# Soil Water Deficit Calculator

> Find how many millimetres of water the root zone needs to return to field capacity from a soil moisture reading, and how depleted it is.

Category: Agriculture · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/soil-water-deficit-calculator

The soil water deficit is the gap between field capacity and current moisture, multiplied by root depth. It is the net irrigation needed to refill the root zone.

Also called: Soil moisture deficit calculator, Root zone depletion calculator.

## Inputs
- Moisture readings are (options: Volumetric %, Gravimetric %; default volumetric)
- Soil bulk density (unit g/cm³; default 1.4)
- Field capacity (unit %; default 32)
- Current moisture (unit %; default 24)
- Wilting point (unit %; default 14)
- Root depth (unit m; default 0.6)
- Area (default 1)
- Area in (options: Hectares, Acres; default ha)

## Results
- Water deficit
- Share of available water used
- Available water left
- Total available water
- Water to refill per hectare
- Water to refill the area

## Formula

```
Deficit (mm) = 1000 (θFC − θ) Zr;  TAW = 1000 (θFC − θWP) Zr;  Depletion = Deficit ÷ TAW;  θ (vol) = θ (grav) × Bulk density
```

Where:
- θ: volumetric water content (m³/m³)
- Zr: root depth (m)
- Bulk density: g/cm³, to convert gravimetric readings

## Worked example
1. Deficit = 1000 × (0.32 − 0.24) × 0.6 = 48 mm.
2. TAW = 1000 × (0.32 − 0.14) × 0.6 = 108 mm, so 48 ÷ 108 = 44.44% is used.
3. Refill volume = 48 mm × 10 = 480 m³ per hectare.

**Result:** The root zone is 48 mm short of field capacity (480 m³/ha), with 44% of its available water used.

## What the result means

Compare the depletion with your crop’s allowable depletion (often 30–60%). When it is reached, apply the deficit divided by your application efficiency.

## Assumptions and limits
- The moisture reading represents the whole root zone.
- Water density is 1 g/cm³ when converting gravimetric readings.
- Single-layer estimate; layered soils need a layer-by-layer sum.

## Questions

### Volumetric or gravimetric?

Most soil moisture probes read volumetric %. Oven-dry samples give gravimetric %; multiply by bulk density to convert.

### Should I apply the whole deficit?

Apply the deficit plus losses. Applying more than the deficit drains below the roots and can leach nutrients.

## Sources
- [FAO Irrigation and Drainage Paper 56, Chapter 8: soil water availability](https://www.fao.org/3/x0490e/x0490e0e.htm)

## Related calculators
- [Irrigation Scheduling Calculator](https://calculators.geekmidst.com/calculators/irrigation-scheduling-calculator)
- [Drip Irrigation Discharge Calculator](https://calculators.geekmidst.com/calculators/drip-irrigation-discharge-calculator)

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/soil-water-deficit-calculator