Proof and Percentage
Overproof, Navy Strength, Cask Strength: How Strong Is Rum?
Rum must be at least 37.5% ABV within the EU, though most bottles sit around 40%. Overproof, Navy Strength, and cask strength all describe stronger bottlings: Navy Strength usually means around 57%, while Overproof can exceed 80%. None of these terms is legally defined.
What Does "Proof" Mean for Rum?
Some bottles show a second figure alongside the percentage: proof. According to legend, British sailors tested their rum by pouring it over gunpowder. If the powder still ignited, the rum was considered "proved." The story is folklore, but the measurement behind it is real. 100 British degrees proof corresponds to 57.15% ABV, the point at which, by the old calculation, full proof was reached.
In the US, a simpler formula applies: proof is exactly double the ABV percentage. 80 proof equals 40% ABV, 151 proof equals 75.5% ABV. British and American figures therefore can't be compared directly. Within the EU, only the percentage by volume, abbreviated % vol, carries legal weight. When a bottle shows a proof figure, it's worth first working out which country's system it follows.
Navy Strength: Why Exactly 57 Percent?
Rums and gins around 57% ABV are often labeled Navy Strength. The figure echoes British proof, where 100 degrees corresponds to 57.15%. It isn't legally defined, it's simply an industry convention. The name recalls the Royal Navy's rum ration, issued for the last time on July 31, 1970, at 54.6% ABV, slightly weaker than today's marketing term suggests.
Navy Rum likewise isn't a protected style. It usually refers to a robust, dark blend of several West Indian rums, often including a share from Guyana. Pusser's received rights to the name from the Admiralty in 1979, along with details of the original blend, and has sold to the public since 1980. Many other brands use the term freely. How the ration came about and why it ended is covered in the History of Rum feature.
Overproof Rum: How Strong Does It Get?
Strictly speaking, anything above British proof, that is, above 57.15% ABV, falls into Overproof territory. In trade use, the term is applied more loosely, with some 60% bottlings carrying it and others well above that. Well-known examples include Wray & Nephew White Overproof from Jamaica at 63% ABV according to the producer, and Sunset Very Strong from St. Vincent at 84.5% ABV. Stroh 80 from Austria sits at 80% ABV, but it's classified as Inländerrum, not rum.
Spirits this strong are mainly tools. In tiki drinks, small amounts add aroma and warmth; in Austria, Stroh goes into dough and punch. They're rarely drunk neat, since at this strength the alcohol numbs the tongue before the aroma registers. Careful handling matters here. The stronger a rum, the more easily it ignites, and flambéed drinks or desserts demand distance from open flame and a steady hand.
Cask Strength: Rum as Strong as It Comes Out of the Barrel
Some bottlings skip water dilution altogether. That's called cask strength, or Fassstärke in German. How strong such a rum turns out depends on the fill strength at barreling, the climate, and time. In tropical warehouses, around eight percent evaporates each year, and this shifts the ratio of alcohol to water along the way. Cask-strength rums therefore have no uniform proof, each barrel brings its own number.
For connoisseurs, cask strength has one advantage: you decide how much water to add. Just a few drops change the aroma, since aromatic compounds distribute differently once the rum is diluted. For beginners, though, a rum this strong can taste too harsh neat. A glass of water alongside helps, as does a small splash in the glass itself. More alcohol doesn't mean more quality, it means more concentration, for better or worse.