API To Density Converter
A 30 degree API crude has a specific gravity of about 0.876, or roughly 875 kg/m3.
Quick answer: Add 131.5 to the API gravity, then divide 141.5 by the result. For example, 10 API is 1 SG, 30 API is 0.8762 SG and 40 API is 0.8251 SG.
The API to density formula
Specific gravity at 60 F equals 141.5 divided by the sum of 131.5 and the API gravity.
Multiply by the density of water to get the density.
Take 10 API. 141.5 ÷ (131.5 + 10) gives 1 SG.
Take 30 API. 141.5 ÷ (131.5 + 30) gives 0.8762 SG.
A scale that runs backwards
API gravity measures how heavy or light a petroleum liquid is compared with water. A liquid above 10 degrees API floats on water, and one below 10 sinks (Wikipedia mirror: API gravity).
The higher the API number, the lighter the oil. Specific gravity at 60 F is 141.5 divided by the API gravity plus 131.5 (Wikipedia mirror: API gravity).
How to convert API to density step by step
- Read the API gravity in degrees.
- Add 131.5 to it.
- Divide 141.5 by that sum to get the specific gravity.
- Multiply by 999.016 to get the density in kg/m3.
Example 1: 10 API
- Start with 10 API.
- 141.5 ÷ (131.5 + 10) = 1.
- The specific gravity is 1 SG.
10 API = 1 SG
Example 2: 30 API
- Start with 30 API.
- 141.5 ÷ (131.5 + 30) = 0.8762.
- The specific gravity is 0.8762 SG.
30 API = 0.8762 SG
Example 3: 40 API
- Start with 40 API.
- 141.5 ÷ (131.5 + 40) = 0.8251.
- The specific gravity is 0.8251 SG.
40 API = 0.8251 SG
The three examples below use common sizes.
Each one shows the sum in full.
API to density conversion table
| API gravity | Specific gravity (60 F) | Density (kg/m3) |
|---|---|---|
| 10 API | 1 SG | 999.02 kg/m3 |
| 15 API | 0.9659 SG | 964.92 kg/m3 |
| 20 API | 0.934 SG | 933.07 kg/m3 |
| 22.3 API | 0.92 SG | 919.12 kg/m3 |
| 25 API | 0.9042 SG | 903.26 kg/m3 |
| 28 API | 0.8871 SG | 886.27 kg/m3 |
| 30 API | 0.8762 SG | 875.3 kg/m3 |
| 31.1 API | 0.8702 SG | 869.38 kg/m3 |
| 35 API | 0.8498 SG | 849.01 kg/m3 |
| 40 API | 0.8251 SG | 824.26 kg/m3 |
| 45 API | 0.8017 SG | 800.91 kg/m3 |
| 50 API | 0.7796 SG | 778.85 kg/m3 |
The highlighted row is the default of the calculator. The last column gives kg/m3.
A quick mental shortcut for API to density
Water has an API gravity of 10, because a liquid with a specific gravity of 1.0 works out to 10 degrees (Wikipedia mirror: API gravity).
Notice that a higher API number gives a lower specific gravity, so the scale runs the opposite way to density.
Crude oil classes
Crude oil above 31.1 degrees API is called light, oil from 22.3 to 31.1 is medium, and oil below 22.3 is heavy (Wikipedia mirror: API gravity).
So a 35 API oil is light, a 25 API oil is medium and a 20 API oil is heavy.
Worked examples
A 40 API oil has a specific gravity of about 0.8251, or about 824.3 kg/m3. A 20 API oil has a specific gravity of about 0.934, or about 933.1 kg/m3.
A 50 API oil has a specific gravity of about 0.7796, or about 778.8 kg/m3.
The reference temperature
API gravity is tied to a reference of 60 F. Water at that temperature is about 999 kg/m3, and this page uses 999.016 kg/m3 for it.
If you want density at another temperature, you need a correction that this page does not include.
Specific gravity and density
Specific gravity has no unit. It is the density of the oil divided by the density of water at the reference temperature.
To get a density, multiply the specific gravity by the density of water.
Going back
To go from specific gravity back to API, use API = 141.5 divided by SG, minus 131.5 (Wikipedia mirror: API gravity).
For an SG of 0.876, that gives about 30.0 degrees API.
Quick conversion answers
| Conversion | Result |
|---|---|
| 10 API | 1 SG |
| 30 API | 0.8762 SG |
| 40 API | 0.8251 SG |
| 22.3 API | 0.92 SG |
Where this conversion is used
Traders and engineers sort crude oils into light, medium and heavy grades.
Students link a hydrometer scale to density.
Common mistakes to avoid
- Expecting a higher API to mean denser oil. It means lighter oil.
- Ignoring the temperature. The scale is tied to 60 F.
- Using the formula for water-like liquids. It is meant for petroleum liquids.
- Mixing specific gravity with density. One has a unit and one does not.
Related converters
More API gravity values
12 API is 0.9861 SG. 18 API is 0.9465 SG. 23 API is 0.9159 SG.
27 API is 0.8927 SG. 33 API is 0.8602 SG. 38 API is 0.8348 SG.
42 API is 0.8156 SG. 55 API is 0.7587 SG. 60 API is 0.7389 SG.
Reading a specific gravity
An oil with a specific gravity of 0.85 has an API gravity of about 35.0, so it counts as light crude.
An oil with an API gravity of 12 has a specific gravity of about 0.9861 and a density of about 985.1 kg/m3, so it is only a little lighter than water.
Comparing two grades
A 25 API oil has a specific gravity of about 0.9042, and a 40 API oil has a specific gravity of about 0.8251. The gap is about 0.079, or about 79 kg/m3.
This shows why a small change in API can be a clear change in density.
Floating and sinking
An oil with an API gravity above 10 is lighter than water and floats on it (Wikipedia mirror: API gravity). A 20 API oil has a specific gravity of about 0.934, so it floats.
Very heavy oils sit close to 10 API, where they are almost as dense as water.
Frequently asked questions
How do I convert API gravity to specific gravity?
Divide 141.5 by the sum of 131.5 and the API gravity.
What is the specific gravity of 30 API?
About 0.8762.
What is the API gravity of water?
10 degrees.
Is a high API oil light or heavy?
Light.
What is a light crude?
Oil above 31.1 degrees API.
What temperature does API use?
60 F.
Sources
- Wikipedia (archived mirror): API gravity (wikipedia.classicistranieri.com)
