How to Calculate Soil Moisture Content

The standard laboratory method

The reference method is ASTM D2216 (AASHTO T 265): weigh a moist sample, dry it in an oven at 110 ± 5 °C until its mass stops changing, and weigh it again. The loss in mass is the water. A microwave oven (ASTM D4643) is a faster alternative for some soils.

• Weigh the clean, empty container: W1.
• Add the moist soil and weigh it straight away: W2.
• Dry it in the oven to constant mass and weigh it again: W3.
• Mass of water = W2 − W3; mass of dry soil = W3 − W1.

The formula — relative to dry weight

Moisture content (%) = mass of water ÷ mass of dry soil × 100 = (W2 − W3) ÷ (W3 − W1) × 100. FHWA's geotechnical manual defines it as the weight of water divided by the weight of soil solids. It is a percentage of the dry soil, not of the wet sample — a common point of confusion — so it can exceed 100% in very wet fine-grained and organic soils.

The Soil Moisture Content Calculator takes the same three masses.

Dry basis vs wet basis

Some sources and instruments report water as a share of the wet sample instead. To convert a wet-basis figure m (as a decimal) to the geotechnical dry basis, use w = m ÷ (1 − m). In the worked example below, 13 g of water in 65 g of wet soil is 20% on a wet basis but 25% on a dry basis. Check which basis a number uses before comparing it with a Proctor optimum.

Typical values

FHWA's geotechnical manual gives 3% to 70% for most natural soils, with saturated fine-grained and organic soils sometimes over 100%. A result far outside what you expect for the soil is worth re-checking against the recorded masses.

Why it matters for compaction

Moisture content strongly affects how well a soil compacts. Every soil has an 'optimum moisture content' at which it reaches maximum dry density for a given compaction effort (determined by a Proctor compaction test) — too dry or too wet, and the same compaction effort achieves a lower density. Field moisture content is often compared against this optimum during construction quality control.

The standard Proctor test (ASTM D698) compacts soil with a 5.5 lbf rammer dropped 12 in, a compactive effort of 12,400 ft-lbf/ft³; the modified test (ASTM D1557) uses a 10 lbf rammer dropped 18 in, 56,000 ft-lbf/ft³. FHWA notes that specifications often require a minimum relative compaction, giving 95% as an example, and AASHTO's recommendations for some subgrade soils also call for compaction within 2% of the optimum moisture content. The field check is covered in Understanding In-Situ Dry Density.

From moisture content to dry density

In a field density test, the moisture content converts the measured wet (bulk) density to dry density: dry density = wet density ÷ (1 + w), with w as a decimal. At 2,140 kg/m³ and 12% moisture, that is 2,140 ÷ 1.12 = 1,911 kg/m³. The In-Situ Dry Density Calculator takes the moisture content from this test.

Common mistakes

• Dividing by the wet mass instead of the dry mass.
• Forgetting to subtract the container mass from W2 and W3.
• Taking the sample out of the oven before its mass stops changing — the leftover water is counted as soil, so the result reads low.
• Leaving the moist sample uncovered before the first weighing, so some water evaporates.