API Gravity Calculator

API gravity is a petroleum-industry scale that expresses how heavy or light a liquid petroleum product is compared with water. The American Petroleum Institute scale is derived from specific gravity measured at a reference temperature of 60 degrees Fahrenheit. Unlike ordinary specific gravity, a larger API number represents a less dense liquid. Petroleum with API gravity above 10 floats on water under reference conditions, while material below 10 is denser than water and tends to sink. The standard conversion divides 141.5 by specific gravity at 60 degrees Fahrenheit and subtracts 131.5. For example, specific gravity of 0.85 produces approximately 34.97 degrees API. The inverse relationship means a small change in specific gravity does not translate to a constant change in API gravity across the scale. Measurements should therefore retain suitable precision and use a temperature-corrected specific gravity rather than an uncorrected field density. Common commercial classifications describe crude above 31.1 degrees API as light, crude from 22.3 through 31.1 as medium, crude from 10 through below 22.3 as heavy, and crude below 10 as extra-heavy. Boundaries can vary among agencies, markets, and contracts, so the classification displayed here is a general reference rather than a contractual grade. Sulfur content, acidity, viscosity, contaminants, and product yield also influence crude quality and value. Temperature matters because petroleum liquids expand as they warm, changing density and specific gravity. Laboratory hydrometers, digital density meters, or sampling procedures typically report or correct values to 60 degrees Fahrenheit. Entering a value measured at another temperature without correction can produce a misleading API result. Pressure and composition can also matter for volatile fluids. Consult the applicable ASTM method, custody-transfer procedure, or operating standard when accuracy has financial or safety consequences. This calculator is useful for quick conversion, educational work, process checks, and approximate crude classification. It assumes the entered specific gravity is dimensionless and already referenced to the standard temperature. The permitted range covers typical petroleum products as well as unusually dense or light liquids, but a mathematically valid result does not prove that a sample is petroleum or that its measurement is reliable. Record the test method, reference temperature, instrument calibration, and uncertainty alongside reported API gravity whenever traceability is required.