How Long Should I Water My Lawn?

The one measurement worth making

Everything on this page is arithmetic except one number: how fast your sprinkler actually delivers water.

It varies enormously between sprinklers, between water pressures, and even between parts of the same lawn. Nobody knows it and it decides the whole answer.

The can test: put several empty flat-bottomed cans out across the area, run the sprinkler for a measured time, and measure the depth collected in each. That gives you inches per hour.

Doing this once is worth more than any calculator, this one included. It also very often reveals that the far corner gets half what the middle does — which no arithmetic would have told you.

Water is measured in inches, like rain

Because it's the only unit that works regardless of area. An inch means an inch everywhere on the ground, whether the bed is tiny or the lawn is enormous.

It's also the same unit rain is measured in — which is what makes them directly comparable and lets you subtract one from the other.

The conversion: an inch over a square foot is 1/12 of a cubic foot, and a cubic foot is 7.48 gallons. So:

• 1 inch over 1 sq ft = 0.623 gallons
• 1 inch over 1,000 sq ft = 623 gallons

That last number is genuinely surprising the first time you see it.

Measuring the area

Enter the length and width of the lawn or bed. Irregular shapes go in as several rectangles added together — don't use the bounding box, which includes beds, paths and driveway you aren't watering.

The area only affects the gallons, not the run time. That's worth understanding: how long to run the sprinkler doesn't depend on how big the lawn is — it depends on how fast the sprinkler applies water, and an inch is an inch whether it falls on 200 sq ft or 2,000.

The area figure tells you the volume, which is what shows up on a water bill.

1 to 1.5 inches a week — including rain

That's the widely agreed figure for most grass, and the important word is including.

A week with an inch of rain needs no watering at all. That's why the rain input subtracts directly, and it's where most of the saving is.

The figure shifts with grass type, season and climate; in cool weather or a dormant period it drops considerably.

Deep and infrequent beats little and often

The most important thing on this page, and the opposite of what most people do.

One or two long soakings a week drive water deep into the soil, and roots follow it down. The result is a lawn that can find its own moisture in a dry spell.

A little every day keeps only the top inch damp. Roots stay shallow, and the lawn becomes dependent on you and fragile in heat.

The weekly total matters more than the schedule — but splitting it across two sessions is fine and splitting it across seven is not.

Water early in the morning

• Evening leaves foliage wet overnight, which encourages fungal disease
• Midday loses a substantial fraction to evaporation before it reaches the soil
• Early morning avoids both, and tends to be when water pressure is best

If you can only water at one time, this is the choice that costs nothing and helps most.

Measure the shallowest can, not the average

Sprinkler coverage is rarely even, and the dry part of the lawn is the part you actually notice.

Working from the shallowest reading over-waters the middle slightly and keeps the edges alive, which is the right trade.

If the spread between cans is large, the fix is sprinkler placement — not run time. No amount of extra minutes will fix a corner the water never reaches.

Water running onto the path

Then you're applying it faster than the soil can absorb it. Common on clay and on any slope.

The fix is cycle-and-soak: run for a shorter period, stop, wait twenty or thirty minutes for it to soak in, then run again. The total is the same, and it all ends up in the soil rather than in the storm drain.

Sandy soils have the opposite problem — they absorb readily but hold little, so they want smaller and more frequent applications than the default assumption here.

Print the watering plan — it has a table for the can test and a weekly log, so the rainfall subtraction actually happens instead of being intended.

Sources: Spokane — the tuna can test · Milorganite — measuring water in your lawn and garden