Soil Moisture

Free Soil Moisture calculator with clear step-by-step results.

About Soil Moisture

Gravimetric moisture is the water lost on oven drying relative to dry soil mass, and multiplying by bulk density converts it to a volumetric basis - which is what turns into millimetres of stored water across a root zone. Subtracting the wilting point gives the fraction plants can actually extract.

Formulas

Gravimetric moisture

Moisture = (wet mass - dry mass) / dry mass

Stored water

mm = gravimetric moisture x bulk density x root depth mm

Worked example

With the default values below, Soil Moisture returns Plant-available water in the root zone of 40.3. Change any input above and every figure here is recalculated in your browser.

Inputs used

Wet sample massminimum 0.1 g: 250 g
Oven-dry sample massminimum 0.1 g: 205 g
Soil bulk densityaccepts 0.5–2.2 g/cm3: 1.35 g/cm3
Root zone depthminimum 10 mm: 300 mm
Gravimetric water at wilting pointaccepts 1–40 %: 12 %

Results

Plant-available water in the root zone: 40.3 mm
Gravimetric moisture content: 21.95% %
Volumetric moisture content: 29.63% %
Total stored water in the root zone: 88.9 mm

Step by step

1Values used
2Gravimetric moisture
3Stored water
4Plant-available water in the root zone= 40.3 mm
5Gravimetric moisture content= 21.95 %
6Volumetric moisture content= 29.63 %
7Total stored water in the root zone= 88.9 mm

Assumptions and limits

The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why divide by dry mass rather than wet?

Dry mass is the stable reference. Using wet mass would change the denominator with every moisture level and make readings incomparable.

What is the wilting point?

The moisture content at which plants can no longer extract water against soil tension, around 15 bar. It ranges from about 5% in sand to over 20% in heavy clay.