Specific Gravity Calculator (relative density & API gravity)

Understanding the specific gravity calculator — concept and principle

This calculator turns a measured density into specific gravity (relative density), the API gravity used for crude oil and fuels, and a few practical figures such as the mass of a litre or a US gallon. Use it when a data sheet gives density but a specification asks for specific gravity, when you need to know whether something floats in water, or when you want to classify a petroleum liquid.

850 kg/m³ against 60 °F water gives SG 0.8508, so the liquid floats and rates 34.8 °API.

How it is calculated

SG = ρ ÷ ρwater

°API = 141.5 ÷ SG − 131.5 (only with the 60 °F reference)

ρ is the density of the substance in kg/m³ (g/cm³ × 1000).
ρwater is the density of the reference water you pick: 999.972 kg/m³ at 4 °C, 999.003 kg/m³ at 60 °F (15.6 °C), 998.207 kg/m³ at 20 °C or 997.048 kg/m³ at 25 °C.
Mass of 1 litre in kg = ρ ÷ 1000; mass of a US gallon (3.785411784 L) is converted to pounds with 1 lb = 0.45359237 kg.

The principle

Specific gravity compares two densities, so the units cancel and the result is a pure number. Because water expands as it warms, its density changes slightly with temperature, and the same oil gives a slightly different SG against 4 °C water than against 25 °C water. That is why a proper SG value always names its reference temperature. The petroleum industry defined API gravity on the 60 °F basis so that water comes out at exactly 10 °API and lighter oils get higher numbers. A substance with SG below 1 is less dense than water and floats; above 1 it sinks.

Worked example

Default inputs: density 850 kg/m³, reference water at 60 °F (999.003 kg/m³).

SG = 850 ÷ 999.003 = 0.8508.
°API = 141.5 ÷ 0.8508 − 131.5 = 34.8.
SG is below 1, so the liquid floats in water.
34.8 °API is above 30, so it falls in the light crude class.
One litre weighs 850 ÷ 1000 = 0.85 kg; one US gallon weighs 850 × 0.003785 ÷ 0.4536 = 7.09 lb.

Each point is the calculator re-run with only “Density of the substance (kg/m³)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Density of the substance (kg/m³)	Specific gravity
212	0.2127
425	0.4254
638	0.6381
850	0.8508
1,062	1.0636
1,275	1.2763
1,700	1.7017

How to read the result

Where does your result fall?

Most liquids and solids lie between about 0.6 and 20 in SG. Gasoline and light oils sit around 0.7–0.8, diesel around 0.82–0.85, sea water a little above 1, and metals much higher. For crude oil, the class boundaries used here are above 30 °API for light, 22–30 °API for medium and below 22 °API for heavy crude. Light crude usually yields more gasoline and diesel per barrel, which is why it is generally priced higher.

Common mistakes

Keep in mind: Results assume a pure, well-mixed sample at the stated temperature. For custody transfer or product specifications, follow the measurement standard your contract or laboratory uses.

Frequently asked questions

Is specific gravity the same as density?

Numerically it is close to the density in g/cm³, because water is almost exactly 1 g/cm³, but SG is a ratio without a unit and depends on the chosen reference.

What is the API gravity of water?

Exactly 10 °API, because 141.5 ÷ 1 − 131.5 = 10.

Why does the reference temperature matter so little?

Water's density changes only about 0.3 % between 4 °C and 25 °C, so the SG changes in the third or fourth decimal place. That still matters in precise work.

Sources: OpenStax, Chemistry 2e (2019), Appendix E, Table E1 "Water Density at Different Temperatures" · Penn State University, FSC 432 Petroleum Refining, "API Gravity"

Checked 9 Nov 2026. Water densities: OpenStax, Chemistry 2e (2019), Appendix E, Table E1 “Water Density at Different Temperatures” (60 °F value linearly interpolated between 15 °C and 20 °C). API gravity and crude classes: Penn State University, FSC 432 Petroleum Refining, “API Gravity”.

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