API gravity calculator
API gravity, specific gravity and density, in any direction — with where the odd 31.1 and 22.3 band figures come from, and why 10° is the one value that cannot tell a correct formula from a broken one. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Everything here is at 60 °F. The API scale is defined at that temperature against water at that temperature, and a hydrometer reading taken anywhere else needs correcting first. The correction tables are in ASTM D1250 and they are not on this page.
The scale runs backwards, and that is deliberate
A bigger API number means a lighter oil. This catches people constantly, and it is not an accident: the scale was built so that the number goes up as the oil gets more valuable. The formula is
°API = 141.5 ÷ SG − 131.5
and it inverts, so SG = 141.5 ÷ (°API + 131.5). Both numbers in it are defined, not measured.
Ten degrees is exactly water — and that is a trap
Put SG = 1 into the formula and you get 141.5 − 131.5 = exactly 10. That is the whole meaning of 10 on this scale: below it, crude is denser than water and sinks. It is the one figure on the scale that is exact, and every reference quotes it as the sanity check.
Which is exactly why it is a bad sanity check. The commonest way to get this formula wrong is to write it upside down — (°API + 131.5) ÷ 141.5 instead of 141.5 ÷ (°API + 131.5). Sweeping three hundred thousand values, the right formula and the upside-down one agree at exactly one point, and it is 10°, because that is where 141.5/x equals x/141.5.
So the number everybody tests with is the one number that cannot tell a correct implementation from a broken one. At 30° the two give 0.8762 and 1.1413 — an oil lighter than water reported as heavier than water, the classification completely reversed. This site's checker tests at 30, and says why.
Why the bands are 31.1 and 22.3
Those figures look arbitrary. They are not. They are round specific gravities put through the formula and rounded to one decimal:
| Specific gravity | Gives | Quoted as |
|---|---|---|
| 0.870 | 31.1437° API | 31.1° |
| 0.920 | 22.3043° API | 22.3° |
| 1.000 | 10° API exactly | 10° |
So the real boundaries are 0.870 and 0.920, and the API figures are the rounded ones — which is the opposite of how they are usually presented. Only the water boundary survives the conversion exactly.
Which water? It matters by half a kilo
You will read everywhere that light crude is “under 870 kg/m³”. That figure assumes water is 1000 kg/m³. The API scale does not. It is defined against water at 60 °F, which is 999.016 kg/m³ — the figure in ASTM D1250.
Work it through properly and 31.1° API is 869.4 kg/m³, not 870. Half a kilo in a thousand, and it is the difference between a number derived and a number assumed. This page uses 999.016 and says so, which is why its densities will disagree very slightly with tables that did not.
And everything here is at 60 °F
The whole scale is defined at 60 °F, against water at 60 °F. A hydrometer reading taken at any other temperature needs correcting before it means anything, because oil expands more than water does. The correction tables are in ASTM D1250 and they are not on this page. If you are doing custody transfer, the correction is the job.
A barrel here is exactly 42 US gallons, the API definition — which is what makes the volume arithmetic exact even where the density is not.