API to Density Calculator: Convert & Find Density

This calculator converts an API gravity value into density (kg/m³) and specific gravity, using the standard American Petroleum Institute formula. You enter the API gravity number, and you get back the corresponding density at 60°F (15.6°C), along with the specific gravity. It is the quickest way to move between the two scales used in the petroleum industry.

Formula

The conversion uses the definition of API gravity:

API = (141.5 / SG) − 131.5

where SG is the specific gravity of the liquid at 60°F (15.6°C) relative to water at the same temperature. Rearranged to solve for specific gravity:

SG = 141.5 / (API + 131.5)

Then density is simply specific gravity multiplied by the density of water at 60°F, which is 999.016 kg/m³ (often rounded to 1000 kg/m³ for practical work). The calculator uses the common rounded value:

Density (kg/m³) = SG × 1000

Variables:

• API – API gravity, in degrees API (°API).
• SG – specific gravity (dimensionless).
• Density – in kilograms per cubic metre (kg/m³).

Worked Example

Using the default value of 35 °API:

1. SG = 141.5 / (35 + 131.5) = 141.5 / 166.5 = 0.849849…
2. Density = 0.849849 × 1000 = 849.8 kg/m³
3. The calculator shows density as 849.8 kg/m³ and SG as 0.8498.

That matches the output of the block above.

Why API Gravity Exists

API gravity was developed by the American Petroleum Institute in the early 20th century as a simple way to compare the relative heaviness of crude oils. It is an inverse scale: higher API means lighter oil. Water sits at 10 °API. Oils with API greater than 10 float on water; those with API less than 10 sink. Most crude oils fall between 15 and 45 °API, with light sweet crudes around 35–45 °API and heavy crudes below 20.

The scale was originally designed so that a change of 1 °API roughly corresponded to a change of about 0.1% in specific gravity, which made it convenient for field measurements. Today it remains the standard in the oil and gas industry, even though density in kg/m³ is more common in engineering calculations and international trade.

How to Read the Result

The density value you get is the density at the reference temperature of 60°F (15.6°C). That is the industry standard for reporting both API gravity and density. If your liquid is at a different temperature, the density will be slightly different because liquids expand when heated. For precise work, you would need to correct for temperature using a volume correction factor (VCF) from API MPMS Chapter 11 or ASTM D1250.

Specific gravity is the same number as the ratio of the liquid’s density to water’s density at the same reference temperature. It is dimensionless, so it has no units. In the calculator, SG is shown as a plain number.

Note that many density values in the oil industry are reported in g/cm³ or kg/L, which are numerically identical to specific gravity. So a density of 0.8498 g/cm³ equals an SG of 0.8498, and equals 849.8 kg/m³.

Common Mistakes and Misconceptions

• Confusing API with density directly. API is not a density; it is an inverse scale. A higher API number means a lower density.
• Using the wrong water density. Some references use 999.016 kg/m³, others use 1000 kg/m³. The difference is about 0.1%, which matters in custody transfer but not for general use. This calculator uses 1000 kg/m³, the most common simplification.
• Forgetting the reference temperature. The 60°F reference is embedded in the formula. If your measurement was at 15°C (59°F), the difference is tiny, but at 20°C it can be significant for precise work.
• Applying the formula to non-petroleum liquids. The 141.5 constant is specific to the API definition. It works for any liquid, but the resulting “API gravityâ€� is only meaningful in the petroleum context.
• Rounding too early. If you round SG to 3 decimals before multiplying by 1000, your density can be off by several kg/m³. Keep full precision until the final step.

What the Formula Can’t Do

This conversion assumes a simple, pure liquid at the reference temperature. Real crude oils are mixtures of hundreds of hydrocarbons, and their density changes with temperature and pressure. The formula gives the density at 60°F and atmospheric pressure only. It does not account for:

• Temperature expansion or contraction.
• Dissolved gases, which lower measured density.
• Water or sediment content in the oil.
• Pressure effects (density increases slightly with pressure).

For custody transfer (buying and selling oil), you need a full laboratory analysis and a temperature correction per API MPMS standards. This calculator is for quick conversions, not for legal or commercial measurement.

How to Choose the Right Input

Enter the API gravity you have, whether from a hydrometer, a lab report, or a product data sheet. If you only have specific gravity, you can convert it to API first using the formula: API = (141.5 / SG) − 131.5. For example, an SG of 0.85 gives API = (141.5 / 0.85) − 131.5 = 166.47 − 131.5 = 34.97 °API.

If you have density in kg/m³ at 60°F, divide by 1000 to get SG, then use the same formula. So a density of 850 kg/m³ corresponds to SG 0.850 and API = (141.5 / 0.850) − 131.5 = 35.0 °API.

For liquids other than crude oil – such as diesel, gasoline, or lubricants – the same formula works, but the API scale is rarely used for those products. You would typically work directly in density or specific gravity.

Frequently Asked Questions

What is the difference between API gravity and density?

API gravity is an inverse measure of how heavy a petroleum liquid is compared to water. Density is mass per unit volume. High API means low density. The two are mathematically linked by the formula above, but they are different scales.

Why is the constant 141.5 in the formula?

The constant comes from the historical definition of API gravity, which was set so that water (SG = 1.0) equals 10 °API. The relationship was chosen to give a convenient range for typical crude oils. The 141.5 value ensures that the scale is linear in specific gravity.

Can I use this calculator for ethanol or water?

Technically yes, the math works for any liquid. But the API scale is not used for those substances. For water, API = 10. For ethanol, the API would be around 57 (density ~789 kg/m³), but you would never see that number in practice. Use density or specific gravity for non-petroleum liquids.

Does temperature affect the conversion?

Yes. The formula is only valid at the reference temperature of 60°F (15.6°C). If your liquid is at a different temperature, you must correct the density to 60°F first, or use a temperature-corrected API value. The calculator assumes your input is already at the reference temperature.

What is the density of water in kg/m³?

At 60°F (15.6°C), pure water has a density of about 999.016 kg/m³. For most engineering work, 1000 kg/m³ is used, which is what this calculator uses. The difference is about 0.1%, which is negligible for most purposes.

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in api | API gravity | °API | 35 | 0
out density | Density at 60°F | kg/m³ | 141.5 / (api + 131.5) * 1000 | 1
out sg | Specific gravity | - | 141.5 / (api + 131.5) | 4