Divide the density by water's density — the units cancel out
Water at 4 °C is 1000 kg/m³ (or 1 g/mL), so in kg/m³ you just divide by 1000. Aluminium at 2700 kg/m³ has a specific gravity of 2.70. Because it is a ratio of two densities, specific gravity has no units and reads the same whether you started in metric or imperial.
Below 1 floats, above 1 sinks
Specific gravity is really a float-or-sink test against water. The dividing line is exactly 1. That is why ice (0.92) rides on top and a coin drops to the bottom.
Anything left of the 1.00 mark floats on water; everything heavier sinks, and the further past 1 it is, the more forcefully.
Specific gravity of common substances
| Substance | Specific gravity | In water |
|---|---|---|
| Gasoline | 0.72–0.77 | Floats |
| Ethanol | 0.79 | Floats |
| Olive oil | 0.92 | Floats |
| Water (4 °C) | 1.00 | Reference |
| Seawater | 1.025 | Sinks |
| Aluminium | 2.70 | Sinks |
| Iron | 7.87 | Sinks |
| Lead | 11.34 | Sinks |
| Mercury | 13.6 | Sinks |
| Gold | 19.3 | Sinks |
Seawater's dissolved salt lifts it just above 1.0, which is why you float more easily in the sea than in a pool.
Two things that throw the number off
- Temperature. Heat expands most substances, dropping their density and so their specific gravity. Oil measured at 15 °C and at 40 °C give different readings, so technical data always states a temperature.
- Gases use a different reference. For gases the standard is air, not water. Do not divide a gas density by 1000 kg/m³ — you would get a near-zero number that means nothing.
Common questions
What is the difference between specific gravity and density?
Density has units, such as kg per cubic metre or g per mL. Specific gravity is that density divided by the density of water, so it has no units at all. Water has a density of 1000 kg per cubic metre and a specific gravity of exactly 1.
Does specific gravity tell me if something floats?
Yes, in water. A specific gravity below 1 floats, above 1 sinks, and exactly 1 stays neutral. Oil at about 0.92 floats; iron at 7.87 sinks straight down.
What density of water should I divide by?
The standard reference is pure water at 4 degrees Celsius, which is 1000 kg per cubic metre or 1 g per mL, where water is at its densest. For most everyday work dividing by 1000 (in kg per cubic metre) is exact enough.
