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Specific Gravity to Density: Complete Engineering Reference for SG to kg/m³ and lb/ft³

Published June 27, 2026 · by Industrial Unit Converter Editorial Team

The fundamental relationship in two sentences

Specific gravity (SG) is the dimensionless ratio of a substance's density to the density of a reference: water at 4°C for liquids and solids, air at standard conditions for gases. Density is mass per unit volume. The conversion is a single multiplication: density = SG × density of the reference substance.

So water at 4°C has SG = 1.000 and density = 1,000 kg/m³ = 62.43 lb/ft³. A liquid with SG = 1.5 has a density of 1,500 kg/m³ = 93.65 lb/ft³. The math is trivial. The nuance is in which reference density you are using and at what temperature.

Specific gravity to density: the conversion formulas

In SI units (water reference at 4°C): Density (kg/m³) = SG × 1,000 Density (g/cm³) = SG × 1.0 (numerically equal: 1 g/cm³ = 1,000 kg/m³)

In US customary (water reference at 4°C): Density (lb/ft³) = SG × 62.428 Density (lb/gal) = SG × 8.345

In both systems (any liquid relative to water at 4°C): The SG number is the same in both unit systems because it's dimensionless. A liquid with SG = 0.85 is 85% of water's density regardless of what units you use for density. This is the entire reason SG exists. It is unit-independent.

Reverse conversion: density to specific gravity: SG = density (kg/m³) ÷ 1,000 SG = density (lb/ft³) ÷ 62.428 SG = density (g/cm³) ÷ 1.0 (numerically equal)

Temperature matters: the reference density changes

Water's density changes significantly with temperature. SG measurements are conventionally referenced to water at 4°C (maximum density), but your process fluid might be at 80°C, where water density is 971.8 kg/m³.

Temperature (°C) Water Density (kg/m³) Water Density (lb/ft³) SG Reference Note
0 999.8 62.42 Ice point
4 1,000.0 62.43 Standard SG reference (maximum density)
20 998.2 62.31 Common lab reference (SG 20/20°C)
50 988.0 61.68 Hot water systems
80 971.8 60.67 Process heating
100 958.4 59.83 Boiling point at 1 atm

SG specifications often include a temperature notation: SG 20/20°C means the substance and the reference water are both at 20°C. SG 20/4°C means the substance at 20°C, water at 4°C. This distinction matters: at 20°C, water density is 998.2 kg/m³ (not 1,000), so an SG(20/20) of 0.9982 means the substance has the same density as water at 20°C. The same substance measured as SG(20/4) would read 0.9982 × 998.2/1000 = 0.9964, a 0.18% difference that matters in precision chemical metering.

Where specific gravity is used in engineering

SG appears in nearly every fluid-handling calculation because it converts dimensionless ratios into actionable engineering numbers:

1. Pump power calculations

Pump brake horsepower (BHP) for centrifugal pumps:

BHP = (Q × H × SG) / (3,960 × η)

Where Q is in GPM, H is in feet of head, SG is dimensionless, and η is pump efficiency (decimal). This single equation explains why SG matters: a pump moving a fluid with SG = 1.2 requires 20% more power than when moving water (SG = 1.0) at the same flow and head. If the SG is wrong, the motor is either oversized (cost) or undersized (failure).

Worked example: A pump delivering 500 GPM at 150 ft head with 75% efficiency, pumping a process liquid with SG = 1.15:

BHP = (500 × 150 × 1.15) / (3,960 × 0.75) = 86,250 / 2,970 = 29.0 BHP → select a 30 HP motor.

If you'd assumed SG = 1.0 (water), BHP = 25.2 HP → the 25 HP motor would trip on overload.

2. NPSH (Net Positive Suction Head)

NPSH available in feet of liquid = (P_atm − P_vapor) / (SG × 0.4335). A fluid with lower SG (lighter than water) gives more NPSH available for the same absolute pressure: the height of a liquid column that exerts 1 psi is inversely proportional to SG. For water (SG = 1.0), 1 psi = 2.31 ft. For gasoline (SG = 0.72), 1 psi = 2.31 / 0.72 = 3.21 ft, the pump has more NPSH margin for the same suction pressure.

3. Pipe pressure drop

The Darcy-Weisbach friction loss equation uses fluid density (not SG directly), but the conversion from SG to density is the entry point for all US customary hydraulic calculations where pipe tables are in feet of water or psi. A fluid with SG = 1.5 produces 50% more pressure drop per foot of pipe than water at the same velocity, a factor that must be included in pump head calculations.

