Calculators / Flow Rate (Q = A × V)
Flow Rate (Q = A × V) calculator
Flow equals the pipe's cross-sectional area times the water's velocity. Exams give velocity in ft/s and ask for gpm or MGD, so you finish with a conversion.
Q (ft³/s) = A (ft²) × V (ft/s) Q (gpm) = Q (ft³/s) × 448.8
Worked example
Water flows through an 8-inch main at 2.5 ft/s. What is the flow in gpm?
- Area = 0.785 × (8 ÷ 12)² = 0.3489 ft²
- Q = 0.3489 × 2.5 = 0.8722 ft³/s
- Q = 0.8722 × 448.8 = 391.5 gpm
Answer: 391.5 gpm
Common mistakes
- Using the diameter in inches in the area formula.
- Mixing up 448.8 (ft³/s to gpm) and 7.48 (ft³ to gal).
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.