Every dose depends on this number. Get it once, get it right.
Chemical doses scale linearly with volume: a 20% volume error means every dose you ever add is 20% off. Above-ground pool boxes list nominal sizes — measuring actual water depth beats the label.
Rectangle: L×W×D. Round: π×(diameter/2)²×depth. Oval: π/4×L×W×D. One cubic metre is 1,000 litres; one cubic foot is 7.48 US gallons.