Water to Add: 65% → 40% ABV
Standard volume method. Verify the final strength with a hydrometer before bottling.
= 1000 × (65 − 40) ÷ 40 = 625 mL per litre
About this dilution
65% is a common off-the-still spirit strength for many pot-still runs, while 40% is the legal minimum and standard bottling strength for most spirits. Bringing 65% down to 40% means adding roughly 625 mL of water for every litre of spirit, increasing the batch to about 1625 mL.
For casual reduction the standard volume method above is accurate enough. When bottling at an exact legal strength, switch to OIML Precise mode in the calculator, which corrects for the slight volume contraction that occurs when ethanol and water mix.
Verify before bottling: a calibrated alcoholmeter is the only way to confirm you have hit 40% ABV. Measure at 20°C.