Methodology & Sources

Every result on DistilCalc is produced from published physical relationships and recognised metrological standards rather than from rounded rules of thumb. The core methods are summarised below so that each figure can be checked against its source. Where a simplified formula is used, the assumption behind it is stated.

The volume of water that must be added to reduce a spirit to a target strength is calculated from a mass-conservation relationship for ethanol. For everyday reduction the standard volume method is applied:

This relationship assumes that volumes are additive. Because ethanol and water contract slightly when mixed, the simple method is treated as an approximation and a precise mode is offered for bottling at an exact strength.

When ethanol and water are combined, the final volume is smaller than the sum of the two starting volumes. To correct for this effect, the precise mode is based on the International Organization of Legal Metrology Recommendation R 22, the international alcoholometric tables that define the density of ethanol-water mixtures at a 20 degrees Celsius reference temperature. The additional water predicted by the precise mode reflects this contraction and is reported separately from the standard result.

Hydrometer and alcoholometer readings are only valid at the temperature for which the instrument is calibrated, which is normally 20 degrees Celsius. Apparent strength is corrected to this reference using the temperature relationships described in the same alcoholometric tables. Readings taken at other temperatures are adjusted before the strength is reported.

For fermented washes, alcohol content is estimated from the change between original gravity and final gravity. A widely used homebrew approximation is applied:

This approximation is accurate for typical wash strengths. For high-gravity ferments the relationship becomes less linear, and the result is therefore treated as an estimate to be confirmed by direct measurement of the distillate.

Refractometer values are reported in degrees Brix. Brix is converted to specific gravity through the standard density relationship for sucrose solutions. When a sample already contains alcohol, the reading is corrected with a wort correction factor, because ethanol changes the refractive index independently of sugar content.

Proof is converted directly from alcohol by volume. United States proof is defined as twice the ABV. The historical United Kingdom proof scale is also supported, on which 100 degrees proof corresponds to approximately 57.1 percent ABV. All conversions are derived from these definitions rather than from lookup tables.

Calculated strengths should always be confirmed with a calibrated hydrometer or alcoholmeter at 20 degrees Celsius before bottling. The tools are intended as planning and reference aids. They do not replace direct measurement, and responsibility for legal compliance and safe practice rests with the user.

• International Organization of Legal Metrology, Recommendation R 22, International Alcoholometric Tables.
• Standard density relationships for ethanol-water and sucrose solutions at a 20 degrees Celsius reference.
• Established homebrew and distilling gravity-to-alcohol approximations.