Reference notes for dry unit weight
1) What is dry unit weight of soil?
Dry unit weight (γd) is the weight of solids per total volume, excluding pore water. It equals ρd·g and is commonly reported in kN/m³ or pcf.
- γd = Ws / V; ρd = Ms / V
- γd = ρd·g; 1 kN/m³ ≈ 6.3659 pcf
2) Converting between γd, ρd, and moisture
For moist soils with water content w (decimal), the dry values follow:
- ρd = ρ / (1 + w)
- γd = γ / (1 + w)
- Given Gs and e: γd = (Gs·γw) / (1 + e)
3) Typical ranges and quick QA checks
- Typical Gs: 2.60–2.75 for most mineral soils.
- Typical e: 0.3–0.7 (sands), 0.5–1.2 (silts/clays) depending on structure.
- γd field: ~14–22 kN/m³ (≈ 89–140 pcf), material dependent.
- Porosity n = e / (1 + e) must be between 0 and 1.
- Saturation Sr ≤ 100%; higher suggests inconsistent inputs.
- wsat ≈ e / Gs; compare with measured water content.
- Relative compaction targets often 90–98% by specification/standard.
Formula used
1) From bulk unit weight and water content:
γ_d = γ / (1 + w)
2) From total density and water content:
ρ_d = ρ / (1 + w)
γ_d = ρ_d · g / 1000 (kN/m³)
3) From specific gravity and void ratio:
γ_d = (G_s · γ_w) / (1 + e)
Extras:
e = (G_s · γ_w / γ_d) − 1
n = e / (1 + e)
S_r = (w · G_s) / e
w_sat = e / G_s
%Compaction = (γ_d(field) / γ_d(max)) × 100
Where: γ is bulk unit weight; w is water content (decimal); ρ is total density; ρ_d is dry density; g is gravitational acceleration; G_s is specific gravity of solids; e is void ratio; γ_w is unit weight of water.
Example data table
| # | Method | Inputs | γd (kN/m³) | γd (pcf) | n | Sr (%) | wsat (%) |
|---|---|---|---|---|---|---|---|
| 1 | γ and w | γ = 19.5 kN/m³, w = 8%, G_s = 2.70 | 18.0556 | 114.9395 | 0.31832 | 46.26 | 17.30 |
| 2 | γ and w | γ = 21.0 kN/m³, w = 15%, G_s = 2.65 | 18.2609 | 116.2465 | 0.29756 | 93.83 | 15.99 |
| 3 | G_s and e | G_s = 2.70, e = 0.65, w = 10% | 16.0527 | 102.1897 | 0.39394 | 41.54 | 24.07 |
How to use this calculator
- Select Unit system and a Method.
- Enter the required inputs shown for the chosen method.
- Optional extras: enter Gs, e, w to compute Sr, n, wsat.
- Optional: supply γd(max) and spec minimum to see status.
- Click Calculate to see results and step-by-step derivation.
- Use Download buttons to export results or examples to CSV or PDF.