How to Test Your Sprinkler Coverage

Two numbers, fifteen minutes

Put five straight-sided tins around the zone, run it for a measured time, and read the depths with a ruler.

That gives you two things nothing else will:

• Your sprinkler's real output rate — the figure to plan with
• How evenly it actually covers the ground — the figure almost nobody measures

This tool measures. The Lawn & Garden Watering Calculator on this site plans — feed it the rate from here and it works out run times and gallons for a lawn of a given size. Two tools, one job each.

Why nobody can look this up for you

A published output rate applies to one head, at one pressure, with one nozzle and one spacing.

Yours depends on your water pressure, the length of pipe feeding it, how many heads share the zone, how they're spaced and how worn they are. Two identical sprinklers on two houses in the same street deliver measurably different amounts.

Fifteen minutes settles it for your garden, and the answer holds until something in the system changes.

The average is the number that lies to you

This is the part worth the whole exercise.

At the defaults the five tins average 0.25 in — a tidy 1.00 in/hr. But the driest caught 0.20 and the wettest 0.30. So:

Water long enough to satisfy the dry patch and everywhere else gets 20% more than it needed. Water to the average and the dry patch stays 20% short, every single time, forever.

A lawn can be watered exactly as long as the arithmetic says and still have a dead corner. The average alone will never tell you.

When a tin catches nothing

Wettest against driest shows a dash, because a ratio against zero has no meaning.

That's not a rounding problem — it's the most useful result this test can give you. Somewhere in that zone is ground receiving no water at all, and no run time fixes it.

Check the tin wasn't sheltered under a shrub or an overhang first. If it was genuinely in the open, you've found a spot the sprinklers don't reach.

Poor uniformity is a sprinkler problem

Not a clock problem. The causes are a short list, and every one is fixed at the head:

• 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 sunk below the grass line
• One simply blocked

Running longer doesn't help. It waters the wet end harder while the dry end stays proportionally just as short, and wastes the difference.

If you genuinely can't fix the coverage, Output rate at the driest spot is the honest planning figure — it waters the whole zone properly at the cost of over-watering the rest. That's a trade you're choosing rather than one happening to you.

Doing the test properly

• 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 out, don't cluster them.
• Put one near the edge of the throw. That's where coverage is worst and where trouble shows first.
• Keep them clear of shrubs and overhangs — a sheltered tin measures the shelter.
• Test in still conditions. A windy test measures the wind as much as the sprinkler.
• Test each zone separately. Head count, spacing and pressure all differ, and a zone at the end of a long run often has noticeably lower pressure.

Retest more often than you'd think

After any change — new nozzles, a replaced head, a pressure adjustment, a repair.

And occasionally anyway. Nozzles wear, heads gradually sink below the grass line, and mains pressure shifts as a neighbourhood grows. A test that was accurate three years ago may not be now, and the drift is slow enough that nobody notices until a patch dies.

Write the rate inside the controller cover. It's the number you'll want next season and won't remember.