4. Storage tank sizing and level measurement

A horizontal cylindrical tank storing a fluid with SG = 0.85 (gasoline) weighs 85% of the equivalent water-filled tank at the same level. The tank structural design (which depends on weight) is directly affected by SG. For level measurement via hydrostatic pressure: P (at level h) = SG × 0.4335 × h, so a 10 ft level of gasoline produces 0.85 × 0.4335 × 10 = 3.69 psi, vs 4.34 psi for water. SG must be known precisely for accurate tank inventory management.

5. Centrifugal separator and cyclone design

Cyclone separators separate particles from gas streams by centrifugal force. The particle's effective density (accounting for the gas density) determines the separation efficiency. For a cyclone separating water droplets from air: ρ_water - ρ_air = 1,000 - 1.225 = 998.8 kg/m³. For a cyclone separating sulfuric acid mist from air: ρ_acid - ρ_air = 1,840 - 1.225 = 1,838.8 kg/m³, about 1.84× more efficient separation because the density differential is larger. The SG of the particle material (relative to the carrier gas) is the key design parameter.

Common material specific gravities

Material SG (relative to water at 4°C) Density (kg/m³) Density (lb/ft³)
Water (4°C) 1.000 1,000 62.43
Seawater (3.5% salinity) 1.025 1,025 63.99
Gasoline 0.72-0.78 720-780 44.9-48.7
Diesel fuel 0.83-0.87 830-870 51.8-54.3
Kerosene / Jet A 0.80-0.82 800-820 49.9-51.2
Crude oil (light) 0.80-0.88 800-880 49.9-54.9
Crude oil (heavy) 0.92-0.98 920-980 57.4-61.2
Sulfuric acid (98%) 1.84 1,840 114.8
Mercury 13.56 13,560 846
Glycerin 1.26 1,260 78.6
Motor oil (SAE 30) 0.875 875 54.6
Concrete (normal) 2.40 2,400 150
Aluminum 2.70 2,700 168.5
Carbon steel 7.85 7,850 490
Gold 19.32 19,320 1,206

API gravity: the oil industry's inverted SG

The petroleum industry uses API gravity (°API) instead of SG because it spreads the density range of crude oils into a more readable scale:

°API = (141.5 / SG) − 131.5

The formula is inverted. Lighter oils have higher API gravity. Water (SG 1.0) = 10 °API. Light crude (SG 0.83) = 39 °API. Heavy crude (SG 0.95) = 17.5 °API. Condensate (SG 0.70) = 70.6 °API.

The API gravity scale was developed by the American Petroleum Institute in 1921 to provide a uniform density scale for the oil industry. The scale was deliberately inverted from SG so that lighter crudes (which are more valuable per barrel because they yield more gasoline and diesel per barrel refined) would have higher API numbers, a quality indicator as well as a density measurement.

Specific gravity of gases

For gases, the reference is air at standard conditions (SG_air = 1.0 at 0°C and 1 atm, density = 1.293 kg/m³). Gas SG is the ratio of the gas density to air density at the same temperature and pressure.

Gas SG (relative to air) Density (kg/m³ at 0°C, 1 atm)
Air 1.000 1.293
Natural gas (methane) 0.554 0.717
Propane 1.52 1.97
Carbon dioxide 1.52 1.96
Helium 0.138 0.1785
Hydrogen 0.0696 0.0899

Gas SG determines buoyancy and dispersion: natural gas (SG < 1) rises and dissipates; propane (SG > 1) sinks and pools in low areas. This is a critical safety distinction for gas detection system placement.

Frequently asked questions

Q: Does specific gravity have units?

No. SG is a dimensionless ratio. This is its primary advantage. A material with SG = 1.2 is 20% denser than water regardless of whether you work in metric or US customary units. When you see "density = 1,200 kg/m³" or "density = 74.9 lb/ft³," you're looking at the same SG = 1.2 material expressed in two different unit systems.

Q: How do I convert specific gravity to density?

Multiply SG by the density of the reference substance at the reference temperature. For water at 4°C: density (kg/m³) = SG × 1,000; density (lb/ft³) = SG × 62.428. For gases relative to air at 0°C, 1 atm: density (kg/m³) = SG × 1.293.

Q: Can I use a hydrometer to measure SG?

Yes. A hydrometer measures SG directly by buoyancy. It floats higher in denser liquids. Hydrometers are calibrated for a specific reference temperature (typically 60°F or 20°C). If your liquid is at a different temperature, apply the temperature correction printed on the hydrometer or use ASTM D1250 (API MPMS Chapter 11) tables for petroleum products.

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