How do you convert ml to oz?

The fluid ounce has no definition of its own. It is a slice of a gallon, and both gallons are exact, so both ounces are too:

1 US fl oz  =  3785.411784 mL ÷ 128  =  29.5735295625 mL
1 UK fl oz  =      4546.09 mL ÷ 160  =  28.4130625 mL

Ten decimal places for the American one, seven for the British one, and no rounding in either. Going back the other way is less tidy. As a fraction, a milliliter is 16000000/473176473 US fluid ounces, and that denominator is 3 × 7 × 11 × 127 × 127 × 127. None of those primes divides into a power of ten, so the decimal never ends: 0.0338140227018 and on.

The two directions

Milliliters to US fluid ounces: divide by 29.5735295625. A 500 mL bottle of water:

500 ÷ 29.5735295625   =  16.907011…   →  16.91 US fl oz

That is why half a liter of water is labeled 16.9 fl oz, a number nobody would pick on purpose.

US fluid ounces to milliliters: multiply by 29.5735295625. A 12 fl oz can:

12 × 29.5735295625    =  354.88235475 mL      exact, then printed as 355 mL

Multiplying by 0.033814 instead of dividing lands in the same place: 500 × 0.033814 is 16.907. That factor is itself rounded, so it is a shortcut and not an equal sign. On 1,000 mL it comes up short by about 0.00002 fl oz, which is not a quantity anyone can pour.

An ounce on a label is not an ounce on a scale

This is the mistake that ruins the cake. The fluid ounce measures space. The ounce on a kitchen scale measures mass, and it is exactly 28.349523125 grams, one sixteenth of the pound in the kg to lbs post. The two only line up for liquids about as dense as water, at roughly 1 gram per milliliter:

1 US fl oz of water  ≈  29.57 g  =  29.57 ÷ 28.35  ≈  1.04 oz by weight
1 UK fl oz of water  ≈  28.41 g  =  28.41 ÷ 28.35  ≈  1.002 oz by weight

The British figure is that close for a reason. The imperial gallon began as the volume of ten pounds of water, which is 160 ounces, which is why it splits into 160 fluid ounces. Water is the only thing this works for. Eight fluid ounces of flour and eight ounces of flour are different amounts, so if a recipe says "8 oz flour" it wants a scale, and no milliliter conversion will save it.

US or UK fluid ounce?

Here the British unit is the smaller one, which surprises anyone who just learned that the UK gallon is bigger:

1 UK fl oz  =  28.4130625 ÷ 29.5735295625  =  0.96076 US fl oz
1 US fl oz  =  29.5735295625 ÷ 28.4130625  =  1.04084 UK fl oz

Both facts are true because the pints are cut differently. A US pint holds 16 fluid ounces. A UK pint holds 20. So the smaller ounce still builds the bigger pint:

US pint  =  16 × 29.5735295625  =  473.176473 mL
UK pint  =  20 × 28.4130625     =  568.26125 mL

On small amounts the 4% gap rarely matters. Over a 750 mL bottle it is 25.36 US fl oz against 26.40 UK fl oz, more than a full ounce apart. If a label is American, assume US. If a recipe is older and British, assume imperial, and check what size its cups are while you are at it.

The rest of the kitchen, in US fluid ounces

Everything above an ounce is a whole number of them, and everything below is a clean fraction:

1 tablespoon  =  1/2 fl oz  =   14.78676478125 mL
1 teaspoon    =  1/6 fl oz  =    4.92892159375 mL
1 cup         =    8 fl oz  =  236.5882365 mL
1 pint        =   16 fl oz  =  473.176473 mL
1 quart       =   32 fl oz  =  946.352946 mL
1 gallon      =  128 fl oz  =  3785.411784 mL

Doing it in your head: divide by 30

Thirty is close enough to 29.57 that dividing by it is the standard shortcut. It runs low by a steady 1.42%. If you care, add 1.5% to the result, which cuts the error to 0.06%:

 500 mL  ÷ 30  =  16.67   + 1.5%  =  16.92    exact 16.907…
1000 mL  ÷ 30  =  33.33   + 1.5%  =  33.83    exact 33.814…

The other way, multiply the ounces by 30 and take off 1.5%. A US pint gives 16 × 30 = 480, minus 7.2 is 472.8 mL, against a true 473.18. Skip the correction and 480 is 1.44% high, which is fine for soup and not fine for a child's cough syrup. For anything dosed, use the syringe that came with it.

Conversion table: ml to oz

Conversion table: oz to ml

Every row here is exact. A whole number of ounces times either factor never needs more decimal places than the factor has, so there is nothing to round:

Where the rounding costs you

On 1,000 mL, each popular shortcut against the exact answer:

Exact      1000 ÷ 29.5735295625  =  33.8140227… US fl oz
0.033814   1000 × 0.033814       =  33.814 US fl oz    short by  0.00002 fl oz
0.0338     1000 × 0.0338         =  33.8 US fl oz      short by  0.014 fl oz
÷ 30       1000 ÷ 30             =  33.33 US fl oz     short by  0.48 fl oz

Rounding halfway through does more damage than any of them. Convert 500 mL to 16.91 fl oz, write that down, then convert it back, and you get 16.91 × 29.5735295625 = 500.09 mL. Nobody spilled anything, and 0.09 mL appeared from nowhere. Keep the whole number until the last step, then round once.

The binary arithmetic underneath has its own opinion. Nine US fluid ounces is 9 × 29.5735295625, which is 266.1617660625 mL and nothing else. A machine working in binary floating point answers 266.16176606249996, because 29.5735295625 has no exact binary form. That trailing mess is the same fault behind 0.1 + 0.2 = 0.30000000000000004. Prism holds 29.5735295625 as the exact number it is, so 9 fl oz converts to 266.1617660625 mL, and 0.1 + 0.2 is 0.3.

Divide it, or let the converter hold the ounces

Dividing by 29.5735295625 or 28.4130625 works in Prism's free calculator, exactly. The volume category of the unit converter (milliliters, liters, US fluid ounces, teaspoons, tablespoons, cups, pints, quarts, US and UK gallons, and cubic meters) is part of the one-time $3.99 unlock. It has no UK fluid ounce, so use the division for that one. No ads, no subscription. See the app.