Calculate gravimetric and volumetric soil moisture content from wet and oven-dry soil sample weights. Essential for irrigation management.
The Soil Moisture Content Calculator determines both gravimetric and volumetric water content from wet and oven-dry soil sample weights. Gravimetric moisture content (θg) expresses water as a percentage of dry soil mass, while volumetric moisture content (θv) expresses water as a percentage of total soil volume.
Gravimetric moisture is the standard laboratory measurement — simply weigh the soil before and after drying at 105°C. Volumetric moisture is more useful for irrigation scheduling because it directly relates to the volume of water in the root zone. Converting between the two requires knowing bulk density.
Accurate moisture monitoring is the foundation of precision irrigation management. Knowing the current soil moisture status relative to field capacity and wilting point allows you to schedule irrigations optimally — avoiding both water stress and over-watering. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process.
Precise soil moisture measurement prevents over-irrigation (wasting water and leaching nutrients) and under-irrigation (stressing crops and reducing yields). This calculator provides the standard method for calibrating field moisture sensors and verifying irrigation schedules. Having a precise figure at your fingertips empowers better planning and more confident decisions. Manual calculations are error-prone and time-consuming; this tool delivers verified results in seconds so you can focus on strategy.
Gravimetric moisture (θg) = (Wet mass − Dry mass) / Dry mass × 100 Volumetric moisture (θv) = θg × Bulk density / ρw Where ρw (water density) = 1.0 g/cm³ So: θv = θg (decimal) × BD (g/cm³) × 100
Result: θg = 25.0%, θv = 32.5%
Water mass = 150 − 120 = 30 g. Gravimetric: 30/120 × 100 = 25%. The soil contains 25 g of water per 100 g of dry soil. Volumetric: 0.25 × 1.3 = 0.325 = 32.5%. The soil holds 32.5% water by volume.
Saturation: all pores filled with water (θv = porosity, typically 40–55%). Field capacity: water held after gravity drainage (24–48 hours after saturation). Wilting point: water remaining when plants can no longer extract moisture. Available water capacity = field capacity minus wilting point.
Field sensors (TDR probes, capacitance sensors, tensiometers, gypsum blocks) provide continuous readings without destructive sampling. However, all sensors must be calibrated against the gravimetric standard for your specific soil. Site-specific calibration curves significantly improve sensor accuracy.
Soil moisture affects nutrient availability through mass flow and diffusion. As soil dries, nutrient movement to roots decreases, even when total nutrient levels are adequate. Maintaining optimal moisture ensures nutrients reach the root surface for uptake.
At 105°C, all free and adsorbed water evaporates, but organic matter does not burn. Higher temperatures decompose OM, which would make the sample lighter than it should be, overestimating moisture content.
Gravimetric is mass of water per mass of dry soil. Volumetric is volume of water per volume of soil. Plants and irrigation systems respond to volumetric water. Converting requires bulk density: θv = θg × BD.
Use a known-volume core sampler (see the Soil Bulk Density Calculator). Oven-dry the sample and divide mass by volume. Typical BD: sandy soils 1.5–1.7, loams 1.2–1.5, clay soils 1.0–1.3 g/cm³.
Most crops grow best when soil moisture is between 50–80% of plant-available water (between field capacity and wilting point). Irrigation is typically triggered at 40–50% depletion of available water.
Microwave drying is faster but less standardized. Research shows that microwave drying at medium power for 10–20 minutes gives results within 1–2% of oven drying. Weigh at intervals to avoid overheating.
For irrigation scheduling, measure at least weekly during the growing season, or use continuous sensors. Before and after irrigation events, spot-check with the gravimetric method to calibrate sensors.