How much water does my sprinkler put down?
Every watering rule you'll read — "an inch a week", "half an inch twice a week" — is in inches of water, and your controller only understands minutes. The conversion between them is your sprinklers' precipitation rate, in inches per hour, and it varies by a factor of five between systems: a spray zone might put down 1.5 in/hr, a rotor zone 0.4. Nobody can tell you yours from the make of the heads. You measure it, once, with some cans — the catch-cup test, or the tuna can test, which is the same thing with a tuna can.
Eight cans after 20 minutes reading 0.4, 0.5, 0.45, 0.3, 0.55, 0.5, 0.35 and 0.6 in average 0.46 in, so the rate is 1.37 in/hr and an inch takes 44 minutes. Measuring by volume instead? 100 ml in a 3.25 in tuna can is 0.74 in of depth — the calculator converts.
How to do the tuna can test
- Gather 6–12 straight-sided containers of the same kind — tuna cans, cat-food tins, catch cups if you have them. Same size matters more than what size.
- Scatter them across one zone: near heads, between heads, the corners, the spot that's always dry. Keep them level.
- Run that zone for a set time — 15–20 minutes for sprays, 30 for rotors — long enough to get a measurable depth.
- Measure each can with a ruler (or pour each into a measuring cup for ml) and type the readings above.
- Repeat per zone. Spray and rotor zones will differ enormously; don't schedule one from the other's number.
Typical precipitation rates by sprinkler type
| Head type | Typical rate | Minutes per inch | Notes |
|---|---|---|---|
| Fixed spray heads | 1.3–2 in/hr | 30–46 | Small areas; put water down fast; runoff on slopes and clay — cycle and soak. |
| Rotary (MP-style) nozzles | 0.4–0.6 in/hr | 100–150 | Spray bodies with rotating streams; slow and even. |
| Gear-driven rotors | 0.4–0.9 in/hr | 67–150 | Large lawns; matched nozzles matter for uniformity. |
| Impact sprinklers | 0.3–0.6 in/hr | 100–200 | Old-school; wind-sensitive. |
| Hose-end oscillating / tripod | 0.3–1 in/hr | 60–200 | Wildly variable — the catch-cup test is the only way to know. |
Published ranges (manufacturer catalogs, Irrigation Association). Use them to sanity-check your test, not instead of it.
What distribution uniformity tells you
The cans never read the same. Distribution uniformity (DU) compares the driest quarter of the cans to the average; a DU of 0.7 means the dry spots get 70% of what the lawn gets on average, and the usual fix — run the zone 1 ÷ 0.7 = 1.4× longer so the dry spots get enough — over-waters everything else by 40%. The calculator prints the DU, grades it, and gives that multiplier, but it's honest about the cap: below about 0.55, the dry corner is a head-spacing or pressure problem and more minutes just make a wetter lawn with the same dry corner. Check that heads reach each other ("head-to-head" coverage), that nozzles are matched, and that the last head on the line isn't starved.
A spray zone at 1.5 in/hr and a rotor zone at 0.5 in/hr want run times three times apart for the same inch. On a shared zone, setting the timer for the rotors floods the spray area and setting it for the sprays starves the rotors — and no controller feature fixes it. If your cans show wildly different depths in two halves of a zone, look for that first.
Estimating the rate from head specs
Before a system exists, or to sanity-check a test, the design formula is PR = 96.25 × GPM per head ÷ (head spacing × row spacing) — the 96.25 converts gallons over square feet into inches per hour. It assumes every head delivers its catalog GPM and the spacing is uniform, which is why it tends to run a little optimistic against the cans.
| Layout | Rate | Minutes per inch |
|---|---|---|
| Spray, 1.5 GPM nozzle, 12 × 12 ft | 1.00 in/hr | 60 |
| Spray, 2.6 GPM nozzle, 15 × 15 ft | 1.11 in/hr | 54 |
| Rotary nozzle, 0.9 GPM, 20 × 20 ft | 0.22 in/hr | 277 |
| Rotor, 3 GPM, 30 × 30 ft | 0.32 in/hr | 187 |
| Rotor, 5 GPM, 40 × 40 ft | 0.30 in/hr | 199 |
Square spacing. For triangular layouts multiply the row spacing by 0.866 — the calculator's checkbox does it.
What to do with the number
Save it and the Lawn Care Planner stops saying "1 inch per week" and starts saying "120 minutes, split in two" for your zone. The run-time calculator takes it further — target inches, days per week, cycle-and-soak for slopes and clay where the rate outruns the soil's ability to absorb it. Either way, the can test is the one measurement that turns a generic watering rule into your watering rule.
Sources: Irrigation Association, Landscape Irrigation Auditor (catch-can method, DUlq); University of Florida IFAS, Measuring Sprinkler Application Rate; Colorado State Extension, Operating and Maintaining a Home Irrigation System; the 96.25 constant from standard irrigation design texts.