How does direct specimen unit weight differ from phase calculation?

Direct specimen calculation measures total weight (or mass converted by standard gravity g = 9.80665 m/s²) divided by total volume (γ = W/V). Dry unit weight is γd = γ / (1 + w). Phase calculation derives these from particle packing (e) and mineral density (Gs). Phase mode is required to calculate saturated and submerged unit weights unless e and Gs are separately known.

Why does degree of saturation sometimes exceed 100%?

The theoretical maximum saturation is 100% (all void space filled with water). If S = (Gs · w) / e > 100%, the input combination is physically inconsistent. This usually indicates lab measurement error in water content or void ratio, specimen swell, organic matter lowering the true Gs, or entrapped air calculation discrepancies. The calculator flags this condition rather than silently altering your inputs.

What unit weight of water is used for SI and US Customary calculations?

In SI units, standard geotechnical practice uses γw = 9.81 kN/m³. In US Customary units, fresh water is taken as γw = 62.4 pcf (lb/ft³). US phase unit weights are computed natively with 62.4 pcf so that the fundamental identity γ' = γsat − γw remains numerically exact.

How is mass in kilograms converted to force in kilonewtons?

Mass is converted to weight force using standard Earth gravity: W (kN) = mass (kg) × 9.80665 m/s² / 1000. Dividing by volume in m³ produces unit weight in kN/m³.

Can saturated and submerged unit weights be determined from direct specimen weight and volume alone?

No. Direct bulk density (W/V) only gives the current state of the specimen. Calculating full saturation (γsat) or buoyant weight (γ') requires knowledge of the solid mineral density (Gs) and void ratio (e) so the total void capacity can be determined.

Soil Unit Weight Calculator

Compute bulk, dry, saturated, and buoyant soil unit weights in kN/m³ or pcf from phase properties (e, Gs, w) or direct specimen weight and volume.

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.

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_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.

Summary

Compute bulk, dry, saturated, and buoyant soil unit weights in kN/m³ or pcf from phase properties (e, Gs, w) or direct specimen weight and volume.

How it works

Choose your calculation method: "Phase Relationships" (using e, Gs, and w) or "Direct Specimen" (using sample weight/mass, volume, and moisture content).
In Phase mode, enter void ratio (e), specific gravity of solids (Gs), and water content (w %). Saturated and submerged weights are calculated alongside total and dry weights.
In Direct Specimen mode, enter sample mass/weight (kN, kg, or lb), total volume (m³, cm³, or ft³), and water content (w %) to compute bulk and dry unit weights.
Toggle between SI (kN/m³) and US Customary (pcf) at any time. Phase conversions use native water densities (γw = 9.81 kN/m³ and 62.4 pcf) so γ' = γsat − γw holds exactly.
Review derived physical indicators, including degree of saturation (S %) and porosity (n %), with physical consistency warnings if voids are overfilled (S > 100%).

Use cases

Verify laboratory bulk and dry unit weights from core, Shelby tube, or compaction mold specimens (ASTM D7263).
Determine total and submerged unit weights for overburden and effective stress profiles.
Calculate lateral earth pressures on retaining structures and basement walls.
Evaluate buoyant uplift pressures on submerged foundations, tanks, and pipelines.
Cross-check borehole logs and geotechnical report laboratory indices for phase consistency.

Frequently Asked Questions

Last updated: 2026-09-20 · Reviewed by Nham Vu

Soil Unit Weight Calculator

Use the Soil Unit Weight Calculator

Summary

How it works

Use cases

Frequently Asked Questions