Fluid ounce to Gallon (UK) Converter

The fluid ounce (symbol: fl oz or fl. oz.) is a unit of volume used primarily in the United States for measuring liquids.

US Fluid Ounce (Standard in America)

US fluid ounce = exactly 29.5735295625 milliliters

Defined as: 1/128 of a US gallon

Mathematical relationships:

• 2 tablespoons = 1 fluid ounce
• 8 fluid ounces = 1 cup
• 16 fluid ounces = 1 pint
• 32 fluid ounces = 1 quart
• 128 fluid ounces = 1 gallon

Imperial Fluid Ounce (UK)

Imperial fluid ounce = exactly 28.4130625 milliliters (about 4% smaller than US)

Defined as: 1/160 of an Imperial gallon

Key difference: The Imperial gallon (4.546 L) is larger than the US gallon (3.785 L), but divided into more fluid ounces, making each Imperial fluid ounce slightly smaller than the US version.

Fluid Ounce vs. Weight Ounce

This is the most common source of confusion:

Fluid ounce (fl oz): Measures volume (capacity/space)

• Used for: liquids (water, milk, juice, oil)
• 1 fl oz = 29.57 mL
• Measured with measuring cups

Ounce (oz): Measures weight/mass

• Used for: solid foods (flour, sugar, meat, cheese)
• 1 oz = 28.35 grams
• Measured with a scale

For water only: 1 fluid ounce of water weighs approximately 1.04 ounces by weight. This near-equivalence was intentional historically but leads to confusion because it doesn't apply to other liquids.

Example of the difference:

• 8 fl oz of water weighs about 8.3 oz (by weight)
• 8 fl oz of honey weighs about 12 oz (honey is denser)
• 8 fl oz of vegetable oil weighs about 7.6 oz (oil is less dense)

Medieval Origins: Weight to Volume

Weight-Based Liquid Trade: In medieval England, wine, ale, and other liquids were originally sold by weight using the avoirdupois ounce (28.35 grams for weight).

Practical Problem: Weighing liquids was impractical for taverns and merchants who needed to pour drinks quickly. They needed a volume measurement.

Solution: The fluid ounce was created to represent the volume that a certain weight of liquid would occupy. For water, this created a convenient relationship: 1 fluid ounce of water weighs approximately 1 ounce.

1707: Wine Gallon Standardization (Colonial America)

Queen Anne's Wine Gallon: In 1707, England standardized the wine gallon at exactly 231 cubic inches. The fluid ounce was defined as 1/128 of this wine gallon.

American Adoption: When the United States gained independence, it retained the wine gallon system, including the fluid ounce as 1/128 of a gallon.

1824: Imperial System Divergence

British Imperial System: In 1824, Britain adopted the Imperial system, replacing the wine gallon with the larger Imperial gallon (277.42 cubic inches instead of 231).

Two Different Fluid Ounces:

• US fluid ounce: Remained 1/128 of the wine gallon (29.57 mL)
• Imperial fluid ounce: Became 1/160 of the Imperial gallon (28.41 mL)

This divergence created confusion that persists today, especially in international recipes and product labeling.

1959: International Agreement

Gallon Defined in Metric: The 1959 international yard and pound agreement defined the US gallon as exactly 231 cubic inches, which equals exactly 3.785411784 liters.

Fluid Ounce Fixed: This automatically defined the US fluid ounce as exactly 1/128 of 3.785411784 liters = 29.5735295625 milliliters.

Modern Usage

US Standard: The United States continues to use fluid ounces extensively for beverages, recipes, and product labeling.

UK Transition: The United Kingdom officially adopted the metric system but still uses Imperial fluid ounces alongside milliliters in some contexts (especially pints in pubs—568 mL).

Global Context: Most of the world uses milliliters and liters exclusively. Only the US, Myanmar, and Liberia primarily use non-metric volume units.

Beverage Industry Standard

Product Labeling: US beverages required to show fluid ounces:

• "Contents: 12 fl oz (355 mL)"
• "Serving size: 8 fl oz (240 mL)"
• Dual labeling helps metric-using countries

Serving Sizes: Standardized for nutrition labels:

• Soft drinks: 8 fl oz serving (though cans are 12 fl oz)
• Juice: 8 fl oz serving
• Energy drinks: varies, often 8 fl oz

Bar and Restaurant:

• Draft beer: served in 12 fl oz, 16 fl oz (pint), 20 fl oz sizes
• Wine by the glass: 5 fl oz, 6 fl oz, 9 fl oz pours
• Mixed drinks: recipes measured in fl oz (jiggers)

Cooking and Recipe Instructions

American Recipes: Nearly all US recipes use fluid ounces and cups:

• "Add 4 fl oz milk (1/2 cup)"
• "Pour in 8 fl oz chicken stock (1 cup)"
• Cups are more common, but fl oz appears on measuring cups

Liquid Measuring Cups: Marked with both:

• Fluid ounces: 2, 4, 6, 8 fl oz markings
• Cups: 1/4, 1/2, 3/4, 1 cup markings
• Milliliters: 50, 100, 150, 200, 250 mL markings

Recipe Scaling: Fluid ounces make doubling/halving easy:

• Original: 8 fl oz
• Double: 16 fl oz
• Half: 4 fl oz

Nutrition Facts and Food Labeling

Serving Size: US nutrition labels show fluid ounces:

• "Serving Size: 8 fl oz (240 mL)"
• "Servings Per Container: About 2 (16 fl oz bottle)"

Comparison Shopping: Unit pricing often per fluid ounce:

• "Price per fl oz: $0.15"
• Helps consumers compare different package sizes

Dietary Guidelines: Recommendations in fluid ounces:

• Water intake: 64 fl oz (8 cups) per day common guideline
• Juice limit: no more than 4-6 fl oz daily for children

Bartending and Mixology

Cocktail Recipes: Professional bartenders measure in fl oz:

• Margarita: 2 fl oz tequila, 1 fl oz lime juice, 1 fl oz triple sec
• Martini: 2.5 fl oz gin, 0.5 fl oz vermouth
• Old Fashioned: 2 fl oz whiskey, plus bitters and sugar

Bar Tools:

• Jigger: double-sided measuring cup (1 fl oz / 1.5 fl oz or 1 fl oz / 2 fl oz)
• Shot glass: 1.5 fl oz standard (though varies)
• Speed pourer: trained bartenders pour accurate fl oz by count

Standard Drink Definition: US defines standard drink as:

• 1.5 fl oz distilled spirits (40% ABV)
• 5 fl oz wine (12% ABV)
• 12 fl oz beer (5% ABV)

Cosmetics and Personal Care

Product Sizing: Cosmetics sold in fluid ounces:

• Perfume: 0.5 fl oz, 1 fl oz, 1.7 fl oz, 3.4 fl oz
• Foundation: 1 fl oz, 1.7 fl oz typical
• Shampoo/conditioner: 8 fl oz, 12 fl oz, 16 fl oz, 32 fl oz

TSA Liquid Limit: Air travel restrictions:

• Maximum container: 3.4 fl oz (100 mL)
• Total carry-on: 1 quart bag (32 fl oz capacity)

Travel Sizes: Products marketed as "TSA-friendly":

• 3 fl oz or smaller containers