Most residential irrigation is sized by eye and scheduled by habit, which is why so many systems simultaneously waste water and underwater plants. The arithmetic is not complicated — area times depth, adjusted for how much of the water actually arrives — but the adjustments are where the real answer lives, and they routinely change the result by a factor of two.
Depth, Not Duration, Is the Target
Plants need a weekly depth of water, measured in inches, the same way rainfall is measured. A cool-season lawn wants roughly an inch a week; drought-tolerant natives want a fraction of that. Run time is not the target — it is what you calculate backwards from depth once you know how fast your heads apply water.
This is why "20 minutes three times a week" is meaningless as advice. Twenty minutes of drip and twenty minutes of large rotor differ by several times in delivered depth. Start from the depth the plants need, divide by the precipitation rate of the heads you actually have, and let that produce the schedule.
Efficiency Is the Multiplier People Skip
Applied water and delivered water are not the same number. Spray heads throw fine droplets that drift in wind and evaporate in heat, and their coverage overlaps unevenly — roughly 60 to 65% of what leaves the head reaches the root zone. Rotors do better at 70 to 75%. Drip, applying water at soil level with no airborne travel, reaches about 90%.
Because the calculator divides by efficiency, this multiplier propagates into everything: weekly gallons, run time, zone count, and cost. Converting a spray zone to drip cuts water use by roughly a third for identical plant health, which is usually the single highest-leverage change available to an existing system.
Soil Decides How, Climate Decides How Much
Soil type does not change the weekly total much — sandy needs about 20% more, clay about 20% less — but it changes the delivery pattern completely. Sandy soil drains fast and stores little, so it wants shorter, more frequent cycles. Clay accepts water slowly, so applying an inch in one 40-minute run produces runoff down the driveway rather than moisture at the roots. That is what cycle-and-soak solves: three short bursts with half-hour rests deliver the same inch with none of the runoff.
Climate is the bigger lever on total volume, ranging from 0.7× in the humid Southeast to 1.5× in the desert Southwest — better than a twofold spread for the same lawn. And it is seasonal: July demand in Denver is roughly twelve times January demand, which is why a fixed year-round schedule is always wrong at least half the year.
Flow Rate Caps Zone Size, and Ignoring It Breaks Coverage
Every zone is limited by the flow your supply can deliver. Load more heads onto a valve than the available GPM supports and pressure drops at every head at once — throw radius shrinks, the spray pattern collapses toward the head, and dry rings appear between heads. The usual symptom is browning in circles that people respond to by watering longer, which wastes more water without fixing the coverage.
Check your actual flow before designing: time how long a 5-gallon bucket takes to fill from an outside spigot, then divide 300 by the seconds to get GPM. Size zones to stay under about 75% of that figure, and run zones in sequence rather than in parallel. The Zone Planner tab does this arithmetic once you enter your measured rate.