What’s the Proof? Knowing the Terms of Alcohol Strength for Rum - Rum Raiders
Skip to main content

Join our newsletter for the latest in rum news and great deals sent right to your inbox!


More to Enjoy

  • Whiskey Raiders
  • Tequila Raiders
  • Gin Raiders

What’s the Proof? Knowing the Terms of Alcohol Strength for Rum

What’s the Proof? Knowing the Terms of Alcohol Strength for Rum

(Photo: phil cruz/Pexels)

Spirits have several terms used to describe the strength of what is contained in the bottle. Whether for sipping or mixing, understanding alcohol percentage or proof can make a big difference in the rum-drinking experience. Several of the terms used to describe rum call back to the time of seafarers, privateers and pirates, when the quality of rum was put into question. This period was the height of rum’s popularity and new terminology developed alongside this spirit. Many brands hold onto these names and traditions when labeling the strength and style of rums. 

There are two main schools of thought when looking at bottle proof of rums and other spirits, the British systems and the American ones. The two methodologies of proving liquor vary slightly but maintain similar guidelines and purposes. 

Gunpowder Proof

The term proof can be a bit of a head-scratcher, partially because it comes from a bygone era and its exact definition has changed a bit over time. In the past, England set greater taxes for spirits with a higher percentage of alcohol. This taxing led to the Government checking alcohol content by setting it ablaze. 

The English Government would determine proof by first soaking gunpowder in rum. If the gunpowder could catch fire, then the rum proved to be higher in alcohol and the government considered it a proof spirit. If the gunpowder exploded then this was considered overproof. However, this method wasn’t the most precise. With the invention of the hydrometer, accuracy became more exact. This resulted in 57.14% ABV becoming known as 100 proof and is the value everything alcohol was compared against. Thus, English proof was created and it became law in 1818.  

Navy Strength

What’s the Proof? Knowing the Terms of Alcohol Strength for Rum

(Photo: Sharon Wahrmund/Pexels)

The gunpowder method of determining strength was also used by sailors. The Navy used to hand out rations of rum, also called tots, to their sailors. These rations were about a half pint and would be distributed by the ship’s purser or pusser, who would also test the strength of the rum. If the rum didn’t light, there may have been some angry sailors to deal with. 

The invention of hydrometers also allowed the British Navy to make adjustments to what they considered proof. They determined that 54.5% ABV or 4.5 degrees below proof was the minimum strength required for gunpowder to ignite. Thus, British Navy Strength became a designation and by 1866 this became the standard.

Imperial Proof 

Britain didn’t always use the alcohol-by-volume system to label spirits. It wasn’t until the 1980s that ABV became commonplace. Prior to this, the proof strength, or 100 proof which today equals 57.14% ABV, was the basis for the imperial proof system of measurement. The label would refer to degrees which is why the British Navy referenced Navy Strength as 4.5 degrees below proof. In degrees, this would be 95.5°. Converting ABV to degrees requires a bit of math. It can be determined by dividing ABV by 57.14 and then multiplying by 100 (ABV/57.14*100). This makes 40% ABV about 70°. Thankfully, this system has since been retired, so no hard mathematical equations are required to find out how strong a rum is. 

Proof in the U.S.

In America, the rule of proof was somewhat simplified over time. Around the mid-1800’s, the US decided that 100 proof equaled 50% ABV. This made it simple to convert proof to ABV by just halving the proof to find the ABV or doubling the ABV to determine the proof. Although, this still causes some confusion and it’s somewhat common for people to call something 80% ABV, even though it’s only 40 percent. The Alcohol and Tobacco Tax and Trade Bureau requires ABV to be present on the label, but not proof. The continued use of proof is optional and a holdover from past traditions. 

Overproof Rum

People are unlikely to still be blowing up gunpowder to determine if something is overproof, but the term is still relatively common and overproof rum is great for cocktails. In the U.S. overproof is not a regulated term but typically anything labeled as such will be over 50% ABV. Traditionally any spirit over 57.14% is considered true overproof. One of the most famous overproof rums produced by Wray and Nephew is 63%, making it undoubtedly overproof.  

Cask Strength or Barrel Strength Rum

What’s the Proof? Knowing the Terms of Alcohol Strength for Rum

(Photo: Los Muertos Crew/Pexels)

Most matured spirits are proofed down or diluted with water after aging and before being bottled. This allows for consistency with blended products. However, cask or barrel-strength rums are not diluted after maturation. Although, some ABV will be lost over time in the cask due to evaporation, depending on the length of aging time. Therefore, the percentages will vary but many cask-strength rums contain a higher ABV than average. 

151 Rum

Labeling a rum 151 is pretty straightforward. The proof of these rums is 151 or 75.5% ABV, making this style of rum an extremely high overproof. This rum is perfect for mixology pyros since the high proof makes it burn really well. 

Bottled-in-Bond Rum

Although more common for whiskey, some rums will adhere to the bottled-in-bond designation which was created in 1897. This is a specific set of rules established in the U.S. to protect the American consumer. During a time of less regulation, the quality of some whiskey began to decrease and cheap knockoffs were becoming commonplace. The Bottle-in-Bond Act ensured that the product was made by the same distiller, at the same distillery, during the one distilling season, and aged for no less than 4 years at a bonded warehouse. It should then be bottled at exactly 100 proof or 50% ABV. This act created insurance of quality and was an ahead of its time form of consumer protection. 

Read Next:

What Is Cask Finishing? The Old School Way to Add Flavor to Rum

What is Navy Strength Gin? The High-Proof Spirit With a History of Gunpowder and Muskets

What Is Still Strength Tequila? Upping the Alcoholic Ante

Here at Rum Raiders, we do more than write about current events in rum. We are the only media property reviewing rums and aggregating the scores and reviews of other significant voices in the rum world in one place. If you’re interested in getting a shot of rum in your morning email, sign up for our Deal of the Day newsletter

This post may contain affiliate links, so we may earn a small commission when you make a purchase through links on our site. This helps support Rum Raiders at no additional cost to you.

Filed Under:

Follow Rum Raiders:

Jessica Gleman is the managing editor of Rum Raiders. She received her Ph.D. at the University College of Dublin in Ireland, where she studied the archaeology of ancient alcohol. Jessica has a passion for the alcohol industry, including agriculture, distillation and mixology. When Jessica is not writing about rum, she is also a travel and food enthusiast who loves going around the world and experiencing various cuisines and cultures. She is enthusiastic about sharing her knowledge and expertise and learning even more about this amazing spirit.