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Pipe Size Calculator

Undersized pipes cause low water pressure and flow problems. This calculator determines the right pipe diameter for a given flow rate, keeping velocity within safe limits to prevent water hammer and noise.

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Results

Enter your measurements to see results

How to Use This Calculator

  1. Enter the required flow rate in gallons per minute.
  2. Select maximum velocity (8 ft/s is standard residential max).
  3. Select pipe material.
  4. Review the recommended pipe size.

How Pipe Size Calculation Works

Water flow through a pipe depends on the pipe's cross-sectional area and the velocity of the water. This calculator divides the required flow rate by the maximum acceptable velocity to find the minimum pipe area, then converts to diameter. Velocities above 8 ft/s in residential systems cause water hammer (banging pipes), noise, and accelerated pipe erosion. Standard nominal pipe sizes are 1/2", 3/4", 1", 1-1/4", 1-1/2", and 2".

Formula

Area = GPM ÷ (Velocity × 449 ÷ 144). Diameter = √(4 × Area ÷ π). Round up to nearest standard pipe size.

Frequently Asked Questions

What pipe size for a typical house?
Most 2–3 bathroom homes need a 3/4" main supply line. Homes with 4+ bathrooms or long runs from the meter should use 1". Individual fixture branches are typically 1/2".
What causes low water pressure with new pipes?
Usually undersized pipe for the demand. When multiple fixtures run simultaneously, velocity spikes and pressure drops. The fix is a larger main line or home-run piping with a manifold.
PEX vs copper — does pipe material affect sizing?
Slightly. PEX has a smoother interior (lower friction) but PEX fittings reduce the internal diameter more than copper sweat fittings. In practice, use the same nominal size for both materials.

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