Reference · Concepts · Fluid Mechanics · ρ · γ · g

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.

Figure 1 — Specific weight γ = ρ·g, hydrostatic pressure column, and density-comparison table

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

Eq. 01 — Specific weight (the standard formula) SI

\gamma = \rho \cdot g

·: γ = specific weight in N/m³ (SI) or lbf/ft³ (imperial)
·: ρ = density in kg/m³ (SI) or slug/ft³ (imperial)
·: g = gravitational acceleration; standard g₀ = 9.80665 m/s² = 32.174 ft/s²

Eq. 02 — Specific weight from specific gravity SI

\gamma = SG \cdot \gamma_{water}

·: SG = specific gravity (dimensionless)
·: γ_water = 9 810 N/m³ at 4 °C = 62.43 lbf/ft³
·: For an SG = 0.85 oil: γ = 0.85 × 9 810 = 8 339 N/m³

Eq. 03 — Hydrostatic pressure from specific weight SI

P = \gamma \cdot h

·: P = pressure at depth h (Pa = N/m²)
·: h = depth below the free surface (m)
·: 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

Reference: specific weight of common substances

Substance (20 °C)	Density (kg/m³)	γ (N/m³)	γ (lbf/ft³)
Air (1 atm)	1.204	11.81	0.0752
Hydrogen (gas)	0.0899	0.882	0.00561
Cork	240	2 354	14.99
Pine wood	510	5 003	31.85
Gasoline	740	7 259	46.20
Ethanol	789	7 740	49.27
Diesel fuel	832	8 162	51.95
Olive oil	920	9 025	57.45
Water (4 °C, reference)	1 000	9 810	62.43
Water (20 °C)	998.2	9 793	62.32
Sea water	1 025	10 055	64.00
Battery acid (35 % H₂SO₄)	1 265	12 410	78.99
Glycerine	1 261	12 370	78.74
Concrete	2 400	23 544	149.83
Aluminium	2 700	26 487	168.55
Iron / steel	7 850	76 999	490.06
Copper	8 960	87 890	559.40
Lead	11 340	111 245	707.99
Mercury	13 546	132 880	845.65
Gold	19 320	189 529	1 206.