Free Soil Moisture calculator with clear step-by-step results.
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.
Gravimetric moisture
Moisture = (wet mass - dry mass) / dry mass
Stored water
mm = gravimetric moisture x bulk density x root depth mm
Dry mass is the stable reference. Using wet mass would change the denominator with every moisture level and make readings incomparable.
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.