API Gravity Calculator

Background.

The oil industry describes how heavy a crude or refined product is with a number that runs backwards. API gravity, defined by the American Petroleum Institute, rises as a liquid gets lighter. Fresh water sits at exactly 10 °API, anything that floats on water scores above 10 and anything that sinks scores below, so a crude at 40 °API is noticeably lighter than one at 20.

The number matters because density tracks value. Lighter crudes generally yield more gasoline, jet fuel and diesel per barrel with less upgrading, so crude is often priced with adjustments for its gravity, and a crude assay reports API gravity next to sulphur content. In the field and at terminals it is commonly read from a glass hydrometer floated in a sample, the method standardised in ASTM D287.

The scale grew out of the older Baumé hydrometer scale and is defined from specific gravity, the density of the oil divided by the density of water, both at 60 °F: °API = 141.5 ÷ SG − 131.5. With SG in a denominator the scale is not linear. Near an SG of 0.85, a change of 0.01 in specific gravity moves API gravity by almost 2 degrees; near an SG of 1.0 the same change moves it by about 1.4.

The 60 °F reference is not a detail. Oil expands as it warms, so a sample measured on a hot day reads lighter than it would at the reference temperature and its apparent API gravity comes out too high. Correcting an observed reading to 60 °F takes the petroleum measurement tables of API MPMS Chapter 11.1. This page assumes that correction has been made: it converts a specific gravity already referred to 60 °F/60 °F into degrees API and does nothing more.

What is api gravity calculator?

API gravity is a measure of how heavy or light a petroleum liquid is compared with water, expressed in degrees API. It is calculated from specific gravity at 60 °F/60 °F, the ratio of the liquid's density at 60 °F to that of pure water at the same temperature, through °API = 141.5 ÷ SG − 131.5. The scale is inverted, so a larger number means a less dense liquid, and it is anchored so that water is exactly 10 °API. Despite the degree sign it is a pure number with no physical unit.

How to use this calculator.

1. 01Enter the Relative density at 60°F/60°F: the specific gravity of the oil corrected to 60 °F and compared with water at 60 °F, as reported on a laboratory or cargo certificate.
2. 02If all you have is a hydrometer reading or a density measured at another temperature, correct it to 60 °F with the API/ASTM measurement tables first. An uncorrected reading from a warm sample gives an API gravity that is too high.
3. 03If your density is in kilograms per cubic metre at 15 °C, dividing by the density of water gives a relative density close to, but not exactly equal to, the 60 °F/60 °F figure, because the reference temperatures differ slightly.
4. 04Read the result in degrees API. Anything above 10 means the liquid is lighter than water; a specific gravity above about 1.076 returns a negative figure.

The formula.

°API = 141.5 ÷ SG − 131.5, where SG is the specific gravity at 60 °F/60 °F. The two constants are chosen so that water, with an SG of exactly 1, lands on 10 °API (141.5 − 131.5 = 10). Rearranged, SG = 141.5 ÷ (°API + 131.5), so the conversion runs just as well in reverse. Because SG sits in the denominator, API gravity falls as density rises, and the scale stretches out for lighter liquids: the rate of change is 141.5 ÷ SG² degrees per unit of specific gravity, about 195.8 at SG 0.85 against 141.5 at SG 1.0. Small density errors therefore matter more for light products than for heavy ones. Any SG above 141.5 ÷ 131.5, about 1.076, produces a value below zero. No temperature or pressure correction is applied, and the answer is given to 12 significant digits.

A worked example.

Frequently asked questions.

References& sources.

1. [1]ASTM D287-22, Standard Test Method for API Gravity of Crude Petroleum and Petroleum Products (BIBLIOGRAPHIC/PAYWALL).
2. [2]API Manual of Petroleum Measurement Standards Chapter 11.1 (BIBLIOGRAPHIC/PAYWALL).
3. [3]BIPM, The International System of Units (SI Brochure), 9th ed., version 3.01, coherent derived units and quantity equations.