Specific Weight of Water — Formula, SI Units & Density Reference
Water specific weight reference covering the γ = ρ · g specific weight formula, the SI unit (N/m³) and imperial unit (lbf/ft³) of specific weight, the specific weight density relationship, and the specific weight calculator workflow used in hydraulic-head, pump-sizing, and pipeline-pressure work. Includes specific weight equation derivations and a reference table of γ values for 25+ engineering fluids and solids. Reviewed by a licensed PE.
Specific weight calculator and conversion workflow
For specific weight calculator workflows, the math is one multiplication: γ = ρ × g. Look up density in the table below, multiply by 9.80665 m/s² (standard gravity), and report in N/m³. For imperial, convert ρ to slug/ft³ then multiply by g = 32.174 ft/s² to get lbf/ft³. We don\'t yet host a dedicated specific weight widget; the related specific gravity calculator is the closest existing tool.
The specific weight formulas
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- γ = specific weight in N/m³ (SI) or lbf/ft³ (imperial)
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- ρ = density in kg/m³ (SI) or slug/ft³ (imperial)
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- g = gravitational acceleration; standard g₀ = 9.80665 m/s² = 32.174 ft/s²
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- SG = specific gravity (dimensionless)
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- γ_water = 9 810 N/m³ at 4 °C = 62.43 lbf/ft³
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- For an SG = 0.85 oil: γ = 0.85 × 9 810 = 8 339 N/m³
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- P = pressure at depth h (Pa = N/m²)
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- h = depth below the free surface (m)
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- For water 10 m deep: P = 9 810 × 10 = 98.1 kPa ≈ 1 atm
Standards and reference data
| Standard / source | Scope |
|---|---|
| BIPM SI 9th Edition (2019) | Defines the kilogram (mass), metre (length), second (time), and the standard acceleration g₀ = 9.80665 m/s² |
| NIST SP 811 | Authoritative SI unit conversion tables — including specific weight conversions between N/m³ and lbf/ft³ |
| ASME / ASTM water property tables | Reference tables of water density, specific weight, viscosity, and surface tension as a function of temperature |
| IAPWS-IF97 | International Association for the Properties of Water and Steam — the reference equations for water properties used in power-plant engineering |
| API MPMS Chapter 11 | Petroleum measurement temperature corrections — applied wherever γ varies significantly with temperature |