Refractive Index Calculations for Spirits and Syrups


Measuring ABV obscured by sugar

The purpose of this calculator is to determine the obscured alcohol content of an alcoholic spirit which also contains sugar. It will also determine the sugar content of the spirit. This page explains how to use the calculator. Those who are interested can read how it works here.

The data needed for the calculation are the density and refractive index of the spirit. The density and the refractive index must both be measured at temperatures between 10C and 30C (50F to 86F) but it is not necessary that they be measured at the same temperature. The instruments needed for these measurements are commonly found in distilleries.

The density can be measured with either a hydrometer or an electronic density meter (EDM). The hydrometers that are commonly found in a distillery are often not calibrated in density units and the AlcoDens LQ software will convert the readings from the available calibration to the density units it requires. The refractive index can be measured by a variety of refractometers and again the AlcoDens LQ software will convert the readings from the available calibration to refractive index units.

As soon as a valid set of inputs has been made for the density and refractive index the calculated results will be displayed. A typical example is shown below. If there is an error in the data input the results fields will be colored red and the fault will be described in the status bar at the bottom of the calculator. The upper part of the results panel shows the properties of the spirit that are either not dependent on the temperature or are usually reported at a specific temperature. These include the Proof of the spirit which is always reported at 60F and the density and refractive index at 20C. In the lower part of the panel the temperature at which the properties are required can be entered.

Obscured ABV determined from density and refractive index

Note that although the refractometer reading was given in Brix units the software has converted this to the actual refractive index, and then combined that with the measured density to determine the true alcohol and sugar content of the spirit.


Measuring ABV with a Brix-calibrated refractometer

The purpose of this calculator is to determine the strength of an alcohol-water spirit by measuring its refractive index (RI). In many cases it is quicker and easier to use a refractometer rather than a hydrometer to measure the strength of an alcohol solution. It will also require a much smaller sample.

This calculator is for spirits containing alcohol and water only. If your spirit also contains sugar then use the alcohol-sugar-water refractive index calculator described above to determine the obscured alcohol strength.

The RI can be measured by a variety of refractometers and it is not necessary that the refractometer is specifically designed for alcohol-water mixtures. In this particular example a brix-calibrated refractometer is used to measure the alcohol content.

As soon as all the information on the type of refractometer and the readings have been entered the calculator will display the strength of the alcohol spirit in Proof and Mass % terms. A standard temperature can also be entered so that the ABV, density and refractive index at a particular temperature can be calculated.

In this particular example the calculator has determined that there are two possible alcohol strengths that would match the measured RI. This is because as the alcohol content of a spirit increases from zero the refractive index also increases until the alcohol content reaches about 80 mass % and then the RI starts decreasing with further increases in alcohol content. For any refractive index above approximately 1.3616 nD there will be two possible alcohol contents found.

ABV measured with brix refractometer

Even though the refractometer being used is calibrated in Sucrose Brix it is able to measure the ABV in a spirit containing alcohol and water only.