How alcohol dilution calculator works
Dilution is one line of algebra: the amount of pure alcohol stays constant, so starting ABV × starting volume = final ABV × final volume. Add water and the same ethanol now hides inside a bigger number. Drop 250 ml of water into 750 ml of 40% ABV spirit and the alcohol — 300 ml's worth — spreads across a full liter: exactly 30% ABV, or 60 proof.
The calculator runs the equation in both directions. Add a known amount of water and it computes the new ABV (plus the US proof, which is simply twice the ABV). Or set a target ABV and it solves backwards for the water needed — the classic cask-strength question: how much water turns 700 ml of 60% cask sample into drinkable 45%? 233 ml, is the answer.
Both inputs at once is the audit mode: give it the water you actually poured and the strength you wanted, and it reports how close you landed, in ABV percentage points. That is handy for bottling small batches, proofing down homemade liqueurs, or splitting a bottle two ways with different tolerances for heat.
One honest caveat: the straight C1V1 = C2V2 model ignores contraction — ethanol and water molecules pack tighter together than either alone, so a mixed volume is about 1–2% smaller than the sum of its parts, and measured ABV wobbles a fraction of a point with temperature. For kitchen and bar purposes the simple model is plenty; certified labs correct for both.