about 568.26 mL. That is why a converter should not treat US and imperial fluid ounces as the same unit.
If a bottle or recipe simply says “oz,” look at its market and source. Drinkware intended for the US market normally uses US fluid ounces. Use the imperial conversion only when the measurement is identified as UK or imperial. When purchasing products for more than one market, ask the supplier to state both the ounce system and the metric capacity in the product specification.
Fluid Ounces and Weight Ounces Are Not Interchangeable
A fluid ounce measures volume. A weight ounce measures mass. Although both may be abbreviated as “oz” in everyday use, they answer different questions.
For a water bottle or tumbler, an ounce rating typically indicates how much liquid the container can hold; it refers to fluid ounces. A food scale that shows ounces measures weight. You cannot convert weight ounces directly to milliliters unless you also know the density of the substance.
This distinction matters for liquids that are denser or lighter than water. The calculators on this page convert liquid capacity only; they do not convert ounces by weight.
What Bottle Buyers Should Confirm About oz and mL Capacity?
The conversion formula is exact, but a commercial bottle size is often a nominal description. A lid can occupy internal space, and the volume measured to the brim may differ from the amount that is comfortable to carry and drink. Production specifications may also define the capacity at a particular measuring point.
Before approving a custom bottle, confirm:
- whether “oz” means US fluid ounces or imperial fluid ounces;
- whether the stated capacity is nominal, brimful, or measured to a specified fill level;
- whether capacity is tested with the final lid or without it;
- which ounce and milliliter values should appear on packaging, product listings, or measuring marks.
For buyers comparing wholesale insulated water bottles, confirming these details before sampling prevents a rounded product name from being mistaken for an exact usable volume.
Frequently Asked Questions
How many milliliters are in 1 oz?
One US fluid ounce equals approximately 29.5735 mL. One UK imperial fluid ounce equals approximately 28.4131 mL. Use the value that matches the original measurement system.
Is 1 oz equal to 30 mL?
Thirty milliliters is a convenient rounded estimate for one US fluid ounce, but the more precise conversion is 29.5735 mL. Use the precise value when checking product capacity or specifications.
How many milliliters are in 8, 12, 16, and 20 oz?
Using US fluid ounces, 8 oz equals 236.59 mL, 12 oz equals 354.88 mL, 16 oz equals 473.18 mL, and 20 oz equals 591.47 mL.
How many fluid ounces are in 500 mL?
Five hundred milliliters equal approximately 16.907 US fluid ounces or 17.598 UK imperial fluid ounces. Choose the result that matches the measurement system you need.
Are fluid ounces and weight ounces the same?
No. Fluid ounces measure volume, while ounces by weight measure mass. A direct conversion from weight ounces to milliliters requires the density of the material.
Which ounce measurement is used for water bottle capacity?
Bottle capacity is measured in fluid ounces. Products intended for the US market normally use US fluid ounces, while imperial fluid ounces should be used only when the specification identifies the UK or imperial system.
The Answer to Remember
One US fluid ounce equals approximately 29.5735 mL, while one UK imperial fluid ounce equals approximately 28.4131 mL. For a quick US conversion, 30 mL per ounce is a useful estimate. For product specifications and capacity comparisons, use the precise conversion and confirm which fluid-ounce system is being used.
