Water to Add: 63% → 40% ABV
Standard volume method. Verify the final strength with a hydrometer before bottling.
= 1000 × (63 − 40) ÷ 40 = 575 mL per litre
About this dilution
63% is typical American barrel-entry strength, while 40% is the legal minimum and standard bottling strength for most spirits. Bringing 63% down to 40% means adding roughly 575 mL of water for every litre of spirit, increasing the batch to about 1575 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.