API to SG Calculator
Convert American Petroleum Institute gravity to specific gravity at 60°F instantly. This premium calculator helps refiners, traders, laboratory teams, fuel distributors, and engineering professionals estimate density behavior, compare fluid classes, and visualize how an oil sample relates to water and common petroleum products.
Interactive API Gravity to Specific Gravity Calculator
Expert Guide to Using an API to SG Calculator
An API to SG calculator converts API gravity into specific gravity, one of the most common density-related values used across petroleum production, transport, refining, laboratory testing, quality control, and commodity trading. While API gravity is an industry-standard way to describe how light or heavy a petroleum liquid is relative to water, many engineering calculations, shipping documents, blending models, and scientific references still rely on specific gravity. That is why a fast and accurate conversion tool is so useful.
The standard relationship is straightforward: a petroleum liquid with higher API gravity is lighter, and a liquid with lower API gravity is heavier. Water sits at a specific gravity of approximately 1.000 at its reference condition, while most crude oils and refined products fall below that value. The classic conversion used by the petroleum industry is:
Specific Gravity at 60°F = 141.5 / (API Gravity + 131.5)
API Gravity = (141.5 / Specific Gravity) – 131.5
This means an oil with an API gravity of 35 is lighter than an oil with an API gravity of 20. Since specific gravity directly represents density relative to water, it becomes especially practical when you need to estimate mass from volume, compare products across technical documents, or interpret lab data from different systems.
What API gravity means in practical terms
API gravity was developed to simplify density communication within the petroleum industry. Instead of reporting every crude or fuel sample only by density, professionals often use API gravity because it makes commercial and operational comparisons easy. A high API number generally indicates a lighter hydrocarbon stream, often associated with higher yields of gasoline, naphtha, and lighter distillates. Lower API values indicate heavier materials, often associated with heavier fuel fractions, vacuum residues, and bituminous streams.
- Above 31.1 API: commonly described as light crude
- Between 22.3 and 31.1 API: commonly described as medium crude
- Below 22.3 API: commonly described as heavy crude
- Very low API values: often linked to extra-heavy oils or bitumen-like materials
These categories are widely used in trading and refinery planning, although exact commercial definitions can vary by market or operator. A conversion to specific gravity helps bridge the commercial language of API gravity with the engineering language of density and mass balance.
Why specific gravity matters
Specific gravity is dimensionless, meaning it expresses a ratio rather than a direct unit like kilograms per cubic meter or pounds per gallon. This ratio compares a substance to water at a specified reference temperature. For petroleum work, the reference is traditionally 60°F. Once you know the specific gravity, you can estimate density and proceed toward tank calibration, loading calculations, blending estimates, and custody transfer support.
- Mass estimation: Convert fluid volume into approximate mass for logistics or process work.
- Product comparison: Compare one hydrocarbon stream with water or other fuels.
- Blending analysis: Use density relationships in formulation and refining studies.
- Quality control: Validate whether a measured sample falls in the expected product range.
- Documentation alignment: Translate between API-based and density-based standards.
How the API to SG calculation works
The formula used in this calculator is the recognized API gravity relationship at standard reference conditions. If you enter an API gravity of 10, the calculated specific gravity will be close to 1.000, which makes sense because 10 API corresponds approximately to water. If you enter an API gravity above 10, the resulting specific gravity will fall below 1.000, indicating a liquid lighter than water. If you hypothetically enter a value below 10, the resulting specific gravity can exceed 1.000, which indicates a material heavier than water.
For example:
- API 40 gives a specific gravity of about 0.825
- API 35 gives a specific gravity of about 0.850
- API 25 gives a specific gravity of about 0.904
- API 15 gives a specific gravity of about 0.966
This relationship is one reason API gravity is intuitive once you work with it regularly: higher API means lower specific gravity and usually lower density.
Typical ranges for common fuels and crude streams
Although exact values vary with composition and temperature, the following table provides realistic industry-style ranges. These figures are helpful for screening and sanity-checking a calculated result. If your result falls far outside an expected range, you may be dealing with a temperature correction issue, a product misclassification, or an input error.
