Free API Gravity Calculator

API gravity is a fundamental metric in the petroleum industry, used to classify crude oil based on its density relative to water. This dimensionless index, developed by the American Petroleum Institute, directly impacts the economic value of crude oil fractions. A dedicated API Gravity Calculator simplifies the conversion between specific gravity and API values, enabling professionals to quickly assess the API gravity of petroleum fluids, including crude oil, gasoline, and kerosene. By using this tool, one can avoid manual calculations and reduce potential errors when evaluating crude oil quality.

Formula and Measurement

The relationship between API gravity and specific gravity (SG) is expressed by the standard formula:

$$\text{SG} = \frac{141.5}{131.5 + \text{API}}$$

Rearranging the terms gives the API to specific gravity conversion formula:

$$\text{API} = \frac{141.5}{\text{SG}} - 131.5$$

Specific gravity itself is defined as the ratio of the density of the liquid ($\rho_{\text{liquid}}$) to the density of water ($\rho_{\text{water}} = 999\ \text{kg/m}^3$ at 60°F):

$$\text{SG} = \frac{\rho_{\text{liquid}}}{\rho_{\text{water}}}$$

In practice, the specific gravity of a petroleum liquid is measured using a hydrometer or an oscillating U‑tube densitometer, following standardized procedures such as ASTM D1298 or D4052. Once the relative density is known, the API gravity is calculated using the equation above.

Comparison with Water

A key reference point is pure water, which has an SG of 1.0. Plugging this value into the formula yields:

$$\text{API}_{\text{water}} = \frac{141.5}{1.0} - 131.5 = 10^\circ \text{API}$$

Liquids with an API gravity greater than 10°API are less dense than water and will float; those with an API gravity below 10°API are denser and will sink. This simple test provides a quick indication of a crude oil’s behavior in water.

API Gravity Classification

The API gravity scale provides a standardized way to categorize crude oils, as shown in the table below.

API Gravity Range (°API)	Classification
< 15	Extra heavy crude
15 – 25	Heavy crude
25 – 35	Medium crude
35 – 45	Light crude
≥ 45	Extra light crude

Crudes in the 35–45°API range (light crude) are typically the most valuable because they yield a higher proportion of high‑demand products. However, extra‑light crudes with an API gravity above 45°API are also valuable but may contain fewer heavy fractions, which can influence refining strategies.

How to Use the Calculator

Using the Petroleum Gravity Calculator is straightforward. Enter the density of the liquid (e.g., in kg/m³) into the tool. The calculator first determines the specific gravity by dividing the liquid’s density by 999 kg/m³, then applies the API formula to display the result.

Example – Gasoline:
Gasoline has a density of approximately 750 kg/m³.

• Specific gravity = 750 / 999 ≈ 0.7508
• API gravity = (141.5 / 0.7508) – 131.5 ≈ 56.98°API

This value confirms that gasoline is a very light petroleum product, falling into the extra‑light category.

Example – Kerosene:
Kerosene has a density of about 775 kg/m³.

• Specific gravity = 775 / 999 ≈ 0.7758
• API gravity = (141.5 / 0.7758) – 131.5 ≈ 50.9°API

These examples illustrate how the calculator efficiently converts density to API gravity, supporting quick decision‑making in drilling, refining, and trading operations.

Summary

Understanding the API gravity of crude oil is essential for evaluating its quality and market value. The API Gravity Calculator eliminates manual computation, making it easy for engineers, traders, and students to obtain accurate results from density or specific gravity data. With its built‑in API to specific gravity conversion and support for various petroleum liquids, this tool serves as a practical resource for the oil and gas industry.