API gravity calculator

An API gravity calculator turns the specific gravity of a petroleum liquid into degrees API using the industry's exact hydrometer formula, with no tables and no hand arithmetic. Enter the specific gravity of your crude oil or refined product — the relative density measured at the standard sixty over sixty degrees Fahrenheit basis — and the tool returns the corresponding degrees API, a light, medium, heavy, or extra-heavy classification, the formula string, and the substitution steps you can paste into a lab notebook or cargo report. Specific gravity of zero or below is physically meaningless for this scale, so the solver rejects it with a clear invalid_input error instead of inventing a number.

What API gravity is and how the formula works

Defined by the American Petroleum Institute and standardized in ASTM D287, it expresses how heavy or light a petroleum liquid is compared with water at the reference temperature of sixty degrees Fahrenheit. Specific gravity here means relative density on the sixty over sixty degrees Fahrenheit basis — the density of the liquid divided by the density of water, both at the reference temperature — so it is a pure dimensionless number with no units attached. API gravity is the density scale the petroleum industry actually speaks. The relationship to specific gravity is exact and algebraic: degrees API equals one hundred forty-one point five divided by the specific gravity, minus one hundred thirty-one point five. Water itself, with specific gravity exactly one, lands at exactly ten degrees API, which makes ten the dividing line: liquids above ten degrees API float on water, liquids below it sink. Light crudes such as West Texas Intermediate sit around forty degrees API, medium grades cluster in the twenties and low thirties, heavy crudes fall between ten and twenty-two, and extra-heavy bitumens drop below ten. Because the formula divides by specific gravity, the scale is undefined when specific gravity is zero and physically meaningless when it is negative; this calculator rejects both with an invalid_input error naming the field, rather than returning an infinite or absurd result. It applies only the closed-form definition — no temperature volume correction factors, no blend rules, no table interpolation — so the answer you get is the pure definitional value you would compute by hand, rounded to twelve decimal places for reproducibility across systems.

How to call the API and read the response

The value must be a finite number, or a strict numeric string without currency symbols or thousand separators, within an absolute magnitude of one trillion, and it must be greater than zero. Send a JSON body with one required field, specific_gravity, which also answers to the aliases sg, value, gravity, relative_density, and density so existing payloads integrate without renaming. On success the response echoes the rounded specific_gravity, then gives api and api_gravity — the same degrees API figure under two keys for convenience — plus unit set to the degrees API symbol, a classification of light, medium, heavy, or extra heavy following the common industry bands, the formula string showing the exact constants used, a plain English message summarizing the result, and a steps array listing the ordered substitution lines from input through classification. Validation failures — a missing field, a non-finite value, a magnitude beyond the limit, or specific gravity of zero or below — return invalid_input with a message that names the offending field, and failed tasks are never charged against prepaid balance. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price. This makes the capability safe to wire into dashboards, mobile apps, batch assay processors, or any client that holds a relative density reading and needs the degrees API figure for display, tickets, or mixed SI and imperial reports.

Practical tips, limits, and when to use this calculator

Send bare numbers when you control the client; string input is accepted only when it matches a strict finite numeric pattern, so reject locale-formatted values on your side before calling. Remember that this tool applies the definitional formula only: custody transfer and official reporting often require temperature-corrected volumes from ASTM D1250 tables, which are out of scope here. Stay within the published absolute limit of one trillion so intermediate division cannot overflow, and treat the classification band as an informal industry convention rather than a contractual boundary. Classic reference points make good sanity checks: specific gravity one must return exactly ten degrees API, a light crude around zero point eight five returns roughly thirty-five degrees API, and a heavy crude near zero point nine three returns about twenty degrees API. If you need the reverse direction, or conversions into absolute density units such as kilograms per cubic metre, grams per cubic centimetre, or pounds per gallon, use the dedicated API gravity converter capability instead — this calculator deliberately does one thing, specific gravity to degrees API, and does it deterministically. This landing serves people searching for an API gravity calculator, degrees API from specific gravity, or a petroleum hydrometer formula check; it is not a general unit converter, not a density tool for other industries, and not a blend or temperature-correction engine. Failed calls return invalid_input and consume no balance, which keeps exploratory scripts and client-side validation probes cheap while you integrate.