API Gravity Calculator - Specific Gravity and Density at 60 °F

An API gravity calculator turns a measured specific gravity or API hydrometer reading into the standard API gravity at 60 °F, the specific gravity, and the liquid density in four common units, so engineers and students can interpret crude samples without redoing the unit math.

• • Crude oil classification: Read a hydrometer SG and check whether a sample lands in the light, medium, heavy, or extra-heavy range.
• • Convert API to density: Translate a lab API value into kg/m³, g/cm³, lb/ft³, and lb/gal so volumetric readings become mass numbers.
• • Field measurement correction: Take an SG read at field temperature and apply the Cragoe equation to bring it back to 60 °F.
• • Refined product checks: Verify diesel, gasoline, or jet fuel samples stay inside the API range expected by the buyer.

The API scale is an inverted, temperature-corrected version of specific gravity that the American Petroleum Institute introduced in 1921, and most lab instruments still read it off a hydrometer, so the API to SG direction is the most common conversion. When you start from mass and volume instead of a hydrometer reading, the Density Calculator gives the kg/m³ and g/cm³ that this calculator then turns into API gravity.

The calculator picks the input direction (API or specific gravity), converts the chosen input to the other scale, applies the Cragoe equation if the reading was made away from 60 °F, then multiplies the corrected SG by the density of water at 60 °F.

• API: API gravity in degrees. Water at 60 °F reads 10 °API.
• SG: Specific gravity of the oil at 60 °F, referenced to water at 60 °F. Dimensionless, so SG = 1 means the liquid matches water at 60 °F.
• T_obs: Temperature of the sample when the hydrometer was read. Most field readings are not at 60 °F, so the Cragoe equation brings the result back to 60 °F.
• alpha: Cragoe thermal expansion coefficient, 0.00045 per °F for most petroleum; 0.00036 for condensates and 0.00050 for heavy crudes.
• rho_water(60 °F): Density of pure water at 60 °F, 999.0 kg/m³, the reference that turns SG into density.

The 141.5 and 131.5 constants were chosen in 1921 so pure water reads 10 °API at 60 °F and most crude oils land between 10 and 70 °API.

The Cragoe equation is SG_60 = SG_T × (1 + α × ΔT). A 30 °F gap with α = 0.00045 per °F lifts SG by 1.3 percent, enough to push a borderline medium-crude sample into the light band.

After the correction, density in kg/m³ is SG_60 × 999.0. The other units are pure conversions: g/cm³ is kg/m³ ÷ 1000, lb/ft³ is kg/m³ × 0.062428, and lb/gal is kg/m³ × 0.008345.

According to Engineers Edge Fluid Data, API gravity is defined as API = (141.5 / SG) − 131.5, where SG is the specific gravity of the oil at 60 °F referenced to water at 60 °F.

After you have API gravity and density, the Reynolds Number Calculator uses the same kg/m³ density to compute the flow regime in the pipe that is carrying the crude.

Four ideas come up every time you work with an API reading: the reference temperature, the inverse direction of the scale, the difference between specific gravity and density, and the Cragoe correction.

Most calculators stop at the 141.5 and 131.5 constants and ignore temperature. The moment a sample is taken on a tank roof in summer or a refrigerated warehouse in winter, the Cragoe step is the difference between a published API value and a measurement that has drifted 1 to 2 °API.

For the vapor side of a separator at the same temperature and pressure, the Ideal Gas Calculator handles the gas density so the liquid API gravity and the gas density stay anchored to the same 60 °F reference.

Decide whether you are starting from an API or a specific gravity reading, enter the value, set the field temperature if it is not 60 °F, and read the result panel.

1. 1 Choose the input mode: Switch the first field to API for a hydrometer °API reading, or to SG for a 60 °F densitometer value.
2. 2 Enter the measured value: Type the API (for example 35) or the SG (for example 0.85) in the row that matches the chosen input mode.
3. 3 Set the observed temperature: Leave T_obs at 60 °F if the value is already corrected. Otherwise enter the field temperature so the calculator runs the Cragoe correction.
4. 4 Adjust the Cragoe coefficient if needed: Keep α at 0.00045 per °F for most crude oils. Drop to 0.00036 for condensates, push to 0.00050 for heavy crudes.
5. 5 Read the result panel: API at 60 °F, SG at 60 °F, density in kg/m³, g/cm³, lb/ft³, and lb/gal, plus the classification.

A shipping lab reports an API of 32.4 on a 30,000-barrel cargo. Entering API = 32.4 with T_obs = 60 °F gives SG ≈ 0.862, ρ ≈ 861.4 kg/m³, and a light-crude classification, so the operator can pre-allocate the cargo.

When the API gravity has to be tied back to composition, the Mole Fraction Calculator turns the per-component mole fractions in a finished blend into the average molecular weight that goes with the measured API value.

A dedicated API gravity calculator saves the time you would spend converting the same number four times in different units, and stops the small constant mistakes that show up when each conversion is done by hand.

• • API, SG, and density in one panel: Enter one number and read API, specific gravity, and density in four units at the same time.
• • Cragoe temperature correction: Apply the Cragoe equation so a reading made off 60 °F still reports the published API value.
• • Both input directions: Switch between API to SG and SG to API by changing the input mode.
• • Crude classification: See the light / medium / heavy / extra-heavy label next to the API number for the same interpretation as a production report.
• • Adjustable Cragoe coefficient: Pick the thermal expansion coefficient that matches the liquid for condensates, bitumens, and refined products.

Pick the same reference (60 °F) and the same standard density of water (999.0 kg/m³) for every value you compare. The calculator pins both to the same anchor.

Use the classification label as a sanity check. Two crudes with API = 30.0 and API = 32.0 both sound close, but the first lands at the top of the medium band and the second moves cleanly into the light band, which can change how a pipeline schedules them.

For laboratory work that starts from a small sample mass instead of a hydrometer reading, the Grams to Moles Calculator converts grams of sample into moles, the input shape that density and API calculations still consume downstream.

Four things change what an API reading really means. Review them before you trust the number on a report.

• • The Cragoe coefficient is a single linear value. It works for refined products and most crudes between 20 and 40 °API, but it can drift on heavy crudes, bitumens, and gas-condensate mixtures. For those, use a full ASTM D1250 table or a densitometer that already reports 60 °F values.
• • The calculator assumes a clean sample. Free water, sediment, or emulsified gas all change the effective SG, so the API reading has to be taken on a representative, degassed, water-free sample.
• • API gravity describes density, not viscosity, gas-oil ratio, or sulfur content. Two crudes with the same API can still have very different refining yields, so the API value should sit alongside viscosity and sulfur for blending or pricing decisions.

If the field temperature varies between the top and the bottom of a tank, take the API reading from a flow-through densitometer or a representative spot sample.

According to Engineers Edge Fluid Data, the Cragoe equation brings a specific gravity reading made at T_obs back to the 60 °F reference by SG_60 = SG_T × (1 + 0.00045 × (T_obs − 60)).