Calculators / Surface Overflow Rate
Surface Overflow Rate calculator
Surface overflow rate is the flow divided by the clarifier's surface area. For a circular clarifier, area = 0.785 × diameter².
SOR (gpd/ft²) = Flow (gpd) ÷ Surface area (ft²) Area = 0.785 × D²
Worked example
A circular clarifier is 60 ft in diameter and receives 2,000,000 gpd. What is the surface overflow rate?
- Area = 0.785 × 60² = 2,826 ft²
- SOR = 2,000,000 ÷ 2,826 = 707.7 gpd/ft²
Answer: 707.7 gpd/ft²
Common mistakes
- Using radius in 0.785 × D². The 0.785 already accounts for it, so use the diameter.
- Leaving flow in MGD.
Check your exam's reference sheet for its exact constants. Some use 3.14 instead of 3.1416, which changes answers in the last digit.