How do you calculate irrigation water requirement?

Multiply irrigated acres by the net depth in inches, then divide by application efficiency as a decimal. Here 80 acres x 0.75 in / 0.85 = about 70.6 acre-inches gross.

What is an acre-inch?

An acre-inch is the volume of water needed to cover one acre one inch deep, roughly 27,154 gallons. It is the standard unit for scheduling and allocation.

Why is gross water more than the net depth needs?

Runoff, wind drift, evaporation, uneven distribution, and deep percolation mean not all pumped water reaches the root zone. At 85% efficiency the 60 acre-inch net target requires about 70.6 gross.

What is a good application efficiency?

Drip and subsurface systems can reach 85 to 95%, well-managed center pivots 80 to 88%, and surface or furrow irrigation often 50 to 70%. Lower efficiency raises the gross requirement sharply.

How do I set the target depth?

Use the crop ET deficit since the last irrigation, the soil's available water holding capacity, or your scheduling target so you refill the root zone without exceeding it.

Does this include rainfall?

No. Subtract effective rainfall from the net depth before entering it, or the tool will size a set the crop may not need.

Agriculture, Soil, Fertilizer & Farm Operations calculator

Irrigation Water Requirement Calculator

Irrigation water requirement is the gross volume of water, in acre-inches, you must pump to deliver a target net depth to a field once system losses are accounted for. Irrigators, crop consultants, and water managers use it to schedule sets, size a pumping window, and budget against a water allocation. Because no system delivers 100% of what it pumps, the gross requirement is always larger than the net crop need, and that gap grows fast on low-efficiency systems. Getting it right avoids both under-watering that stresses the crop and over-pumping that wastes water and energy.

What this calculator does

Calculate irrigation water requirement from field acres, target irrigation depth, and application efficiency.
Use it to plan irrigation sets, pump volume, water allocation, or seasonal irrigation scheduling.
It computes the gross acre-inches to pump, plus the net crop water target and the loss allowance, from acres, depth, and efficiency.

Formula used

Water required = irrigated acres x target depth / application efficiency

Inputs explained

Irrigated field area: Use acres receiving water in this set or irrigation event.
Net irrigation depth to apply: Use crop ET deficit, soil water holding capacity, or irrigation schedule target.
Irrigation system application efficiency: Account for runoff, wind drift, evaporation, distribution uniformity, and deep percolation.

How to use the result

Use it when planning an irrigation set or event, or when checking whether a water allocation covers the season's applications.
It assumes uniform efficiency across the field and does not model in-season rainfall, so pair it with an ET-based schedule for timing.

Current U.S. benchmarks

U.S. manufacturing runs at 75.6% of capacity (Federal Reserve, Jun 2026). New factory orders are up 2.3% year over year (Census).
Industrial natural gas averages $4.9 per Mcf (EIA, Apr 2026), down 7.7% from a year earlier, with industrial electricity at 8.66 cents per kWh. Process heating and refrigeration budgets track both.

Common questions

How do you calculate irrigation water requirement? Multiply irrigated acres by the net depth in inches, then divide by application efficiency as a decimal. Here 80 acres x 0.75 in / 0.85 = about 70.6 acre-inches gross.
What is an acre-inch? An acre-inch is the volume of water needed to cover one acre one inch deep, roughly 27,154 gallons. It is the standard unit for scheduling and allocation.
Why is gross water more than the net depth needs? Runoff, wind drift, evaporation, uneven distribution, and deep percolation mean not all pumped water reaches the root zone. At 85% efficiency the 60 acre-inch net target requires about 70.6 gross.
What is a good application efficiency? Drip and subsurface systems can reach 85 to 95%, well-managed center pivots 80 to 88%, and surface or furrow irrigation often 50 to 70%. Lower efficiency raises the gross requirement sharply.
How do I set the target depth? Use the crop ET deficit since the last irrigation, the soil's available water holding capacity, or your scheduling target so you refill the root zone without exceeding it.

Last reviewed 2026-05-12.