Pipe Sizing Calculator
Calculate pipe sizing accurately for your build. Get material quantities, waste allowances, and project cost breakdowns.
Formula
D = sqrt(4Q / (pi x V_max)) where Q = flow rate, V_max = maximum velocity
The minimum pipe inside diameter is calculated from the continuity equation. Convert the flow rate from GPM to cubic feet per second, then divide by the maximum allowable velocity to get the minimum cross-sectional area. The diameter is then found from the area using the circle formula. Select the next available standard pipe size with an inside diameter at least as large as the calculated minimum.
Worked Examples
Example 1: Residential Water Supply Main
Problem: Size a pipe for 15 GPM with a maximum velocity of 5 ft/s for a house supply line.
Solution: Q = 15 x 0.002228 = 0.03342 cfs\nA_min = 0.03342 / 5 = 0.006684 sq ft\nD_min = sqrt(4 x 0.006684 / pi) x 12 = 1.11 inches\nSelect 1-1/4 inch (ID = 1.38 in)\nActual velocity = 3.20 ft/s
Result: Recommended pipe size is 1-1/4 inch with actual velocity of 3.20 ft/s
Example 2: Commercial HVAC Chilled Water
Problem: Size a pipe for 50 GPM chilled water with a maximum velocity of 6 ft/s.
Solution: Q = 50 x 0.002228 = 0.1114 cfs\nA_min = 0.1114 / 6 = 0.01857 sq ft\nD_min = sqrt(4 x 0.01857 / pi) x 12 = 1.85 inches\nSelect 2 inch (ID = 2.067 in)\nActual velocity = 5.14 ft/s
Result: Recommended pipe size is 2 inch with actual velocity of 5.14 ft/s
Frequently Asked Questions
How do I determine the correct pipe size?
Pipe sizing starts with the required flow rate in GPM and a maximum allowable velocity. The minimum pipe inside diameter is calculated from the flow rate and velocity using the continuity equation: Area = Flow / Velocity. Then select the next standard pipe size with an inside diameter equal to or larger than the calculated minimum. For residential water supply, velocity should not exceed 5-6 ft/s. For commercial mains, up to 8 ft/s is acceptable. Fire protection systems may allow 10 ft/s.
What velocity should I use for pipe sizing?
The maximum velocity depends on the application. Residential water supply lines should stay below 5 ft/s to minimize noise and water hammer. HVAC hot and chilled water systems typically design for 4 ft/s in branches and 6-8 ft/s in mains. Steam lines operate at much higher velocities: 100-120 ft/s for superheated steam. Compressed air systems typically use 20-30 ft/s. Always check local codes as they may specify maximum velocities for different applications.
How does pipe material affect sizing?
Pipe material affects sizing through roughness (friction loss) and available sizes. Copper and PEX have very smooth interiors, resulting in lower friction losses and potentially allowing smaller pipe sizes for the same flow. Steel and iron pipes have rougher interiors, especially after years of service, requiring larger sizes to maintain adequate pressure. Material also determines the available wall thicknesses and standard sizes. Always size based on the actual inside diameter of the specific pipe type you are using.
Can I share or bookmark my calculation?
You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.
Is Pipe Sizing Calculator free to use?
Yes, completely free with no sign-up required. All calculators on NovaCalculator are free to use without registration, subscription, or payment.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.