Irrigation Pivot Sizing Calculator

A center pivot rotates around a fixed point in the middle of the field, so its reach in any one direction is only half the field's shorter dimension — the calculator takes your field length as that shorter side and divides it by 2 to get the radius. A standard quarter-section field is 2,640 feet on a side, which is why the default gives a 1,320-foot pivot radius.

Application efficiency represents the fraction of water that actually reaches and stays in the crop root zone — the rest is lost to evaporation, wind drift, or runoff — so the calculator divides your peak crop water need by efficiency to get the gross amount you must actually pump. A lower efficiency (like an impact sprinkler at 75-85%) means you have to apply more gross water to deliver the same net amount to the crop, which raises the required system flow rate (GPM), not lowers it.

Rotation time is the outer tower's full-circle travel distance (2π × radius) divided by its travel speed, and it directly sets how much water gets applied per pass — a slower rotation means more time under each part of the pivot's sprinklers, so lower travel speeds apply a deeper watering per revolution while faster speeds apply a shallower amount more frequently. This is why the tool ties outer tower speed to application depth per revolution rather than treating them as independent settings.

An end gun is a booster nozzle on the outer end of the pivot that throws water beyond the wheel track's circular reach into the field's corners, and the calculator assumes it adds roughly 5% more irrigated area on top of the basic circular coverage. This is only relevant if your system actually has an end gun installed — if it doesn't, the total coverage and percentage-of-quarter-section figures will be modestly overstated for your real setup.