How do I Change API Gravity to Specific Gravity?

You can convert this oil's API gravity to specific gravity.

API gravity is a scale developed by the American Petroleum Institute to measure the specific gravity of petroleum products. Pegged to a reference temperature of 60 degrees Celsius, specific gravity (SG) is related to API gravity by this formula: SG = 141.5/(API gravity + 131.5). Hence, an API gravity of 10 corresponds to a specific gravity of 1, which is that of pure water. API gravity decreases with increasing specific gravity. It's a simple matter to evaluate the above expression to determine the specific gravity corresponding to a given API gravity.

Things You'll Need

• Pencil
• Paper
• Calculator

Instructions

1. Write down the API gravity you wish to convert to a specific gravity. For example, say you have a somewhat heavy oil with API gravity of 21 degrees. Write that on your paper.

2. Add 131.5 to obtain the denominator in the equation: 21 + 131.5 = 152.5.

3. Divide 141.5 by your answer: 141.5/152.5 = 0.9279. This solves the specific gravity equation. Write down the result.

4. Check your work at an online conversion site such as the Conversion Calculators site offered by PetEng Calculators. Near the bottom of the page, under "API Gravity Conversion," Enter "21" in the box under "°API." Click on the "Calculate" button and read the answer in the box beneath "Specific Gravity." The answer is 0.9279, which agrees with your calculation.

Tips & Warnings

• Specific gravity calculations are based on a reference temperature of 60 degrees Celsius. To obtain an accurate answer you must correct the gravity to account for the actual temperature.

References

• Integrated Publishing: Calculate Specific Gravity

Resources

• PetEng Calculators: Oilfield Units Conversion Calculators
• PetEng Calculators: Corrected Oil Gravity & Volume Correction Factor