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The weight of the solids in a unit volume of soil, from the laboratory's dry density.
A laboratory reports dry DENSITY, in Mg/m³, g/cm³ or pcf. Multiply a Mg/m³ or g/cm³ figure by 9.81 to get kN/m³ — 1.66 Mg/m³ is 16.28 kN/m³ — while a pcf reading goes in as it stands, because a pound of mass and a pound of force share the number at standard gravity.
The mass of pore water as a percentage of the mass of dry solids.
From an oven-dry test on the same specimen as the density. It is a percentage of the DRY mass, so a peat can exceed 100 per cent without anything being wrong; a sheet that gives water content as a share of the wet mass needs converting first, w = wet share ÷ (1 − wet share).
The density of the soil grains relative to water — about 2.65 to 2.70 for most mineral soils.
Measured to BS 1377-2 or ASTM D854 where the report gives it; otherwise 2.65 for sands and 2.70 for clays is the usual assumption and should be written down as one. Organic soils run much lower, because organic matter is far lighter than mineral grain.
Moist (bulk) unit weight
122.7 pcf
The moist figure is the weight of the ground as dug, water included — the one retained soil carries above the water table. Below the water table a wall or footing check takes the buoyant figure and carries the water separately at full pressure.
- Void ratio (e)
- 0.61
- Porosity (n)
- 38.08 %
- Degree of saturation (S)
- 80.19 %
- Air voids
- 7.54 %
- Dry unit weight
- 103.64 pcf
- Saturated unit weight
- 127.42 pcf
- Buoyant (submerged) unit weight
- 64.97 pcf
They open the calculator with your figures already in it
Soil Phase Relationships Calculator (Unit Weights and Degree of Saturation): 123 pcf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Phase relationships of a soil element of solids, water and air: void ratio e = Gs·γw/γd − 1, porosity n = e/(1 + e), degree of saturation S = w·Gs/e, moist unit weight γ = γd(1 + w), saturated unit weight γsat = (Gs + e)·γw/(1 + e) and buoyant unit weight γ′ = γsat − γw (Craig's Soil Mechanics, chapter 1; the quantities as BS 1377-2 and ASTM D7263 define them)
Inputs used
- Dry Unit Weight (γd)
- 103.64 pcf
- Water Content (w), per cent
- 18.4
- Specific Gravity of Solids (Gs)
- 2.68
Intermediate steps
- Void ratio (e)
- 0.61
- Porosity (n)
- 38.08 %
- Degree of saturation (S)
- 80.19 %
- Air voids
- 7.54 %
- Dry unit weight
- 103.64 pcf
- Saturated unit weight
- 127.42 pcf
- Buoyant (submerged) unit weight
- 64.97 pcf
Confidence note: The moist figure is the weight of the ground as dug, water included — the one retained soil carries above the water table. Below the water table a wall or footing check takes the buoyant figure and carries the water separately at full pressure.
What this calculation does not cover
- The unit weight of water is fixed at 9.81 kN/m³ (62.4 pcf), which is fresh groundwater. A pore fluid that is brine, a tailings liquor or a contaminated leachate is heavier, and the saturated and buoyant figures need working with its own value.
- Phase relations describe one specimen. The dry density, the water content and the specific gravity have to come from the same sample; mixed across samples they can produce a saturation over 100 per cent, which this page reports as impossible rather than rounding it away.
- The saturated and buoyant weights assume the voids are completely full below the water table. A zone of partial saturation just above it, or gas in an organic clay, sits between the moist and saturated figures.
- It converts what the laboratory measured; it does not measure the ground. A density from a disturbed or remoulded sample describes the sample, and fill placed and compacted on site has its own figures, set by the compaction specification rather than by the natural ground beside it.
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This result is a specification — 122.7 pcf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-22 · v1.0.0
Regulatory standards & verification citations1
- Phase relationships of a soil element of solids, water and air: void ratio e = Gs·γw/γd − 1, porosity n = e/(1 + e), degree of saturation S = w·Gs/e, moist unit weight γ = γd(1 + w), saturated unit weight γsat = (Gs + e)·γw/(1 + e) and buoyant unit weight γ′ = γsat − γw (Craig's Soil Mechanics, chapter 1; the quantities as BS 1377-2 and ASTM D7263 define them)
Which documents these citations point at
Standards referenced: ASTM D7263 (ASTM International, United States).
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