The 1824 Split: One Gallon Became Two
For most of the 18th century, Britain and its American colonies used the same gallon: the Queen Anne wine gallon of 1707, defined as a cylinder 7 inches in diameter and 6 inches deep — 231 cubic inches. Colonists measured rum, wine, and molasses in this gallon. When Samuel Adams wrote about British taxes on tea and Madeira, the "gallon" he referenced was the very same unit a London merchant used. Independence did not, at first, change the measuring cup.
Then came 1824. The UK Parliament, in a fit of rationalization, passed the Weights and Measures Act of 1824 — one of the great consolidating statutes of British imperial governance. Among its many provisions, the Act abolished the wine gallon, the ale gallon, and the corn gallon (all different sizes, all in simultaneous use) and replaced them with a single imperial gallon. Its definition was elegant: the volume occupied by exactly 10 pounds avoirdupois of distilled water, weighed in air at 62°F, with the barometer at 30 inches of mercury. This works out to roughly 277.42 cubic inches — about 4.54609 liters.
The United States, 48 years into independence and not particularly interested in adopting British parliamentary reforms, kept the 231 cubic-inch wine gallon. That gallon — 3.785411784 liters — became the US liquid gallon. It is still the legal gallon for all liquid measure in the United States.
Now the cascading consequence. A fluid ounce is a fraction of a gallon. In the US system, there are 128 fluid ounces to the gallon. In the UK imperial system, there are 160 imperial fluid ounces to the gallon. (Why 160? Because an imperial fluid ounce was defined as the volume of one avoirdupois ounce of water — roughly 28.4 ml — and a gallon weighs 10 pounds, or 160 ounces. The US fluid ounce was also supposed to equal one ounce of water by weight, but since the US kept the smaller 231 cubic-inch gallon, its fluid ounce landed at about 29.6 ml.) The result:
1 UK imperial fluid ounce = 28.4130625 ml
Difference: 1.16 ml, or 3.92%
These numbers are exact. The 1959 International Yard and Pound Agreement, which fixed the foot and pound, also settled the inch-to-centimeter ratio at exactly 2.54. From that, the cubic inch is exactly 16.387064 cm³, from which the gallon and the fluid ounce acquire their exact metric values. There is no measurement uncertainty in any of these numbers. They are definitions.
The Formula
ml = US fl oz × 29.5735295625
This calculator uses the US fluid ounce — the standard for all FDA nutrition labeling, all US consumer product packaging, and effectively all English-language recipes published in North America. If you are holding a can of soda, a bottle of vanilla extract, or a jar of baby food sold in the United States, the "fl oz" on the label means 29.57 ml each. If you are holding the same product purchased in London, "fl oz" means 28.41 ml — but in practice, UK food labels have been exclusively metric since 2000, and the imperial fluid ounce survives mainly in British cocktail recipes and older cookbooks.
Worked Examples
8 fl oz → ml (a US cup)
8 × 29.5735295625 = 236.59 ml. This is the legal US customary cup — half a US pint, 1/16 of a US gallon. But the FDA rounds a "cup" to 240 ml for nutrition labeling. And the metric cup used in Australia, Canada, and New Zealand is exactly 250 ml. A 236.6 ml cup, a 240 ml "FDA cup," and a 250 ml metric cup are three different volumes sharing one name. Any cups to ml conversion must specify which cup is meant. This page converts US customary cups (236.6 ml) by default.
12 fl oz → ml (a soda can)
12 × 29.5735295625 = 354.88 ml. Yet every 12 fl oz soda can in the United States is filled to 355 ml. The filling line is metric; the label is in fluid ounces. The 0.12 ml difference — about 2 drops — is within the fill tolerance of the canning line. But the Nutrition Facts panel will say "12 fl oz (360 ml)" because the FDA's household measure rounds 1 fl oz to exactly 30 ml. The can contains 355 ml. The label says 360 ml. Both are legally correct.
1.7 fl oz → ml (airline mini liquor bottle)
1.7 × 29.5735295625 = 50.27 ml. The TSA liquid rule (3-1-1: 3.4 fl oz / 100 ml per container) uses a different rounding: 100 ml exactly equals 3.3814 fl oz. The "3.4 fl oz" travel-size limit on shampoo bottles is actually 100 ml, and the TSA instruction card just rounds. A 100 ml bottle labeled "3.4 fl oz" in the US would be "3.52 fl oz" in a UK airport — same bottle, different fluid ounce.
Where the 4% Gap Bites
Infant Formula
Pediatric formula is the highest-stakes fluid ounce conversion in daily life. A standard newborn feed is 2 to 3 fl oz every 3 hours. If a British expat in the US uses a UK-published feeding chart that assumes 28.4 ml per fluid ounce, but prepares the formula using a US measuring cup calibrated to 29.6 ml, every feed is 4% too concentrated. Over a single day: roughly 24 fl oz → 710 ml (US) vs 682 ml (UK), a difference of 28 ml — about one extra ounce of water or formula powder per day. Over a month: nearly a liter. For a 3.5 kg newborn on calorie-dense preemie formula at 24 kcal/oz, a systematic 4% over-concentration can push daily caloric intake above the safe ceiling. Preterm infant feeding protocols specify formula-to-water ratios in grams per 100 ml — the word "ounce" appears nowhere in the NICU. At home, with sleep-deprived parents mixing powder and water at 3 AM, it appears on every scoop and every bottle marking.
Cocktail Recipes
A Manhattan recipe that calls for 2 fl oz of rye whiskey means 59.1 ml in a New York bar and 56.8 ml in a London bar. The drink is 4% stronger in the US version. This is not usually a problem — cocktails are forgiving — until you scale to batch production. A wedding caterer making 150 Manhattans from a UK-published recipe book, measuring with US jiggers, multiplies the 4% unit error across 300 ounces of whiskey: 8.87 liters instead of 8.52 liters, a 350 ml surplus. Half a bottle of rye vanishes into the ether of unit confusion. The International Bartenders Association resolved this in 2015 by publishing all standard cocktail specs in milliliters only, with fluid ounce equivalents in parentheses marked "US."
Nutrition Labeling Across Borders
The FDA's 1993 Nutrition Labeling and Education Act (NLEA) established that 1 fl oz = 30 ml for the purposes of the Nutrition Facts panel. This was an intentional simplification — the agency knew the exact factor was 29.57 but chose 30 because dividing by 30 in your head is something a grocery shopper might actually do. The result: a 20 fl oz bottle of Gatorade is labeled "20 fl oz (600 ml)." The actual contents are 591.5 ml. The label overstates the volume by 8.5 ml — a 1.4% inflation. The FDA explicitly permits this rounding because the quantity is in the bottle regardless of what the label says, and the agency's priority was preventing consumer confusion about serving sizes. The UK Food Standards Agency takes the opposite approach: all prepackaged food labels must state net quantity in metric, with imperial equivalents optional but prohibited from being more prominent. A UK-bought bottle of the same Gatorade simply says "591 ml" — no fluid ounces appear anywhere.
Common Fluid Ounces to Milliliters
| US fl oz | ml (exact) | ml (FDA label) | Where you see it |
|---|---|---|---|
| 0.5 fl oz | 14.79 ml | 15 ml | One tablespoon (US). A standard bar spoon. |
| 1 fl oz | 29.57 ml | 30 ml | The anchor. Standard shot glass. NLEA household unit. |
| 1.7 fl oz | 50.27 ml | — | Airline mini liquor bottle. International standard. |
| 2 fl oz | 59.15 ml | 60 ml | Standard cocktail pour. Cough syrup cup. |
| 3.4 fl oz | 100.55 ml | — | TSA 3-1-1 liquid limit. Exact metric limit is 100 ml. |
| 4 fl oz | 118.29 ml | 120 ml | Half cup. Baby food jar. Juice box for toddlers. |
| 5 fl oz | 147.87 ml | 150 ml | Standard wine pour at a restaurant (~150 ml). |
| 8 fl oz | 236.59 ml | 240 ml | One US cup. Coffee serving. Water glass. |
| 12 fl oz | 354.88 ml | 360 ml | Soda can. Beer bottle (standard US longneck). |
| 16 fl oz | 473.18 ml | 480 ml | US pint. Sports drink bottle. Tallboy beer. |
| 16.9 fl oz | 500 ml | — | Standard European water bottle, labeled for US market. |
| 20 fl oz | 591.47 ml | 600 ml | Vending machine soda bottle. |
| 25.4 fl oz | 750 ml | — | Standard wine bottle. "Fifth" — 1/5 of a US gallon ≈ 757 ml, standardized globally to 750 ml. |
| 33.8 fl oz | 1,000 ml | — | 1 liter. Nalgene bottle. Soda liter bottle. |
| 128 fl oz | 3,785.41 ml | — | One US gallon. Milk jug. Gasoline can. |
Fluid Ounce vs Ounce: When the Word "Ounce" Is Ambiguous
The word "ounce" without a modifier is dangerous. One fluid ounce of water weighs roughly one avoirdupois ounce — by design. Both systems defined the fluid ounce as the volume of one ounce of water. But that equivalence holds only for water at 4°C (the temperature of maximum density). For every other liquid, the volume-weight relationship depends on density:
- Honey (density ~1.42 g/ml): 1 fl oz weighs about 1.48 oz (avoirdupois). A 12 fl oz jar labeled "Net Wt. 12 oz" is wrong — the contents weigh 17.8 oz.
