The terms “proof” and “concentration” in the context of alcohol refer to two different ways of expressing the amount of alcohol in a liquid, typically used in different contexts and for different purposes.

**1. Concentration: **

– The concentration of alcohol is usually expressed as a percentage, either by volume or by weight.

– “Alcohol by volume” (ABV) is the most common measure and is used worldwide. It represents the volume of pure ethanol present in 100 units of volume of the solution at 20°C. For example, a beverage that is 40% ABV contains 40% pure ethanol by volume.

– “Alcohol by weight” (ABW) is less common and represents the weight of ethanol as a percentage of the total weight of the solution. Since ethanol is lighter than water, the ABW will always be lower than the ABV for the same solution.

**2. Proof:**

– Proof is a measure of the strength of the alcohol that is primarily used in the United States. Historically, it was a test for the amount of alcohol in a spirit, but now it’s simply a numerical value.

– In the U.S., proof is calculated as twice the ABV. So, a beverage that is 40% ABV is 80 proof. This means that the number for proof is always double the percentage of ABV.

– The origin of this system dates back to the 18th century and was based on a test to determine if alcohol was of a sufficient level of purity. The term “proof” came from the practice of “proving” the alcohol content by igniting it. If it burned with a certain type of flame, it was considered “proof” that the alcohol content was at a certain level.

– It’s important to note that the definition of proof can vary in other countries. For example, in the UK, the proof system was different and more complex, based on a specific gravity scale. However, the UK now primarily uses ABV.

In summary, while concentration (expressed as ABV or ABW) is a direct measure of the amount of alcohol in a liquid, proof is a more traditional measure used mainly in the United States, representing a numerical scale based on the ABV.

The use of “proof” in the UK historically differed significantly from the system used in the United States. The UK’s proof system, which is no longer in official use, was based on a specific gravity scale and was a bit more complex than the straightforward doubling method used in the U.S.

**1. Historical Background**: The term “proof” originated from a test used to determine whether distilled spirits were of acceptable strength. In the UK, this involved “proving” the alcohol by pouring it on gunpowder and attempting to ignite the gunpowder. If the gunpowder could still burn, the spirit was said to be “above proof” and taxed at a higher rate. If it did not ignite, it was “under proof”. Spirits that just allowed the gunpowder to ignite were “100 degrees proof”.

**2. Specific Gravity-Based System:** The UK proof system was based on a specific gravity scale. A spirit with 12/13 the specific gravity of pure water at the same temperature was defined as 100 degrees proof. This system was somewhat more complicated than the U.S. system, which simply doubles the alcohol by volume (ABV) percentage.

**3. Conversion to ABV**: In the UK system, 100 degrees proof was equivalent to an ABV of about 57.15%. This means that a spirit with an ABV of 57.15% would be exactly 100 proof in the UK system. To convert from UK proof to ABV, one could use the formula: ABV = (Proof / 1.75). Conversely, to convert from ABV to UK proof, the formula would be: Proof = ABV * 1.75.

**4. Modern Usage:** The UK officially abandoned the proof system in 1980, switching to ABV as a more straightforward and internationally recognized measure. ABV is now used on all alcoholic beverages sold in the UK, aligning with the standard used in most other countries.

The shift from proof to ABV in the UK was part of a broader move towards standardization and simplification in the measurement and labeling of alcoholic beverages. The ABV system is easier to understand for consumers and aligns with scientific measures of concentration.