| Material | Typical API Gravity Range | Approximate Specific Gravity Range at 60°F | Operational Note |
|---|---|---|---|
| Gasoline | 50 to 70 | 0.787 to 0.711 | Very light product with lower density and high volatility. |
| Jet Fuel / Kerosene | 35 to 50 | 0.850 to 0.779 | Middle distillate range, typically denser than gasoline. |
| Diesel | 30 to 40 | 0.876 to 0.825 | Common transport fuel with moderate density. |
| Light Crude Oil | 31.1 and above | Below about 0.870 | Often preferred for higher light product yield. |
| Medium Crude Oil | 22.3 to 31.1 | About 0.920 to 0.870 | Broad commercial category in refining and trading. |
| Heavy Crude Oil | Below 22.3 | Above about 0.920 | Requires more intensive upgrading or processing. |
Reference statistics and density conversions
Specific gravity becomes even more useful when paired with direct density estimates. Since water is approximately 999 to 1000 kilograms per cubic meter near standard reference conditions, multiplying specific gravity by roughly 999 to 1000 can give a practical density estimate in kilograms per cubic meter. For many engineering approximations, practitioners use 999.016 kg/m³ at 60°F as a convenient water reference. That is the value used in many quick field calculations, while highly precise laboratory work may use stricter standards and correction tables.
| API Gravity | Specific Gravity at 60°F | Approximate Density kg/m³ | Approximate Density lb/gal |
|---|---|---|---|
| 10 | 0.999 | 998 | 8.33 |
| 20 | 0.934 | 933 | 7.78 |
| 30 | 0.876 | 875 | 7.30 |
| 40 | 0.825 | 824 | 6.87 |
| 50 | 0.780 | 779 | 6.50 |
When to use an API to SG calculator
This calculator is especially helpful when reading assay reports, validating product certificates, preparing transport paperwork, checking inventory estimates, or translating data between different standards and stakeholders. For example, a crude purchaser may receive assay data in API gravity, while an internal engineering model may expect specific gravity or density. Similarly, a laboratory team may have a hydrometer-derived API reading but need a specific gravity estimate for a downstream mass calculation.
Common use cases include:
- Estimating cargo mass from storage volume
- Classifying crude streams as light, medium, or heavy
- Comparing refined products by relative density
- Supporting tank farm inventory planning
- Interpreting petroleum assay sheets
- Preparing educational or technical reports
Important temperature considerations
One of the most common sources of confusion is temperature. API gravity and specific gravity are reference-based values, not universal constants independent of temperature. Petroleum liquids expand as temperature rises and contract as temperature falls. The standard API relationship normally assumes a 60°F basis. If your field measurement was taken at a significantly different temperature, the raw observed density may need to be corrected to standard conditions before a strict comparison is made.
That means this calculator is best understood as a standard-condition converter, not a full petroleum volume correction engine. If you need highly accurate custody transfer, tax reporting, regulatory compliance, or contractual settlement values, use the applicable ASTM or API correction standards and certified tables rather than a simple educational calculator.
How to interpret your result correctly
After the calculation, focus on three numbers: specific gravity, density estimate, and optional mass estimate. Specific gravity tells you the fluid’s relative heaviness versus water. Density in kilograms per cubic meter or pounds per gallon gives you a practical engineering value. If volume is entered, the mass estimate helps with planning and rough logistics. For instance, a 1000-liter volume of a fluid with a density of 850 kg/m³ corresponds to about 850 kilograms of mass. This is an approximation but often very useful for screening work.
Also pay attention to product context. A specific gravity of 0.84 might be completely normal for a jet fuel-like stream but may be unexpectedly low or high for another category. The comparison chart below the calculator exists to make that quick visual check easier.
Limitations of simplified conversions
No simple converter can replace full laboratory density analysis or standards-based temperature correction. Composition matters. Sulfur content, paraffinicity, aromatic content, dissolved gases, and blending behavior can all influence a product’s physical properties. A single API value is useful, but it does not fully describe viscosity, vapor pressure, flash point, or distillation behavior. Therefore, this calculator should be used as a first-line decision tool, not as the only source for commercial or safety-critical conclusions.
Authoritative resources for further reading
If you want to go deeper into petroleum statistics, density references, and measurement science, review these reliable sources:
- U.S. Energy Information Administration (EIA) definition of API gravity
- National Institute of Standards and Technology (NIST) for measurement science and reference practices
- U.S. Department of Energy for broader petroleum and energy context
Best practices for professionals using API and SG data
- Always confirm the temperature basis of the reported value.
- Use standard-condition data for apples-to-apples comparison.
- Check whether your workflow needs density, specific gravity, or corrected volume.
- Validate unusual results against expected product ranges.
- For custody transfer and compliance, rely on applicable industry standards and certified data.
In short, an API to SG calculator is a practical bridge between petroleum trading language and engineering calculation language. It lets you move quickly from an API gravity number to specific gravity, density, and rough mass estimates, which supports better decisions in operations, laboratory workflows, education, and technical reporting. Used carefully and with proper awareness of temperature and standards, it is one of the most convenient petroleum property conversions available.