Water Addition Calculator
Enter your current volume, ABV, and desired target ABV
Calculations follow the OIML R 22 alcoholometric standard. Methodology & sources →
How the Dilution Formula Works
When you dilute a spirit, the total mass of alcohol stays the same — you're just adding more liquid around it. The Standard mode uses the classic volumetric formula:
This works well for most home distilling. However, ethanol and water are not ideal liquids — when they mix, hydrogen bonds form between molecules, causing the total volume to contract by roughly 1–3%. Precise (OIML) mode corrects for this using official density tables:
Where ρ values are densities looked up from OIML R22 tables. This is the same calculation used by customs and excise authorities worldwide for legal verification of spirits strength.
When to Use Each Mode
Use Standard mode when doing a rough batch dilution and you will measure the final ABV with a hydrometer before bottling anyway. It is accurate enough for most purposes and gives results in seconds.
Use Precise (OIML) mode when you need to hit a specific regulated ABV for bottling — such as exactly 40.0%, 43.0%, or 46.0% ABV. The difference between modes is typically 10–30 mL per litre, which can push a batch above or below a legal threshold. For serious bottling, always use Precise mode and still verify with a hydrometer at 20°C.
