Dry Density Calculator
Bulk (wet) density counts both the soil solids and the pore water in a unit volume, so it changes with how wet the sample is.
Enter Values
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How to use this calculator
- Enter the bulk (wet) density of the soil in kg/m³ — the total mass (solids plus pore water) divided by the total volume, e.g. from a sand-replacement or nuclear field density test.
- Enter the gravimetric moisture content w as a percentage (mass of water ÷ mass of dry solids × 100), typically from an oven-dry test.
- Read the dry density in kg/m³ and t/m³ (g/cm³); compare it against the maximum dry density from your Proctor test to assess compaction.
How it works
Bulk (wet) density counts both the soil solids and the pore water in a unit volume, so it changes with how wet the sample is. Dry density counts only the mass of solids in that same volume, giving a moisture-independent measure of how tightly the soil skeleton is packed. Because the gravimetric moisture content w is defined as the mass of water divided by the mass of dry solids, a unit of wet soil has mass proportional to (1 + w) for every unit of dry solid mass — so dividing the bulk density by (1 + w) strips out the water and leaves the dry density.
The working formula is ρd = ρ ÷ (1 + w), where ρ is the bulk (wet) density, ρd is the dry density and w is the moisture content expressed as a decimal (12% → 0.12). The result is reported in kg/m³ and also as t/m³ (numerically equal to g/cm³). Dry density is the value geotechnical engineers compare against the maximum dry density from a standard or modified Proctor compaction test to compute the degree of compaction (relative compaction) of earthworks.
Worked example
Wet density 2,000 kg/m³ at 12% moisture. A field density test on a compacted fill returns a bulk (wet) density of 2,000 kg/m³ and an oven-dry moisture content of 12%. Convert w to a decimal: 12 ÷ 100 = 0.12. Then ρd = 2,000 ÷ (1 + 0.12) = 2,000 ÷ 1.12 = 1,785.71 kg/m³ (1.7857 t/m³). The water accounts for 2,000 − 1,785.71 = 214.29 kg per cubic metre. This dry density is what you compare against the maximum dry density from a Proctor test to report degree of compaction.
Common mistakes
- Entering w as a decimal (0.12) instead of a percentage — this field expects the percentage value 12, and the tool divides by 100 internally.
- Confusing gravimetric moisture content (water ÷ dry solids) with volumetric water content or with degree of saturation — only gravimetric w works in ρd = ρ ÷ (1 + w).
- Using the dry density in place of the bulk density (or vice versa) in the input; the tool needs the total wet mass per unit volume, before the water is removed.
Frequently asked questions
What is the difference between bulk density and dry density?
Bulk (wet) density is the total mass of soil — solids plus pore water — divided by the total volume. Dry density is the mass of the dry solids only divided by that same total volume. Dry density removes the effect of moisture, so it is the value used to judge how well a soil has been compacted.
What is the formula for dry density?
Dry density ρd = bulk (wet) density ρ ÷ (1 + w), where w is the gravimetric moisture content expressed as a decimal. For example, at 12% moisture w = 0.12, so ρd = ρ ÷ 1.12.
Why does dry density matter for compaction?
Earthworks specifications are written in terms of dry density because it isolates the packing of the soil skeleton from the moisture present. You compare the field dry density against the maximum dry density from a Proctor test; the ratio (as a percentage) is the degree of compaction, or relative compaction.
Is this calculator a substitute for a laboratory or engineering assessment?
No. It reproduces the standard ρd = ρ ÷ (1 + w) relationship for guidance only. Field and laboratory density testing, and any compaction acceptance decision, must follow the relevant standard (AS, ASTM, ISRM) and be verified by a competent geotechnical professional.
Related tools
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- Density Mass Volume Calculator
- Swell Factor Calculator
- Bulk Density Calculator
- Compaction Percent Calculator
- Moisture Content Calculator
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