A US fluid ounce is 1/128 of a US gallon.
Fluid ounces to Imperial fluid ounces Converter — fl oz to imp fl oz
Convert Fluid ounce (fl oz) to Imperial fluid ounce (imp fl oz) using the exact conversion factor (1 fluid ounce = 1.040842731 imperial fluid ounce). See the formula, worked examples, conversion table, unit comparison, history, and common uses.
Fluid ounces to Imperial fluid ounces converter
Converter
Volume Converter
Convert metric, US customary, imperial, cooking, dry, and industrial volume units.
Context values
Some unit families need one extra value for a physically accurate result.
An imperial fluid ounce is 1/160 of an imperial gallon.
Result = input x 2.95735296e-5 / 2.84130625e-5.
Volume uses the cubic meter as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Fluid ounces to Imperial fluid ounces
Fluid ounce and Imperial fluid ounce appear side by side at fuel pumps and on beverage labels, since the US measures liquids in gallons while most of the world uses liters. Fluid ounce is typically associated with united States (beverage, nutrition labeling), while Imperial fluid ounce is typically associated with united Kingdom (historically; less common in current UK retail than metric units).
Formula
imperial fluid ounces = fluid ounces × 1.04084273079
This factor comes from each unit's defined relationship to the category's base unit: 1 fluid ounce equals 0.0000295735295625 base units, and 1 imperial fluid ounce equals 0.0000284130625 base units, so dividing one by the other gives the direct fluid ounce-to-imperial fluid ounce factor of 1.04084273079.
Simple example
1 fl oz × 1.040842731 = 1.040842731 imp fl oz
1 fluid ounce = 1.040842731 imperial fluid ounces.
Real-world example
10 fl oz × 1.040842731 = 10.40842731 imp fl oz
10 fluid ounces = 10.40842731 imperial fluid ounces.
Conversion table
| Fluid ounce (fl oz) | Imperial fluid ounce (imp fl oz) |
|---|---|
| 1 fl oz | 1.040842731 imp fl oz |
| 5 fl oz | 5.204213654 imp fl oz |
| 10 fl oz | 10.40842731 imp fl oz |
| 25 fl oz | 26.02106827 imp fl oz |
| 50 fl oz | 52.04213654 imp fl oz |
| 100 fl oz | 104.0842731 imp fl oz |
Reverse conversion: Imperial fluid ounce to Fluid ounce
1.040842731 imp fl oz × 0.9607599404 = 1 fl oz
1.040842731 imperial fluid ounces = 1 fluid ounces.
fluid ounces = imperial fluid ounces × 0.960759940404
For a page dedicated to this direction, see Imperial fluid ounce to Fluid ounce.
Fluid ounce vs. Imperial fluid ounce
| Aspect | Fluid ounce | Imperial fluid ounce |
|---|---|---|
| Family | US customary unit | UK/imperial customary unit |
| Typical use | US beverage container sizing (soda, water bottles) | UK/Commonwealth beverage measurement in historical or specialized contexts |
| Where it's used | United States (beverage, nutrition labeling) | United Kingdom (historically; less common in current UK retail than metric units) |
Understanding the Fluid ounce
Fluid ounce (fl oz) measures volume. The US fluid ounce is 1/128 of a US gallon: exactly 29.5735295625 milliliters. It is classified as a US customary unit.
Where it's used: United States (beverage, nutrition labeling)
Uses of the Fluid ounce today
- US beverage container sizing (soda, water bottles)
- US nutrition and medication dosing labels
History of the Fluid ounce
The US fluid ounce follows from the same English wine-gallon system as the rest of the US liquid volume units, defined as a consistent fraction of the gallon.
Historical origin. Fluid ounce was not the invention of a single named person; it developed through the process described above.
Understanding the Imperial fluid ounce
Imperial fluid ounce (imp fl oz) measures volume. The imperial fluid ounce is 1/160 of an imperial gallon: exactly 28.4130625 milliliters — slightly smaller than the US fluid ounce despite the larger imperial gallon, because it divides that gallon into more parts. It is classified as a UK/imperial customary unit.
Where it's used: United Kingdom (historically; less common in current UK retail than metric units)
Imperial fluid ounce vs. US fluid ounce. Confusingly, the imperial fluid ounce (28.41 mL) is slightly smaller than the US fluid ounce (29.57 mL), even though the imperial gallon is larger overall — because the imperial gallon is divided into 160 fluid ounces rather than the US gallon's 128.
Uses of the Imperial fluid ounce today
- UK/Commonwealth beverage measurement in historical or specialized contexts
History of the Imperial fluid ounce
The imperial fluid ounce follows from Britain's 1824 imperial gallon, divided into 160 parts rather than the 128 used in the US system, producing its counterintuitively smaller size relative to the US fluid ounce.
Historical origin. Imperial fluid ounce was not the invention of a single named person; it developed through the process described above.
Fluid ounce — sources
- NIST Handbook 44, Appendix C — General tables of units including the US fluid ounce definition.
Imperial fluid ounce — sources
- UK Weights and Measures Act 1824 — Basis for the imperial fluid ounce via the imperial gallon.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many imperial fluid ounces are in 1 fluid ounce?
1 fluid ounce equals 1.040842731 imperial fluid ounces, using the exact defined conversion factor rather than an estimate.
How do I convert fluid ounce to imperial fluid ounce?
Multiply the fluid ounce value by 1.040842731. The converter above does this instantly to whatever precision you set.
How do I convert imperial fluid ounce back to fluid ounce?
Use the reverse factor: 1 imperial fluid ounce equals 0.9607599404 fluid ounces. You can also use the swap control in the converter above to flip the direction instantly.
What is a fluid ounce?
Fluid ounce (fl oz) measures volume. The US fluid ounce is 1/128 of a US gallon: exactly 29.5735295625 milliliters. It is classified as a US customary unit.
What is a imperial fluid ounce?
Imperial fluid ounce (imp fl oz) measures volume. The imperial fluid ounce is 1/160 of an imperial gallon: exactly 28.4130625 milliliters — slightly smaller than the US fluid ounce despite the larger imperial gallon, because it divides that gallon into more parts. It is classified as a UK/imperial customary unit.
Is the fluid ounce to imperial fluid ounce conversion exact?
Yes. The imperial fluid ounce is 1/160 of an imperial gallon: exactly 28.4130625 milliliters — slightly smaller than the US fluid ounce despite the larger imperial gallon, because it divides that gallon into more parts. That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.
What is the difference between imperial fluid ounce vs. us fluid ounce?
Confusingly, the imperial fluid ounce (28.41 mL) is slightly smaller than the US fluid ounce (29.57 mL), even though the imperial gallon is larger overall — because the imperial gallon is divided into 160 fluid ounces rather than the US gallon's 128.
Where is the fluid ounce still used today?
United States (beverage, nutrition labeling)