Convert US gallons to US fluid ounces

One US gallon is 128 US fluid ounces. To convert any amount, multiply by 128. Going the other way, one US fluid ounce is 0.0078125 US gallons.

Multiply the US gallon figure by 128. So fl oz = gal × 128. For a quick mental estimate, multiplying by about 128 gets you within a percent or so, which is close enough for most everyday purposes.

One US gallon is a large jug of milk. The US gallon is exactly 231 cubic inches, or 3.785 litres. The imperial gallon still used in some UK contexts is about 20% larger, which makes fuel-economy comparisons misleading.

One US fluid ounce is a small shot glass. The US fluid ounce is 1/128 of a US gallon, about 29.6 mL. The imperial fluid ounce is 28.4 mL — close enough to confuse, far enough apart to ruin a recipe at scale.

US customary. The imperial gallon still used in some UK contexts is about 20% larger than the US gallon, which makes fuel-economy figures misleading when the unit is not stated. Imperial fluid ounces are slightly smaller than US ones, and imperial pints are 20 fluid ounces against the US 16. Cups are worse still: US customary is about 237 mL, US legal is 240, metric is 250 and Japanese is 200 — and recipes almost never say which.

Yes. Since the 1959 international yard and pound agreement, the imperial and metric systems are related by exact definitions rather than measurements — the inch is exactly 25.4 mm and the pound exactly 0.45359237 kg. Everything derived from those, including this conversion, is therefore exact. Round at the end rather than partway through, since rounding gal before converting introduces an error the conversion then multiplies.