- Olive oil (density ~0.92 g/ml): 1 fl oz weighs about 0.96 oz. The difference is 4%. A recipe that calls for "3 oz olive oil" is ambiguous — is that fluid ounces (89 ml, ~82 g) or weight ounces (85 g, ~92 ml)?
- Mercury (density 13.6 g/ml): 1 fl oz weighs about 14.2 oz — nearly a pound. A 1 fl oz vial of mercury is surprisingly heavy the first time you pick it up.
- Ethanol (density ~0.789 g/ml): 1 fl oz weighs about 0.83 oz. A "1 oz shot" of 80-proof vodka weighs 23.3 grams (0.82 oz) — but the bartender measures it by volume, not by weight.
The distinction matters in baking, where flour and sugar are measured by weight in professional kitchens and by volume in home kitchens, producing different results from the same recipe. It matters in pharmacy, where an "ounce" of cough syrup is always a fluid ounce (30 ml rounded), but an "ounce" of zinc oxide ointment is an avoirdupois ounce (28.35 g). The abbreviation "fl oz" exists specifically to prevent this confusion. When you see just "oz" on a liquid product, assume it means fluid ounces if the product is a beverage and weight ounces if it's a solid. When you see "fl oz," the "fl" is your confirmation that volume was intended.
Engineering Context
Fluid ounces are a legacy unit, but they appear in enough industrial contexts that engineers who work at the US-metric interface need to know the exact factor cold. In inkjet printing, ink consumption is specified in picoliters per nozzle and fl oz per 1,000 pages — a 4% unit error in the spec sheet translates directly to the ink cartridge yield estimate printed on the box. In pharmaceutical manufacturing, liquid oral dosage forms (cough syrups, pediatric antibiotics) have fill volumes specified in both milliliters and fluid ounces; the FDA's Current Good Manufacturing Practice regulations require the metric value to be the master and the fl oz value to be the conversion, never the reverse. In US beer brewing, the barrel (31 US gallons) is the legal unit of production for federal excise tax purposes, and every barrel contains 128 × 31 = 3,968 fl oz, which equals 117.35 liters. A microbrewery that rounds its barrel to "about 117 liters" and uses that shortcut across a 1,000-barrel production run will misreport its tax liability by 350 liters of beer — not enough to trigger a TTB audit, but enough to fail a voluntary compliance check. The gallons to liters conversion is the parent unit; every fluid ounce inherits its metric value from the gallon definition. In countries where only the metric system is legal for trade, fluid ounces appear exclusively on export packaging — and the conversion printed on the box must match the metric fill exactly, because the metric value is the one a customs inspector will verify with a graduated cylinder.
More: ml to fl oz (× 0.0338) ml to cupsmiddot; ml to cups · cups to ml · liters to ml · gallons to liters · Guide
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Frequently Asked Questions
Are fluid ounces the same in the US and UK?
No. 1 US fluid ounce = 29.5735 ml. 1 UK imperial fluid ounce = 28.4131 ml. The US fl oz is 4% larger. Both are 1/128 (US) or 1/160 (UK) of their respective gallons, and the US gallon (3.785 L) is 17% smaller than the imperial gallon (4.546 L). The two fluid ounces are therefore different for two reasons: the gallon is different, and the number of subdivisions is different. A US recipe that says "8 fl oz milk" means 237 ml. A UK recipe that says "8 fl oz milk" means 227 ml. Use this calculator for US fluid ounces; multiply the result by 0.96076 to convert to imperial fluid ounces.
Why does my measuring cup say 240 ml = 1 cup, but this calculator says 237 ml?
Because there are three different "cups" in common use. The US customary cup (used in this calculator and most American recipes) is 236.59 ml. The FDA's nutrition labeling "cup" rounds to exactly 240 ml — your measuring cup may show this number for compatibility with the Nutrition Facts panel. The metric cup used in Australia, Canada, and New Zealand is exactly 250 ml. None is wrong. Each serves a different regulatory purpose. For cooking, 237 vs 240 vs 250 ml is rarely noticeable. For a recipe that calls for 3 cups of flour in a bread formula, a 250 ml cup adds 40 extra ml of water compared to a 237 ml cup — enough to change the dough hydration by 3 percentage points and noticeably alter the crust. Bakers work in grams for this reason. See our cups to ml converter for the full breakdown of cup variants.
Why do some bottles of water say 16.9 fl oz instead of 17?
Because 16.9 fl oz = 500 ml exactly. The bottle is filled to a metric target — 500 ml — and the label converts the metric fill to US fluid ounces for the American market. 500 ÷ 29.5735 = 16.907 fl oz, rounded to 16.9. This is why "weird" fluid ounce values keep appearing on imported products: 16.9, 33.8, 50.7. They are clean metric numbers (500 ml, 1 L, 1.5 L) wearing a fluid ounce costume for US retail shelves. The manufacturer did not choose 16.9 — the math did.