Sprinkler Precipitation Rate Calculator

How it works

Rate (mm/h) = head flow (L/min) × 60 ÷ (head spacing × row spacing in m²) [matched-precipitation nozzles]; catch-can: mm collected ÷ minutes × 60; run time = target mm ÷ rate × 60

A sprinkler zone’s precipitation rate is the depth of water it lays down per hour, and it is what turns an agronomic need — say 25 mm a week for a lawn — into minutes on the controller. It can be calculated from the nozzle flow and head spacing, since every litre per hour spread over a square metre is a millimetre of depth, or measured directly with tuna cans set out across the zone, which also exposes dry spots. Spray heads typically apply 25–45 mm/h, rotors 8–15 mm/h — which is why mixing them on one zone fails. If the rate exceeds what the soil can absorb, water runs off; the fix is cycle-and-soak: several short runs with pauses.

Worked example

Full-circle heads at 6 L/min on a 4.5 × 4.5 m grid: 360 L/h over 20.25 m² = 17.8 mm/h, so 12 mm takes 40 minutes — split into two 20-minute cycles on loam, which only absorbs about 12 mm/h.

Frequently asked questions