How to Calculate Water Flow Rate: Complete Guide with Calculator
Learn how to calculate water flow rate through pipes. Step-by-step guide with flow rate calculator, formulas (Hazen-Williams, Darcy-Weisbach), and examples.
Step-by-Step Guide
1. Understand Flow Rate Formula
Basic formula: Q = A × V, where Q is flow rate (GPM), A is pipe cross-sectional area (π × r²), and V is velocity (ft/s). This is the continuity equation fundamental to fluid mechanics.
2. Measure Pipe Diameter
Determine the inside diameter of your pipe. For standard sizes: 1/2" = 0.622", 3/4" = 0.824", 1" = 1.049". Use actual inside diameter, not nominal size.
3. Determine Velocity
For water supply, maintain velocity between 4-8 ft/s. Lower velocities (2-4 ft/s) for drainage. Use Hazen-Williams or Darcy-Weisbach equations for pressure-driven flow calculations.
4. Use Hazen-Williams Formula
For water distribution: V = 1.318 × C × R^0.63 × S^0.54, where C is roughness coefficient (140 for copper, 150 for PEX), R is hydraulic radius, S is slope.
5. Calculate Flow Rate
Multiply area by velocity: Q = π × (d/2)² × V × 7.48 (conversion to GPM). Our Water Flow Rate Calculator handles all conversions and formulas automatically.