Irrigation Calculators

Irrigation math for lawn, garden, and field: how long to run your system, how much water a zone needs, drip emitter flow, sprinkler precipitation rates, and what it all costs — with editable example inputs and honest sourcing.

Irrigation

Irrigation Run Time Calculator

How long to run sprinklers or drip to apply the water depth your lawn or crop needs, from precipitation r...

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Irrigation Water Usage Calculator

Estimate gallons of water your lawn, garden, or field needs per watering and per season from area and tar...

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Drip Irrigation Calculator

Sizing and run time for drip systems: total emitter flow from emitter count and flow rate, plus run time ...

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Sprinkler Precipitation Rate Calculator

Find your sprinkler zone's precipitation rate in inches per hour from flow rate and coverage area, then s...

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Irrigation Cost Calculator

Estimate the cost to water your lawn, garden, or field from gallons used and your water rate, with monthl...

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How to choose

How to choose an irrigation calculator

Start from what you are trying to size. Sprinkler coverage? The sprinkler calculator works from spacing and precipitation rate. Drip lines? The drip irrigation tool converts emitter flow and zone size into run time. Pump and pressure questions? The pump calculator relates head, flow, and power. Each tool's page states its formula first.

The math

How these calculators work

The irrigation tools use standard horticultural relationships — precipitation rate from nozzle flow and spacing, application run time from soil-available water and root depth, pump power from flow and total dynamic head. Every assumption is an editable input, and results compute locally with no account.

Honest limits

What these tools do not do

These tools do not schedule valves for you or model pipe networks hydraulically. They size the core numbers — application rate, run time, pump demand — with visible math.

Quick answers

Common questions

How long should I run my sprinklers?

Run time depends on your system's precipitation rate and your soil's infiltration and storage. The sprinkler run time calculator walks the calculation from measured catch-can output or nozzle specifications.

How many drip emitters do I need per plant?

It depends on plant water demand and emitter flow rate. The drip irrigation calculator computes the count from your emitter spec and the zone's total flow budget.

What size pump do I need?

Pump size follows from the flow you need and the total dynamic head — elevation lift plus friction losses. The pump calculator shows the relationship so you can test different operating points.

Do these tools handle water restrictions or budgets?

Yes, indirectly: the application-rate outputs tell you exactly how much water a schedule delivers, which is the number restriction schedules are written against.

Are results valid for any climate?

The math is climate-independent; the crop water demand you enter is not. Use a local evapotranspiration figure from your extension service for best accuracy.

A worked run time example, step by step

Take a small lawn zone with four rotary nozzles, each rated 1.5 gpm, on 12 ft by 12 ft square spacing. Step 1: total zone flow is 4 x 1.5 = 6 gpm. Step 2: precipitation rate for one nozzle covering a 144 sq ft cell is (96.3 x 1.5) / 144 = 1.00 in/hr. That 96.3 constant converts gpm over square feet into inches per hour; if your spacing is 14 ft, the same nozzle gives (96.3 x 1.5) / 196 = 0.74 in/hr, so wider spacing slows application by about a quarter.

Step 3: pick a water target. For a cool-season lawn in mild weather, 0.75 inch per week is a common starting figure. Run time = target / rate = 0.75 / 1.00 = 45 minutes. If catch cans in that zone instead collect 0.5 inch in 60 minutes (measured rate, which is usually lower than the spec-sheet rate), run time becomes 0.75 / 0.5 = 90 minutes. The gap between the two answers is the efficiency loss to wind, drift, and overlap error, and it is why measured catch-can output beats nozzle math when both are available.

Step 4: check infiltration. Clay soils commonly absorb around 0.2 in/hr, so a 45-minute run at 1.0 in/hr delivers more than the soil can take in at once. Splitting it into two 23-minute cycles with an hour between lets the water soak down instead of running off the surface.

Boundary cases and where the math breaks

The run time formula divides by precipitation rate, so a rate near zero blows up: a zone of only one or two mist nozzles on 20 ft spacing can compute a rate under 0.2 in/hr, which produces run times of many hours. At the limit, rate = 0 is a genuine no-output case and the result is meaningless, not 'run forever'. On the other side, rates above roughly 2 in/hr almost always mean the soil is the constraint, not the math: infiltration caps useful application and cycling is mandatory.

Negative or nonsense inputs deserve their own check. A negative emitter flow or negative spacing is physically impossible; the honest handling is to reject it, not silently compute. A target of 0 inches returns 0 minutes, which is technically right and useless. Entering evapotranspiration in mm while spacing is in feet silently inflates demand by about 25x, so unit consistency is the most common real-world failure.

The pump calculation has its own edge. Total dynamic head is a sum of positive terms (lift, friction, pressure), but if a user enters static lift as 0 and forgets pressure head entirely, the result understates power badly: at 10 gpm, 127 ft of TDH needs about 10 x 127 / 3960 = 0.32 water horsepower, or roughly 0.46 brake horsepower at 70 percent pump efficiency. Drop the 40 psi sprinkler pressure term (92 ft of head) and the same calculation suggests a pump under a tenth of a horsepower, which will not prime the system.