Sprinkler Precipitation Rate Calculator (Inches per Hour)

This calculator finds the precipitation rate of a sprinkler zone: how many inches of water per hour it puts down. Enter the total flow of the zone in gallons per minute, the area it waters in square feet and a depth to apply; you get inches and millimetres per hour and the minutes to apply that depth.

Formula

Precipitation rate (in/h) = 96.25 × total flow (gpm) ÷ area (ft²). The constant 96.25 converts gallons per minute spread over square feet into inches per hour: 1 gpm = 60 gal/h = 8.02 ft³/h, and 8.02 ft³ over 1 ft² is 96.25 in deep. Millimetres per hour = in/h × 25.4. Time = depth ÷ rate × 60.

• gpm: sum of the nozzle flows in the zone (from the nozzle chart), or a metered flow
• a: area covered by the zone in square feet
• d: inches of water you want to apply

Worked Example

A zone of nozzles totalling 10 gpm over 900 ft², applying 0.5 in.

1. Rate = 96.25 × 10 ÷ 900 = 1.07 in/h
2. In mm: 1.0694 × 25.4 = 27.16 mm/h
3. Time for 0.5 in = 0.5 ÷ 1.0694 × 60 = 28.05 minutes, shown as 28:03

How to use the result

Compare zones: spray heads often apply water much faster than rotors, so zones of different head types need different run times. Use the rate in the sprinkler run time calculator to set the timer.

Limits and common mistakes

• This is the average (total area) rate; uneven coverage means some spots get more and some less. A catch-cup test shows the real pattern.
• Nozzle chart flows assume a set water pressure; at other pressures the flow changes.
• Count only the area the zone actually wets.

Frequently Asked Questions

What is a typical precipitation rate?

Spray heads are often around 1-2 in/h and rotors around 0.4-1 in/h, but check your own nozzles.

Where does 96.25 come from?

It is 60 minutes × 231 in³ per gallon ÷ 144 in² per ft², the factor from gpm per ft² to in/h.

How do I find the zone flow?

Add the gpm of every nozzle in the zone from the maker’s chart, or read a water meter while only that zone runs.

Can I use metric?

The mm/h result is the metric rate; enter flow in US gallons per minute and area in square feet.

in gpm | Total flow of the zone | gal/min | 10 | 0 | 1000
in a | Area the zone waters | ft² | 900 | 1
in d | Depth to apply | in | 0.5 | 0 | 5
out pr | Precipitation rate | in/h | 96.25 * gpm / a | 2
out mm | Precipitation rate | mm/h | pr * 25.4 | 2
out mins | Time to apply that depth | h:mm:ss | d / pr * 60 | time