Four ways to say how heavy a liquid is
Density is the physical quantity; the other three are scaled conventions that grew up in different industries. Everything hinges on specific gravity — density relative to water:
The API scale is deliberately inverted — higher API means lighter oil — and pinned so that water reads exactly 10 °API. It exaggerates the small SG differences between crude oils into a wider, more workable range.
Worked example
A diesel with density 850 kg/m³:
- SG = 850 ÷ 1000 = 0.850
- °API = 141.5 ÷ 0.850 − 131.5 = 166.47 − 131.5 = 34.97 °API
- Above 31.1 °API → classified as light
API gravity reference
| °API | SG | Category |
|---|---|---|
| 10 | 1.000 | water |
| < 22.3 | > 0.920 | heavy crude |
| 22.3–31.1 | 0.870–0.920 | medium crude |
| > 31.1 | < 0.870 | light crude |
| ~40 | 0.825 | typical diesel |
| ~60 | 0.739 | gasoline |
Field notes
- Gravity is temperature-dependent. API is defined at 60 °F (15.56 °C); a hot sample reads lighter. Custody transfer applies volume-correction factors (VCF) to standardise — an online converter cannot replace those tables.
- Why instrumentation cares: hydrostatic level, DP flow and mass-flow inference all need density. A drifting API means a drifting level or flow reading — see our DP level and mass-volumetric tools.
- Baumé has two scales — one for liquids denser than water, one for lighter. Using the wrong branch gives nonsense; this tool auto-selects by SG.
Frequently asked questions
How do I convert specific gravity to API gravity?
°API = (141.5 ÷ SG) − 131.5, where SG is specific gravity at 60/60 °F. Water (SG 1.0) is 10 °API; lighter oils have higher API numbers.
What is the API gravity of typical crude oil?
Light crude is above 31.1 °API, medium 22.3–31.1, heavy below 22.3. Water is 10 °API; anything above floats on water, below sinks.
What is specific gravity versus density?
Density is mass per volume (kg/m³). Specific gravity is density relative to water, so it is dimensionless: SG = density ÷ 1000 (using water at 1000 kg/m³).
What is the Baumé scale?
An older gravity scale still seen on some chemical datasheets. For liquids heavier than water: °Bé = 145 − (145 ÷ SG). For lighter: °Bé = (140 ÷ SG) − 130.
Provided for reference and education; not for custody transfer without temperature correction. See our disclaimer.