Formula
Gallons = area (sq ft) × inches × 0.623. Litres = gallons × 3.785. Example: 1 inch over 32 sq ft = about 20 gallons (75 litres).
Most vegetables need about 1 inch per week from rain plus watering, but sandy soil dries faster and clay holds water longer — check soil moisture with a finger test before watering on a schedule.