SeriesCalc Logo

SeriesCalc

🛢️ API Gravity Calculator

Calculate API gravity, specific gravity, and density of petroleum liquids per API standards.

Inputs

°API

Results

API Gravity
0.00 °API
Specific Gravity (SG)
0.0000
Density (kg/m³)
0.0 kg/m³
Density (lb/gal)
0.000 lb/gal
Oil Classification

Density Comparison

Water (SG=1)Oil (SG=0.00)Floats on water

Calculator Description

The API gravity calculator converts between the petroleum industry's API-degree scale, specific gravity (SG) and density of crude and refined petroleum liquids. API gravity is an inverse scale — the lighter the liquid (SG below 1), the higher the API number — and is used for grading, trading and refinery design.

What this calculator finds

Enter specific gravity (SG at 60°F) to get API gravity, or enter API gravity to obtain SG and density, with classification into light/medium/heavy/extra-heavy crude.

Why it matters

  • Grading crude oil and pricing it (lighter crudes command a premium)
  • Estimating refinery yield and converting tank/pipe volumes to mass
  • Assessing buoyancy/sink behaviour for marine transport and storage

Formula

API Gravity Equations

API gravity is defined relative to specific gravity at 60°F (15.6°C), an inverted scale with water fixed at SG = 1.0.

1. API Gravity (°API)

∘API=141.5SG−131.5^{\circ}\text{API} = \dfrac{141.5}{SG} - 131.5

2. Specific Gravity (SG at 60°F)

SG=141.5∘API+131.5SG = \dfrac{141.5}{^{\circ}\text{API} + 131.5}

Density conversion: ρ = SG × 999.0 kg/m³ (or SG × 62.4 lb/ft³), where 999.0 kg/m³ is the density of water near 60°F.

  • °API — API gravity (dimensionless scale, water ≈ 10°API)
  • SG — Specific gravity = liquid density / water density (at 60°F, dimensionless)
  • ρ — Liquid density [kg/m³ or lb/ft³]

Crude oil classification

  • Light Crude — > 31.1 °API (lighter than water)
  • Medium Crude — 22.3 ~ 31.1 °API
  • Heavy Crude — 10 ~ 22.3 °API
  • Extra Heavy — < 10 °API (sinks in water, e.g. bitumen)

How the formula works

  • At SG = 1.0 (water) °API = 10; as SG decreases (lighter liquid) °API rises sharply.
  • API 10 is the water reference: above it the liquid floats on water, below it sinks.

Worked example

For crude with SG = 0.85: °API = 141.5 / 0.85 − 131.5 ≈ 35.0°API, a light crude, with density 0.85 × 999.0 ≈ 849 kg/m³.

Useful Tips

Practical tips

  • Use SG corrected to 60°F (SG₆₀/₆₀); for other temperatures apply ASTM-IP correction tables.
  • Light crudes yield more gasoline/kerosene; heavy crudes give more residue/asphalt.
  • When trading by volume (barrels), always convert to mass using density.

Limitations & cautions

  • This scale is petroleum-specific; do not confuse it with a generic density-to-SG conversion (which may reference 4°C water).
  • Blended liquids are not volume-additive (volume shrinkage), so simple ratio conversion introduces error.