Sprinkler Coverage Test Calculator

How this is calculated

THIS TOOL MEASURES; IT DOES NOT SCHEDULE. It turns a catch test into two figures: your sprinkler's real output rate, and how uniform its coverage is. Once you have the rate, the Lawn & Garden Watering Calculator on this site takes it and works out run times and gallons for your lawn - that is the planning half, and this is the measuring half. ⚠️ NO PUBLISHED OUTPUT RATE DESCRIBES YOUR SPRINKLER. A quoted figure applies to one head, at one pressure, with one nozzle and one spacing. Yours depends on your water pressure, the length of the pipe run, how many heads share the zone, how they are spaced and how worn they are - two identical sprinklers on the same street deliver measurably different amounts. Fifteen minutes with some tins settles it, and the answer stays good until something changes. ⚠️ THE UNIFORMITY MATTERS MORE THAN THE AVERAGE, AND ALMOST NOTHING MEASURES IT. If the driest tin catches much less than the average, no run time is correct for the whole zone: set the clock for the dry patch and you drown the wet end, set it for the wet end and the dry patch stays brown. The average alone hides that completely, which is why a lawn can be watered exactly as long as the arithmetic says and still have a dead corner. WHAT TO DO ABOUT POOR UNIFORMITY IS A SPRINKLER JOB, NOT A CLOCK JOB. The usual causes are heads spaced too far apart, mismatched nozzles on one zone, pressure too low or too high for the heads, a head sunk below the grass line, or one simply blocked. All of those are fixed at the sprinkler. Running longer to compensate waters the wet end harder and does not fix the dry one. STRAIGHT-SIDED TINS ONLY. A tapered container concentrates or spreads the water as it fills and reads wrong. Tuna tins, small food tins and purpose-made catch cups all work. Spread them across the zone rather than clustering them, put at least one near the EDGE of the throw where coverage is usually worst, and keep them out from under shrubs and overhangs. TEST EACH ZONE SEPARATELY. Head count, spacing and pressure differ between zones, and a zone at the end of a long run often has noticeably lower pressure than the one nearest the supply. One zone's rate does not transfer to the next. RETEST AFTER ANY CHANGE - new nozzles, a new head, a pressure adjustment, or a repair. And retest occasionally anyway: nozzles wear, heads sink, and pressure changes as a neighbourhood grows. DO THE TEST IN STILL CONDITIONS if you can. Wind moves water off target and a windy test measures the wind as much as the sprinkler.

Common questions

How is this different from the watering calculator?

This one measures and that one plans. This tool turns a catch test into your sprinkler's real output rate and tells you how evenly it covers the ground. The Lawn & Garden Watering Calculator takes a rate and works out run times, gallons and how to split sessions for a lawn of a given size. Use this one first, take the rate it gives you, and put that into the other. Splitting them that way keeps each tool doing one job properly rather than one tool doing both approximately.

Why can't I just look up my sprinkler's output?

Because a published figure applies to one head, at one pressure, with one nozzle and one spacing - and yours depends on your water pressure, the length of pipe feeding it, how many heads share the zone, how they are spaced and how worn they are. Two identical sprinklers on two houses in the same street deliver measurably different amounts. Fifteen minutes with five tins settles it for your garden, and the answer stays good until something in the system changes.

What is a good uniformity figure?

Higher is better and 100% would mean every tin caught the same, which no real sprinkler achieves. Rather than chase a target number, use it comparatively: measure, make a change at the heads, measure again and see whether it improved. What matters practically is the shortfall figure - if the driest spot gets 20% less than the average, then watering long enough to satisfy it means everywhere else receives 20% more, and you can decide whether that is an acceptable trade or a coverage problem worth fixing.

The wettest-against-driest figure shows a dash. Why?

Because one of your tins caught nothing at all, and a ratio against zero has no meaning. That is not a rounding problem - it is a coverage gap, and it is the most useful result this test can give you. Somewhere in that zone is ground receiving no water whatever, which no run time fixes. Check whether the tin was sheltered by a shrub or an overhang first, and if it was genuinely in the open, you have found a spot the sprinklers do not reach.

What causes poor uniformity?

Usually one of a short list, and all of them are fixed at the sprinkler rather than at the controller: heads spaced too far apart for their throw, mismatched nozzles on the same zone, pressure too low or too high for the heads fitted, a head that has sunk below the grass line, or one simply blocked. Running the zone longer to compensate does not help - it waters the wet end harder while the dry end stays proportionally short, and it wastes the difference. Fix the coverage, then set the clock.

Where should I put the tins?

Spread across the zone rather than clustered, and at least one near the EDGE of the throw, which is where coverage is usually worst and where the trouble shows first. Keep them out from under shrubs, overhangs and anything else that shelters them, since a sheltered tin measures the shelter rather than the sprinkler. Straight sides matter more than the exact container: tuna tins, small food tins or purpose-made catch cups all work, while a tapered container concentrates or spreads the water as it fills and gives a confidently wrong reading.

How often should I retest?

After any change - new nozzles, a replaced head, a pressure adjustment or a repair - and occasionally anyway. Nozzles wear, heads gradually sink below the grass line, and mains pressure changes as a neighbourhood grows or as demand shifts through the season. A test that was accurate three years ago may not be now, and the symptom of drift is gradual enough that nobody notices until a patch dies. Testing in still conditions matters too; a windy test measures the wind as much as the sprinkler.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Sprinkler Coverage Test Calculator at https://www.bfcbrilliance.com/tools/sprinkler-coverage-test-calculator.

What I entered:
- How long you ran the zone (min): ___
- Tin 1 depth (in): ___
- Tin 2 depth (in): ___
- Tin 3 depth (in): ___
- Tin 4 depth (put this one at the edge) (in): ___
- Tin 5 depth (in): ___

What it calculated:
- Your sprinkler's real output rate: ___
- Average depth caught: ___
- Driest tin: ___
- Wettest tin: ___
- Uniformity - driest against average: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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