How to Calculate Soil Moisture Content
The weight of water in a sample, as a percentage of its dry weight
The standard laboratory method
The most common laboratory method (commonly run per a standard such as ASTM D2216) weighs a moist soil sample, dries it in an oven at a controlled temperature until its weight stops changing, then weighs the now-dry sample. The difference in weight is the mass of water that was in the sample.
The formula — relative to dry weight
Moisture content (%) = (weight of water ÷ weight of dry soil) × 100. This is a key detail: moisture content is expressed as a percentage of the dry soil's weight, not the original wet sample's total weight — a common point of confusion.
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.
Worked Example
A moist soil sample weighs 250 g; after oven-drying it weighs 210 g.
Weight of water: 250 − 210 = 40 g
Moisture content = 40 ÷ 210 × 100 ≈ 19%
Related Calculators
Related Guides
From the Blog
Sources
ASTM D2216
Standard Test Method for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass.