Calculate flow rate, velocity, pressure drop, and recommended pipe size for water and plumbing systems.
Choose a calculation mode using the tabs: Flow Rate (find GPM and pressure drop for a known pipe diameter), Find Pipe Size (determine the minimum diameter for a desired flow rate), or Pressure Drop (calculate head loss and PSI drop for a given pipe and flow). Enter your values, select the pipe material, and click Calculate. Results update the diagram above automatically.
Getting pipe sizing wrong is one of the most expensive mistakes in residential and commercial plumbing. Undersized pipes create excessive pressure drops — a 1-inch water main might look fine on paper, but at 20 GPM it can generate over 10 PSI of friction loss per 100 feet, starving fixtures and stressing the system. Oversized pipes waste money on material and installation, and in hot water systems they increase wait time and heat loss.
HVAC engineers sizing chilled-water loops typically target velocities between 2–6 ft/s to balance friction losses against pump energy costs. Fire suppression designers must meet NFPA 13 flow requirements, where pipe sizing directly affects sprinkler coverage. Even a simple home irrigation system benefits from proper sizing — a ¾-inch supply line can support up to 4 zones at moderate pressure, while pushing 5 zones causes annoying low-pressure issues at heads farther from the source.
This calculator uses the Darcy-Weisbach equation — the gold standard for friction head loss — combined with the Colebrook-White friction factor approximation, giving results accurate to within 1–2% for turbulent water flow.
The core relationships are:
Where D = diameter (ft), L = length (ft), V = velocity (ft/s), g = 32.174 ft/s², ε = pipe roughness (ft), ν = kinematic viscosity.
For residential cold water, 2–5 ft/s is the recommended range. Hot water recirculation loops are often kept below 3 ft/s to reduce noise and erosion on the copper pipe walls. Most plumbing codes allow up to 8 ft/s as an absolute maximum.
Nominal pipe size (NPS) is a label, not a measurement. For example, a ½-inch Schedule 40 PVC pipe has an actual OD of 0.840 inches and an ID of 0.622 inches. Always use the actual inner diameter for flow calculations — check a pipe schedule chart for your material and schedule.
No — this tool is designed for water (liquid) flow using Darcy-Weisbach. Gas line sizing involves compressibility, specific gravity corrections, and different pressure standards (AGA equations). Use a dedicated gas pipe sizing chart or calculator for natural gas or propane systems.
The Swamee-Jain approximation is accurate to within ±3% of the Colebrook-White equation for Reynolds numbers between 5,000 and 100,000,000 and roughness ratios of 0.000001 to 0.05. For practical engineering calculations in water supply and HVAC, this is more than sufficient precision.