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Generated on: July 20, 2026 at 09:33 PM

API Gravity ⇄ Specific Gravity Converter

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Convert°API → SG
API gravity40
Specific gravity0.9

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API Gravity ⇄ Specific Gravity Converter

Convert between API gravity (°API) and specific gravity, with the density at 60 °F and the crude-oil classification. °API = 141.5/SG − 131.5.

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\begin{aligned} \mathrm{SG} &= \dfrac{141.5}{\mathrm{API} + 131.5} \\ &= \dfrac{141.5}{ 40 + 131.5 } \\ &= ? \end{aligned}

Details

\begin{aligned} \rho &= \mathrm{SG} \times 999.016 \\ &= ? \times 999.016 \\ &= ?\,\text{kg/m³} \end{aligned}

At an API gravity of 40 °API, the specific gravity is ... and the density is about ... at the 60 °F reference temperature.

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API Gravity and Specific Gravity

API gravity is a measure of how light or heavy a petroleum liquid is compared with water, defined by the American Petroleum Institute. It restates a liquid's specific gravity on a degree scale (°API) that the oil industry uses to describe and price crude oil. This converter moves between the two representations in either direction and reports the corresponding density and crude-oil classification.

Two scales for one quantity

Specific gravity (SG) is the ratio of a liquid's density to the density of water at the same reference temperature. Water is therefore exactly 1.000, a liquid denser than water is above 1, and a lighter one is below 1. Crude oils and refined products are almost all lighter than water, so their specific gravities cluster in a narrow band just below 1.

API gravity spreads that narrow band across a wider, more readable range of degrees, and it inverts the direction: a higher API gravity means a lighter, less dense liquid. Water, at SG 1.000, corresponds to 10 °API. The two scales carry the same information, so a value on one converts exactly to the other.

Formula

The definitions are a matched pair, each the inverse of the other, both evaluated at the 60 °F (15.56 °C) reference temperature:

°API=141.5SG−131.5SG=141.5°API+131.5\begin{aligned} \text{°API} &= \frac{141.5}{\mathrm{SG}} - 131.5 \\ \mathrm{SG} &= \frac{141.5}{\text{°API} + 131.5} \end{aligned}

Once the specific gravity is known, density follows directly. At 60 °F the density of water is about 999.016 kg/m³, so:

ρ=SG×999.016 kg/m3\rho = \mathrm{SG} \times 999.016\ \text{kg/m}^3

Worked example

Take a light crude with an API gravity of 40 °API. Its specific gravity is:

SG=141.540+131.5=141.5171.5=0.8251\begin{aligned} \mathrm{SG} &= \frac{141.5}{40 + 131.5} \\ &= \frac{141.5}{171.5} \\ &= 0.8251 \end{aligned}

and its density at the reference temperature is:

ρ=0.8251×999.016=824.3 kg/m3\begin{aligned} \rho &= 0.8251 \times 999.016 \\ &= 824.3\ \text{kg/m}^3 \end{aligned}

Running the calculation the other way, a sample with SG 1.000 — the same density as water — returns exactly 10 °API, the anchor point of the scale.

Light, medium, and heavy crude

API gravity is one of the primary ways a crude stream is characterized, because lighter crudes generally flow more easily and yield a larger fraction of high-value products such as gasoline. The commonly used bands are:

  • Light crude — above 31.1 °API
  • Medium crude — 22.3 to 31.1 °API
  • Heavy crude — below 22.3 °API
  • Extra-heavy crude — below 10 °API

Extra-heavy crude and bitumen fall below 10 °API, meaning they are denser than water and will not float on it. The converter labels the result with the band that the API gravity falls into, in whichever direction the conversion runs.

To work directly in density units — kg/m³, g/cm³, lb/ft³ and others — use the Density Converter. The Mass/Weight Converter and Volume Converter cover the quantities that combine with density to give the mass or volume of a batch.

Frequently Asked Questions (FAQ)

What is API gravity?

API gravity is a scale defined by the American Petroleum Institute that expresses how light or heavy a petroleum liquid is relative to water. It is derived from specific gravity by °API = 141.5/SG − 131.5, measured at 60 °F (15.6 °C). Because the relationship is inverse, a higher API gravity means a lighter, less dense liquid: water sits at 10 °API, while most crude oils fall between about 10 and 45 °API.

How do API gravity and specific gravity differ?

Specific gravity is the ratio of a liquid density to the density of water at the same reference temperature, so water is exactly 1.000. API gravity rescales that ratio into degrees, spreading the small density differences between crude oils across a wider, more workable range.

Both carry the same information: °API = 141.5/SG − 131.5 and SG = 141.5/(°API + 131.5). A specific gravity below 1.000 (lighter than water) gives an API gravity above 10.

What counts as light, medium, or heavy crude?

The common industry bands, by API gravity, are light crude above 31.1 °API, medium crude from 22.3 to 31.1 °API, heavy crude below 22.3 °API, and extra-heavy crude below 10 °API. Lighter crudes flow more readily and typically yield a larger share of high-value products such as gasoline, so API gravity is one of the main pricing and refining characteristics of a crude stream.

Why is the density referenced at 60 °F?

The API and specific-gravity definitions fix the reference temperature at 60 °F (15.56 °C) because liquid density changes with temperature. Standardizing the measurement lets values from different samples and locations be compared directly. At that temperature the density of water is about 999.016 kg/m³, which is why density here is the specific gravity multiplied by 999.016.

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