Sprinkler Precipitation Rate Calculator
The precipitation rate (in/hr) is how fast your sprinklers add water. It depends on flow (GPM) and the area they cover — the standard 96.3 constant does the conversion.
Formula: precipitation rate (in/hr) = 96.3 × GPM ÷ area (sq ft). The constant 96.3 = 60 min/hr × 12 in/ft ÷ 7.48 gal/ft³ (the universal sprinkler-design constant). Run time = target inches ÷ rate × 60. Use: size zones so all heads in a zone match type (matching precipitation), then set run time to replace ET minus rainfall.
FAQ
What is the 96.3 in sprinkler design?
It converts GPM and square feet into inches per hour. 96.3 = 60 minutes/hour × 12 inches/foot ÷ 7.48 gallons per cubic foot. It is the standard precipitation-rate constant used by irrigation designers.
Why do my zones water unevenly?
Heads of different types (spray vs rotary) have very different precipitation rates. If they share a zone, part of the area gets too much water and part too little. Group same-type heads in the same zone.
What is a good precipitation rate?
Fixed spray heads typically apply 1.0-1.5 in/hr; rotary heads 0.3-0.6 in/hr; drip far lower. Match the rate to your soil's infiltration — sandy soil absorbs faster than clay, which is prone to runoff.
Figures are editable example defaults for planning, not guarantees. Water needs vary by crop, soil, weather, season, and local climate — check ET data (NOAA, your state CIMIS/AgriMet network) and verify with your local extension service or agronomist.