Determining & Adjustment of the Alcohol Strength

Whisky is bottled with various alcohol percentage but legally, the minimum must be 40%. The percentage on alcoholic beverages are usually referring to the volume of alcohol the liquid has. It also can be measured in percent of weight, but it became common in the past to use percent by volume, or to be specific ‘alcohol by volume’ (ABV).

Did you see a cowboy in a western movie, drinking half a bottle of Whisky at a saloon and then walking straight out to win a duel? That wasn’t far from reality. In the ‘Wild West’ light Whiskies with 15 to 25% alcohol strength were regulary served. The wilder the saloon, the less alcohol in the bottles. Everybody was diluting a little bit more until the Whisky was as strong as a good Wine. Today, Whisky with less alcohol is official named Whisky Liqueur.

But how can the percentage of alcohol be determined, if there is no information available? Or how can you water down the Malt in your glass to a certain number in strength?

Convenient Tools for the Dilution at Home

If you like to buy Whiskies with cask strength and dilute them at home, we offer you some small and easy programs, which can help. Further down, we also show manually methods, if a PC is not available.

For Windows: eWhiskyReducer V1.45 (c) E. Kriese, 2011 Freeware 1,6 MB

This program offers an expert mode. This version calculates with exact volume contraction and therefore has very small deviations in the results. For Windows systems.

For Apple: WhiMix V0.8 (c) M. Uhl, 2007 Freeware 128 kB

For Excel (Macro): Pilipps Whisky Calculator (C) V1.3, 2011, Freeware 178 kB

For Windows: eWhiskyMerger V1.05 (c) E. Kriese, 2011 Freeware 1,6 MB

Allows to mix two Whiskies with different alcohol percentages.

Measuring in the Distillery

The alcohol content can be determined with the help of the spindle. In every spirit safe of Scottish distilleries there are a spindle and a thermostat. The spindle is a floatation panel, which shows the density of a liquid. Since the density is depending on the temperature, the read density value must be corrected according to the temperature in a chart. This way, the exact alcoholic content can be measured.

Measuring of the Strength in Proof

But what has been done in Scotland 200 years ago? Well, they used the tools they had on hand. The Whisky was mixed with gun powder and set on fire. Did the mix burned down quickly with a bright flame, the Whisky had a so called ‘Proof’ alcoholic strength. - Since it was proofed.

But did the mix weakly and slowly burned down, the alcohol was ‘Under Proof’. Did the liquid almost entirely exploded, it was ‘Over Proof’. Even today this procedure is reflected in the names, like Springbank 100 Proof or Glenfarclas 105 Proof.

With the help of a spindle the alcoholic content can be measured way more specificly and resulted in a set number in comparison to the ones of the arbitrary ‘Proof’. 100 Proof in Scotland are about an alcohol percentage of 57%. Every increase of 5 Proof is equivalent to 3%. To complicate matters, the Americans define 100 Proof different. There, it equals to only 50% vol.. In the USA, you can calculate alcoholic content if you divide the Proof number with two. Is this because of the American pragmatism or because the gun powder in the Wild West was stronger? Who knows.

Today Whisky and especially Scottish Whisky is bottled with these strengths:

Strength in Vol%Strength in Proof 1Strength in US Proof

1 numbers are not exact.

As proper Middle-Europeans, we stick to the metric system with the percent of volume in the following part.

Reducing the Strength for Drinking

If you want to dilute Whisky, which relation between water and Whisky is right, to get an exact percentage of alcohol? Is the ratio 1:1, it is easy. The alcohol content is halved. Is there an easy formula, to determine the alcoholic content of an arbitrary mixing proportion? And is there a formula to calculate the amount of water that is needed to reduce the strength of a Whisky to a certain amount?

There are two ways to calculate the alcoholic content:

1. Through the absolute alcohol content

2. With the help of the Pearson’s square

1. Determining the Percent of Alcohol with the absolute Alcohol Content


A glass of 4cl Whisky with 46%. How high is the alcohol content, if 2cl water was added? The mixing ratio is: Whisky : water = 2:1


The absolute alcohol content: 4cl * 46% = 0,04 litre * 0,46 = 0,0184 litre

After adding 2cl Water the amount of liquid increased to 6cl. The alcohol strength is calculated, if the absolute alcohol content is in relation to the whole amount of liquid. 0.0184 litre / 0,06 litre = 0,3067 = 30,67%

In this table you can find the calculation for the common alcoholic strengths of Whisky.


2. Determining the Percentage of Alcohol with the Pearson’s Square

Example: A glass of 4cl Whisky with 57%. How much cl water must be added to get an alcohol percentage of 40%?

Result: The calculation is based on the Pearson’s square. This formula is an easy tool.

The Pearson’s square contains five squares, where a missing field on a diagonal can be calculated with the other two. There are two equations:

If there are missing fields on the right side, you can rearrange the equation to solve the missing field:

But enough theory, here is the answer to the example:

You have to add 1.7cl water to reduce the alcohol percentage of the Whisky to 40%. The whole amount of Whisky with 40% therefore is 5.7cl.

The Pearson’s square can not only be used for the ratio of Whisky and water. Even the result of mixes with different strengths can be determined.

If these calculation seem confusing, the free programs from the beginning are also still available.

3. Diluting Whisky

1. Pour in Whisky with cask strength (57%) until it reaches the widest part of the glass (amounts to 4cl Whisky)

2. Dilute with water to drinking strength: 1,7cl water ≈ 3 ½ tea spoons (1 tea spoon = 0,5 cl)

3. Result: 5,7cl Whisky with drinking strength (40%). Ready to be enjoyed!