Center Pivot Irrigation Calculator — Depth per Revolution & System Capacity
Calculate how much water a center pivot applies per revolution (mm or inches), its irrigated area, system capacity per day, flow per hectare and end-tower speed from radius, flow and revolution time.
How it works
Area = π × r² · Metric: depth (mm) = flow (L/s) × 3,600 × hours ÷ area (m²) · US: depth (in) = flow (gpm) × 60 × hours ÷ (27,154 × acres) · Net depth = gross × efficiency · End speed = 2πr ÷ hours
A center pivot spreads the pump’s flow over a circle, so the depth applied depends only on flow, revolution time and area: running faster puts on less water per pass. One acre-inch is 27,154 US gallons and one millimetre on one square metre is one litre, which is all the conversion needs. System capacity — the depth the pivot can apply in a full day — should match peak crop water use (often 6–9 mm or 0.25–0.35 in per day for maize); if it is lower, the soil profile has to supply the difference during hot spells.
Worked example
A 400 m pivot covers π × 400² = 502,655 m², or 50.27 ha. Pumping 50 L/s for a 36-hour revolution applies 50 × 3,600 × 36 = 6,480,000 L, or 12.9 mm gross; at 85% efficiency 11.0 mm reaches the soil. Running non-stop it can apply 8.6 mm/day (0.99 L/s/ha), and the end tower moves at 69.8 m/h.
Sources: Washington State University Extension — Useful conversions and formulas for irrigation (D = Q × T ÷ A) · University of Tennessee Extension — Understanding center pivot application rate
Frequently asked questions
How do I change the depth a pivot applies?
With the percentage timer: a lower setting slows the end tower and puts on more water per revolution, a higher setting speeds it up and applies less. The total water per day stays the same — only the depth per pass changes.
How much flow does a center pivot need?
A common design rule is 6–8 mm/day (roughly 0.7–0.9 L/s per hectare, or 5–7 gpm per acre) for maize and other high-water crops in hot climates. Lower capacities are workable on deep, high-water-holding soils.
Why does the outer end of a pivot apply water faster?
The outer spans irrigate far more area per metre of pipe, so their sprinklers deliver more water in a narrow strip. If that instantaneous rate exceeds the soil’s infiltration rate, water runs off — a frequent problem on clay soils and slopes.
How many hectares does a center pivot irrigate?
The area is π × radius². A quarter-section pivot of about 400 m (1,300 ft) covers roughly 50 ha (125 acres); adding an end gun increases the effective radius.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.