What is plant-available water capacity?

Plant-available water (PAW) capacity is the amount of water a soil can hold between field capacity and wilting point, expressed in inches of water per foot of soil depth. Sandy soils hold about 0.75 in/ft, loams about 1.5 in/ft, and clays about 2.0 in/ft.

How is the water volume calculated?

Volume = Area × Wetting Depth × Soil PAW Factor. The result is the net water needed to refill the root zone. The calculator converts area and depth to consistent units before multiplying.

What flow rate should I enter?

Use the combined output of your irrigation system. For drip systems, add up the flow of all emitters (e.g., 100 emitters × 1 gph = 100 gph = 1.67 gpm). For sprinklers, check the head specifications or use a catch-cup test.

Does this account for evaporation or runoff losses?

No. This calculator gives net water needed to fill the soil pore space. For practical scheduling, increase the calculated volume by 10–20% to account for distribution uniformity losses, evaporation, and runoff.

How deep should I irrigate?

Match the wetting depth to your crop's effective root zone: 12 inches for lawns and shallow vegetables, 18–24 inches for row crops, and 36–48 inches for trees and deep-rooted perennials.

Can I use this for center-pivot or flood irrigation?

Yes. Enter the field area and your system's flow rate. For flood irrigation where flow rate is unknown, use the volume result to estimate how long to open the gate based on your measured canal or pipe flow.

Irrigation Water Calculator

Enter your field area, soil type, and target wetting depth to calculate total water volume needed and system runtime for drip or sprinkler irrigation.

Irrigation Parameters

Field / Garden Area

Soil Texture

Target Wetting Depth

Root zone depth: lawns 12 in, row crops 18–24 in, trees 36–48 in.

System Flow Rate (optional — for runtime estimate)

Fill in the parameters and click Calculate to see results.

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Summary

The irrigation water calculator estimates the volume of water required to bring a field or garden to field capacity based on area, target soil moisture depth, and soil type. Each soil texture has a known plant-available water capacity (in inches of water per foot of soil depth). The tool converts that into gallons or liters and, if you enter your system's flow rate, calculates how long to run your drip or sprinkler irrigation system.

How it works

Enter the irrigated area in acres, square feet, or square meters.
Select the soil texture (sandy, loamy, clay, etc.) — each has a preset water-holding capacity.
Enter the target wetting depth (how deep you want to moisten the root zone).
Optionally enter your system's flow rate (gallons per minute or liters per hour) to get a runtime estimate.
Click Calculate to see the total water volume and estimated system runtime.

Use cases

Plan drip irrigation schedules for vegetable gardens or orchards.
Estimate how long to run a sprinkler system to wet a lawn or crop field.
Compare water requirements across different soil textures before planting.
Size a water tank or pump to supply a single irrigation cycle.
Calculate water costs for irrigating a specific field area.
Optimize root-zone irrigation depth to reduce water waste.
Convert irrigation prescriptions from inches of water to gallons or liters.
Evaluate the effect of soil type on irrigation frequency and volume.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu