What is the unit weight of soil?

Unit weight is weight per unit volume, in kN/m³ or lb/ft³ (pcf). Most natural soils have a moist unit weight of about 16–21 kN/m³ (100–135 pcf). Loose sand is about 14.5 kN/m³ dry, dense sand about 18, glacial till about 21, and soft organic clay only 6–8 kN/m³ dry.

What is the formula for soil unit weight?

γ = (Gs + S·e)·γw ÷ (1 + e), where Gs is the specific gravity of solids, e the void ratio, S the degree of saturation (0 to 1) and γw the unit weight of water. Using S = Gs·w/e, it can also be written γ = Gs·γw·(1 + w) ÷ (1 + e).

What is the difference between dry, saturated and submerged unit weight?

Dry unit weight counts only the solids (S = 0): γd = Gs·γw/(1 + e). Saturated unit weight has every void full of water (S = 1): γsat = (Gs + e)·γw/(1 + e). Submerged (buoyant) unit weight is γ' = γsat − γw and is used for effective stress below the water table.

How do I calculate dry unit weight from a lab sample?

Divide the specimen weight by its volume to get the moist unit weight, then divide by (1 + w). A 400 kg specimen in 0.2 m³ with 15% water weighs 400 × 9.80665 ÷ 1000 ÷ 0.2 = 19.613 kN/m³ moist and 19.613 ÷ 1.15 = 17.06 kN/m³ dry.

Can I get saturated unit weight from weight and volume alone?

Not without Gs. Weight and volume give only the current state. If you also know Gs, the void ratio follows from e = Gs·γw/γd − 1 and the saturated and submerged unit weights can then be calculated, which this tool does when you enter Gs.

Why does the degree of saturation sometimes exceed 100%?

S = Gs·w/e cannot physically exceed 100%. A higher value means the water content, void ratio or Gs is inconsistent, often because of measurement error, swelling, or organic matter lowering the true Gs. The calculator flags it instead of changing your inputs.

What unit weight of water is used?

γw = 9.81 kN/m³ in SI and 62.4 pcf in US units. Each system uses its own γw so that γ' = γsat − γw holds exactly; the two systems therefore differ by about 0.1%.

Soil Unit Weight Calculator

γ = (Gs + S·e)·γw ÷ (1 + e). With S = 0 this is the dry unit weight, with S = 1 the saturated unit weight, and γ' = γsat − γw below the water table. Typical soils weigh 14–21 kN/m³ dry (90–135 pcf) and 17–23 kN/m³ saturated.

Calculation Method

Void Ratio (e)

Ratio of void volume to solid volume (V_v / V_s).

Specific Gravity of Solids (G_s)

Ratio of solid mineral density to water. Typically 2.60–2.80.

Moisture Given As

Water Content (w, %)

Moisture content by dry mass (w = M_w / M_s × 100).

Standard Phase Presets (S ≤ 100%)

Calculated Results

Output System:

Total / Bulk Weight

γ (moist)

—

kN/m³

Total weight per unit volume including natural pore water.

Dry Unit Weight

γ_d

—

kN/m³

Weight of mineral solids only (γ_d = γ / [1 + w]).

Saturated Unit Weight

γ_sat

—

kN/m³

Unit weight with 100% of pore voids filled with water.

Submerged Unit Weight

γ' (buoyant)

—

kN/m³

Effective unit weight below water table (γ_sat − γ_w).

Governing Equations & Physical Bounds

γ = (G_s + S · e) · γ_w / (1 + e) (general)

γ_d = G_s · γ_w / (1 + e)

γ = γ_d · (1 + w) [or γ = W / V in direct lab tests]

γ_sat = (G_s + e) · γ_w / (1 + e)

γ' = γ_sat − γ_w (buoyant effective weight)

n = e / (1 + e) | S = (G_s · w) / e

Standard constants: γ_w = 9.81 kN/m³ (SI), 62.4 pcf (US Customary). Standard Earth gravity for mass-to-weight: g = 9.80665 m/s². Calculations assume mineral soil grains are incompressible under atmospheric test conditions.

Typical unit weights of soil

Natural-state void ratio, saturated water content and dry unit weight after Das (Principles of Geotechnical Engineering). Saturated and submerged values are derived with γ_w = 9.81 kN/m³; site-specific tests always govern.

Soil	e	w_sat %	γ_d kN/m³	γ_d pcf	γ_sat kN/m³	γ′ kN/m³
Loose uniform sand	0.80	30	14.5	92	18.9	9.1
Dense uniform sand	0.45	16	18.0	115	21.0	11.2
Loose angular-grained silty sand	0.65	25	16.0	102	19.9	10.1
Dense angular-grained silty sand	0.40	15	19.0	121	21.8	12.0
Stiff clay	0.60	21	17.0	108	20.7	10.9
Soft clay	0.90–1.40	30–50	11.5–14.5	73–92	17.2–19.1	7.4–9.3
Loess	0.90	25	13.5	86	18.1	8.3
Soft organic clay	2.50–3.20	90–120	6.0–8.0	38–51	13.5–15.0	3.7–5.2
Glacial till	0.30	10	21.0	134	23.3	13.5

Typical G_s of soil minerals: quartz 2.65, kaolinite 2.6, illite 2.8, montmorillonite 2.65–2.80, calcite 2.72, dolomite 2.85. Organic matter lowers G_s.

Worked examples

Phase relations: medium sand with e = 0.65, G_s = 2.65 and w = 24% has S = 2.65 × 0.24 ÷ 0.65 = 97.8%. Then γ_d = 2.65 × 9.81 ÷ 1.65 = 15.76 kN/m³, γ = 15.76 × 1.24 = 19.54 kN/m³, γ_sat = (2.65 + 0.65) × 9.81 ÷ 1.65 = 19.62 kN/m³ and γ′ = 19.62 − 9.81 = 9.81 kN/m³.

Saturation given: the same sand at S = 50% has γ = (2.65 + 0.5 × 0.65) × 9.81 ÷ 1.65 = 17.69 kN/m³ and w = 0.5 × 0.65 ÷ 2.65 = 12.3%.

Lab specimen: 400 kg in 0.2 m³ with w = 15% gives γ = 19.61 kN/m³ and γ_d = 17.06 kN/m³. The dry density is 400 ÷ 1.15 ÷ 0.2 = 1,739 kg/m³, so with G_s = 2.65, e = 2.65 × 1,000 ÷ 1,739 − 1 = 0.524, S = 2.65 × 0.15 ÷ 0.524 = 75.9%, γ_sat = 20.43 kN/m³ and γ′ = 10.62 kN/m³.

Summary

This geotechnical calculator evaluates soil unit weights two ways. Phase relationships use void ratio (e), specific gravity of solids (Gs) and either water content (w) or degree of saturation (S) in the general equation γ = (Gs + S·e)γw/(1 + e). Direct specimen mode divides the measured weight by the volume (ASTM D7263) and, if Gs is known, back-calculates e, n and S so the saturated and submerged unit weights can also be found. Results are given in kN/m³ or pcf, with bulk and dry density, and a table lists typical values for common soils.

How it works

Choose Phase Relations (e, Gs and w or S) or Direct Specimen (weight or mass, volume and w, plus Gs if known).
Phase mode: γd = Gs·γw/(1 + e), γ = (Gs + S·e)·γw/(1 + e) with S = Gs·w/e when you enter water content, γsat = (Gs + e)·γw/(1 + e) and γ' = γsat − γw.
Direct mode: γ = W/V and γd = γ/(1 + w). With Gs, the tool also finds e = Gs·γw/γd − 1, n = e/(1 + e), S = Gs·w/e and the saturated and submerged unit weights.
Switch between kN/m³ (γw = 9.81) and pcf (γw = 62.4). Bulk and dry density are shown in kg/m³ and g/cm³.
A warning appears when S works out above 100%, which means the inputs are physically inconsistent.

Use cases

Checking bulk and dry unit weight of core, Shelby-tube or compaction-mold specimens (ASTM D7263).
Getting total and submerged unit weights for vertical and effective stress profiles.
Estimating unit weight from a soil description when only typical values are available.
Lateral earth pressure and retaining-wall design inputs.
Buoyancy and uplift checks for submerged foundations, tanks and pipelines.
Cross-checking borehole log indices (e, w, Gs, S) for phase consistency.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu

Soil Unit Weight Calculator

Use the Soil Unit Weight Calculator

Typical unit weights of soil

Worked examples

Summary

How it works

Use cases

Frequently Asked Questions