Agricultural Field Calculators
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Center Pivot Irrigation Calculator — GPM, Runtime & Acre-Inches

Calculate gallons per acre, acre-inches applied, total water volume, and pivot revolution runtime for any center pivot system. Enter flow rate and pivot radius to find your application depth and scheduling parameters.

Calculate gallons per minute (GPM), application depth, runtime, and acre-inches for center pivot irrigation systems. The Application Rate tab converts between depth and runtime for a known field and flow rate. The System Design tab calculates the required system flow for a given field size, target depth, and desired runtime. Used by irrigation designers, agronomists, and irrigated crop farmers.

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Irrigation Water Usage

Calculate total water volume and runtime from depth, or applied depth from a known runtime.

Fill in the fields and press Calculate to see your results.

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Disclaimer — Irrigation calculations are estimates based on standard hydraulic formulas. Actual system performance depends on pump efficiency, pipe losses, field topography, and soil conditions. Consult a licensed irrigation engineer for system design.

Frequently Asked Questions

A center pivot is a large irrigation system that rotates around a fixed center point, watering a circular field. Water is pumped from a well or canal through a lateral pipe supported by wheeled towers, spraying water onto crops as it slowly rotates. A typical pivot irrigates 100–130 acres in a full circle. They're common across the US Great Plains, Pacific Northwest, and many irrigated regions worldwide. The key design variables are flow rate (GPM), application depth (inches), and how long one full revolution takes.

Acre-inches = (GPM × runtime hours × 60) ÷ (field acres × 27,154). MyFarmCalc calculates this automatically from your pivot radius, speed, and nozzle flow rate.
A pivot with a 1,320-foot radius covers approximately 125.7 acres (π × 1,320² ÷ 43,560). Corner areas add about 34 unirrigated acres per quarter section.
A common starting point is 8–10 GPM per acre for corn. A 125-acre pivot applying 1 inch per week needs approximately 1,000–1,250 GPM running continuously.
Acres = π × radius² ÷ 43,560. A 1,320-ft pivot covers ≈125.7 acres. A 2,640-ft (half-mile) pivot covers ≈502.7 acres.
Low-pressure drop nozzles at crop canopy: 90–95%. High-pressure impact heads: 70–80%. LEPA (Low Energy Precision Application) drag socks: up to 98%.
Injection rate (gal/min) = (gal/acre × pivot GPM) ÷ (acres × 60). For 3 gal/acre UAN on a 125-acre pivot at 1,000 GPM: 3 × 1,000 ÷ (125 × 60) = 0.4 gal/min.

Modern center pivot systems achieve 85–92% application efficiency — meaning 85–92% of applied water reaches the root zone. To calculate fertilizer or chemigation application rates through the pivot, see the Fertilizer Rate Calculator.. Older high-pressure impact sprinkler pivots may be 70–80% efficient. Low-energy precision application (LEPA) systems can reach 95%+ efficiency.

A typical quarter-section pivot (128 acres) applying 1 inch per week needs about 750–900 GPM from the well. Use the Field Size Calculator to find your exact irrigated acres, dry corners, and end-gun coverage before sizing your pump. Use the formula: GPM = (acres × inches/week × 18,857) ÷ (hours of operation per week). Most commercial center pivots pump 700–1,200 GPM depending on acreage and desired application rate.