Why does my measured ABV differ from a dilution calculator?

The common water formula, V × (starting ABV ÷ target ABV − 1), estimates the difference between target final volume and starting spirit volume. It treats that difference as the water to add. Real ethanol–water mixtures occupy less volume than the separate components would suggest at the same temperature.

For example, 1 L at 60% gives a 500 mL water estimate for 40% ABV. Mixing those quantities does not guarantee exactly 1.5 L of finished spirit at exactly 40%. A contraction-aware calculation uses suitable density or alcoholometric data, rather than a fixed extra percentage for every batch.

Density measurements depend on temperature. Read the reference temperature marked on your alcoholmeter or its documentation, and use the correction procedure appropriate to that instrument and product. Do not assume all instruments use the same reference temperature.

For US proofing, TTB explains that Table 1 converts corrected hydrometer and thermometer readings to proof at 60°F. That specific US reference should not be silently applied to an instrument calibrated on another basis.

• Confirm that starting strength was entered as ABV, rather than US proof.
• Check that spirit and water volumes use the same units.
• Mix the batch thoroughly before taking a representative sample.
• Let the sample stabilize at a suitable measurement temperature.
• Check instrument calibration and read it according to its instructions.
• Account for sugar or other dissolved ingredients that can make a plain alcoholmeter reading unsuitable.

Water addition cannot correct a data-entry error or an unsuitable measurement method. Resolve those issues before chasing a new target.

Record starting volume and strength, instrument and temperature, estimated water, each actual addition, and the final measured volume and strength. This separates the plan from what happened in the tank and makes a discrepancy easier to trace later.

The free proofing calculator is a planning aid. The Stillbench Toolkit pairs theoretical proofing with production records and cost planning; it does not remove the need to verify finished spirit.

TTB: interpolation of proof from Table 1 explains the US temperature-correction basis. TTB: What is Density? describes density and points to ethanol–water density tables across temperature ranges. Use the measurement procedures applicable to your equipment and operation.