Donaldson R F Garden Irrigation Water Calculator
Work out how many litres a garden or bed needs this week, from its area, a watering depth and any rainfall already received.
25mm (about 1 inch) a week is a commonly cited starting point for established plants in dry weather — it's a general guideline, not a precise target for every plant, soil or season. Adjust it to what you're actually growing.
How it is worked out
1mm of water spread over 1m² is exactly 1 litre (1m² × 0.001m = 0.001m³ = 1 litre) — so this is a straightforward unit conversion once you know the area and the depth you want to apply. Net depth needed = target depth − rainfall already received (not less than zero). Litres needed = area × net depth. If you give a number of plants, it also divides the total evenly to show litres per plant — useful as a rough guide, though larger or more established plants will often need more than a small or new one.
Source: the 1mm-over-1m²-equals-1-litre conversion is an exact SI unit identity. The 25mm/week figure is a widely cited rule of thumb from general gardening guidance, not a single official published standard — treat it as a starting point.
Working out rainwater you could collect instead? Try the Rainwater Harvest Volume Calculator.