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A US customary cup equals 8 fl oz, but a US legal cup (used on nutrition labels) equals 8.115 fl oz, a metric cup equals 8.454 fl oz, and an Imperial cup equals 9.608 fl oz. Confusing these definitions in recipes or industrial formulations introduces dosage errors of up to 20%. This converter handles all four cup standards and distinguishes between fluid ounces (volume) and weight ounces (mass), applying liquid density ρ when converting between the two measurement domains.

Weight ounces depend on the substance. Water at 4°C has ρ ≈ 1.0 g/mL, so 1 US fl oz of water weighs 1.043 oz. Milk, oil, and honey differ significantly. This tool approximates mass conversions assuming the density you select. For precision work, measure density with a hydrometer.

Formulas

Volume-to-volume conversion uses a base-unit approach. Every unit is defined by its factor f in millilitres:

V_out = V_in × f_inf_out

Where f_in is the millilitre value of one input unit and f_out is the millilitre value of one output unit. For example, US cup → US fl oz: V_out = V_in × 236.58829.574 = V_in × 8.

For volume-to-mass (weight ounces), multiply the volume in mL by the liquid density ρ (g/mL) then convert grams to ounces:

m = V_in × f_in × ρ × 128.3495

Where m is mass in avoirdupois oz, V_in is the input value, f_in is mL per input unit, ρ is density in g/mL, and 28.3495 is grams per avoirdupois ounce.

Reference Data

Cup Standard	Volume (mL)	US fl oz	Imp fl oz	Context
US Customary Cup	236.588	8.000	8.327	US recipes, cookbooks
US Legal Cup	240.000	8.115	8.447	FDA nutrition labels
Metric Cup	250.000	8.454	8.799	Australia, NZ, Canada
Imperial Cup	284.131	9.608	10.000	UK older recipes
Japanese Cup (gō)	180.390	6.101	6.351	Japanese cooking
US Fluid Ounce	29.574	1.000	1.041	US volume measure
Imperial Fluid Ounce	28.413	0.961	1.000	UK volume measure
Avoirdupois Ounce (weight)	28.350 g (mass, not volume)			US/UK weight
Troy Ounce (weight)	31.103 g (mass, not volume)			Precious metals
Common Liquid Densities (ρ at ~20°C)
Water	0.998 g/mL			Reference standard
Whole Milk	1.030 g/mL			Dairy
Heavy Cream	0.994 g/mL			Dairy
Olive Oil	0.913 g/mL			Cooking oil
Vegetable Oil	0.920 g/mL			Cooking oil
Honey	1.420 g/mL			Viscous sweetener
Maple Syrup	1.330 g/mL			Sweetener
All-Purpose Flour	0.529 g/mL (packed)			Dry good (approx)
Granulated Sugar	0.845 g/mL			Dry good (approx)
Ethanol (pure)	0.789 g/mL			Spirits base

Frequently Asked Questions

The statement "1 cup = 8 oz" only holds for US customary cups measured in US fluid ounces. A US legal cup (FDA standard) is 240 mL, yielding approximately 8.115 US fl oz. A metric cup is 250 mL (≈8.454 US fl oz), and an Imperial cup is 284.131 mL (≈9.608 US fl oz). Furthermore, "8 oz" as weight depends on the liquid's density - 8 fl oz of honey weighs roughly 11.9 oz, not 8 oz.

US legal cups (240 mL) appear on FDA Nutrition Facts panels and in government dietary guidelines. US customary cups (236.588 mL) are used in traditional American recipes and measuring cup sets. The difference is 1.44%, which compounds in large-batch cooking or pharmaceutical formulations. If your source is a nutrition label, use legal cups. If it is a cookbook, use customary cups.

Fluid ounces measure volume; weight ounces measure mass. The bridge between them is density (ρ). Water at 20°C has ρ ≈ 0.998 g/mL, so 1 US fl oz of water weighs about 1.043 oz. Olive oil (ρ ≈ 0.913 g/mL) is lighter - 1 US fl oz weighs only 0.953 oz. Honey (ρ ≈ 1.42 g/mL) is heavier - 1 US fl oz weighs about 1.482 oz. Always select the correct substance density for accurate mass results.

This tool handles volume-to-volume conversions accurately for any substance. For volume-to-weight conversions of dry ingredients, results are approximate because dry goods have variable packing density. All-purpose flour ranges from about 0.48 to 0.58 g/mL depending on sifting. The tool uses a typical packed value of 0.529 g/mL. For precision baking, weigh dry ingredients directly on a kitchen scale.

A US fluid ounce equals 29.5735 mL. An Imperial fluid ounce equals 28.4131 mL - about 4% smaller. This means 1 Imperial pint (20 Imp fl oz) is 568.261 mL, while 1 US pint (16 US fl oz) is 473.176 mL. Using the wrong fluid ounce standard in a British recipe adapted for American kitchens introduces systematic error in every measurement.

For volume-to-volume conversions, temperature has no effect because both sides are volume. For volume-to-weight conversions, temperature changes density. Water's density drops from 0.9998 g/mL at 4°C to 0.9584 g/mL at 100°C - a 4% shift. For room-temperature cooking (15-25°C), the error is under 0.3% and negligible for most applications. For industrial or scientific work at extreme temperatures, use a measured density value.