API Gravity & Fluid Blending
Mix different hydrocarbon streams together and calculate the resulting API gravity, specific gravity, and viscosity of the final blend.
Hydrocarbon Streams
Enter the volume, specific gravity (or API gravity), and optional viscosity for each fluid stream. SG and API will auto-convert as you type.
Reference Information
Blend Classification
| Classification | API Range |
|---|---|
| ● Extra Heavy | < 10.0° |
| ● Heavy | 10.0° - 22.3° |
| ● Medium | 22.3° - 31.1° |
| ● Light | 31.1° - 50.0° |
| ● Condensate | > 50.0° |
Global Benchmarks
API Gravity is an inverse measure of density. Higher API means lighter fluid. Viscosity blending uses the non-linear Refutas index (VBI) based on mass fractions.
About This Tool
The API Gravity & Fluid Blending Calculator is a vital engineering utility for predicting the physical properties of hydrocarbon mixtures. Whether you are blending crude oils in a terminal, mixing product streams in a refinery, or calculating pipeline hydraulics, predicting the final specific gravity and viscosity is critical.
While density (and thus specific gravity and API gravity) blends relatively linearly based on volume or mass, kinematic viscosity is highly non-linear. This calculator employs the industry-standard Baird & Refutas equation to accurately predict the viscosity of complex petroleum blends.
Engineering Methodology & Equations
This calculator utilizes two primary mathematical models based on established thermodynamic principles:
1. API Gravity & Density (ASTM D287)
The relationship between Specific Gravity (SG) and API Gravity is defined by the equation:
API = (141.5 / SG) - 131.5. When blending streams, the calculator converts all inputs to mass and volume to calculate the true blended density.
2. Viscosity Blending (Refutas Method)
Because mixing a heavy oil (100 cSt) with a light oil (10 cSt) does not result in a 55 cSt blend, we must use the Viscosity Blending Index (VBI):
VBI = 10.975 × ln(ln(v + 0.8)) + 14.534
The calculator converts the kinematic viscosity (v) of each stream into its VBI, blends the VBI using mass fractions, and then mathematically reverses the formula to find the final kinematic viscosity of the blend.
Frequently Asked Questions
Yes, this calculator supports dynamic multi-stream blending for up to 10 distinct fluid streams simultaneously.
No, kinematic viscosity does not blend linearly. A 50/50 mix of a high-viscosity and low-viscosity fluid will result in a blend that is heavily skewed toward the lower viscosity. This is why we use the Refutas VBI index.
Related Fundamentals
Input / Output Units
Specific to the active calculator.