Pipe Sizing & Pressure Drop Calculator

Estimate flow velocity and friction pressure loss for PEX, Copper Type L, and PVC Sch 40 pipe using Hazen–Williams — compare sizes and materials before you order pipe or lay out a branch run.

Pipe Details

Flow Rate (GPM)

Pipe Length (ft)

Material

Nominal Size

Live Results

Velocity

4.53 ft/s

Good velocity — within recommended range

Pressure Drop

0.0008 psi

Negligible drop — verify inputs, run may be very short

Inner Diameter

0.671 in

Hazen–Williams C

140

Material

PEX

Nominal Size

3/4"

Hazen–Williams estimates assume steady flow in full pipes. Verify sizing against local plumbing code and manufacturer specifications.

How to Use This Pipe Sizing Calculator

Enter the flow rate in GPM. Input the design flow rate in gallons per minute for the pipe run — fixture demand, branch circuit load, or irrigation zone total. Typical residential branch lines range from 2–8 GPM depending on fixture count and simultaneous use.
Enter the pipe length. Measure the total developed length of the pipe run in feet, including fittings as equivalent length if your jurisdiction requires it. Longer runs increase friction loss and pressure drop proportionally.
Select pipe material. Choose PEX, Copper Type L, or PVC Sch 40. Each material uses a Hazen–Williams C-factor that reflects interior smoothness — smoother pipes have higher C values and lower friction loss at the same flow.
Select nominal pipe size. Pick the nominal diameter from 1/2", 3/4", 1", or 1-1/4". The calculator uses published inner diameters for each material and size combination to compute velocity and pressure drop.
Review velocity and pressure drop. Results update instantly. Compare velocity against the 2–5 ft/s guideline range and check whether total pressure drop fits your available head pressure for the run.

Formulas & Example

Velocity and pressure drop use standard US plumbing forms of the Hazen–Williams relationship with inner diameter in inches, length in feet, flow in GPM, and C-factors for each pipe material.

Velocity (ft/s) = (0.408 × GPM) ÷ ID²
Pressure Drop (psi) = 0.001131 × Length × (GPM ÷ C)^1.852 × ID^−4.87

C-factors:
  PEX:           140
  Copper Type L: 150
  PVC Sch 40:    140

Worked Example

5 GPM through 50 ft of 3/4" PEX (ID = 0.671 in, C = 140):

Velocity = (0.408 × 5) ÷ 0.671² = 4.53 ft/s
Pressure Drop = 0.001131 × 50 × (5 ÷ 140)^1.852 × 0.671^−4.87 = 1.47 psi

Result: 4.53 ft/s velocity, 1.47 psi drop — within the 2–5 ft/s guideline

Upsizing to 1" PEX would lower velocity and pressure drop for the same flow. If velocity exceeds 5 ft/s or drop exceeds available head, try the next nominal size or a smoother material with a higher C-factor before reducing run length.

Frequently Asked Questions

What is the Hazen–Williams equation?▾

The Hazen–Williams formula estimates head loss and pressure drop in pressurized water pipes based on flow rate, pipe diameter, pipe length, and a roughness coefficient (C-factor). It is widely used in plumbing and fire protection for turbulent flow in full pipes. This calculator applies the standard form with GPM, inches, feet, and psi units common in US residential and light commercial work.

What is a good pipe velocity for residential plumbing?▾

Many designers target 2–5 ft/s for domestic water distribution. Below 2 ft/s, sediment can settle in horizontal runs; above 5 ft/s, erosion, noise, and water hammer risk increase — especially at valves and elbows. Hot water recirculation lines often aim for higher velocity to limit wait time, while main lines may accept slightly lower velocity when head pressure is limited.

Why do PEX, copper, and PVC have different pressure drops?▾

Each material has a different inner diameter for the same nominal size and a different Hazen–Williams C-factor. Copper Type L uses C = 150 with slightly larger IDs than PEX at the same nominal label. PVC Schedule 40 has the largest IDs listed here but C = 140. A larger inner diameter reduces velocity and friction loss; a higher C-factor also reduces calculated drop for the same flow.

Does pipe length affect pressure drop?▾

Yes — pressure drop is directly proportional to pipe length in the Hazen–Williams equation. Doubling the run length doubles the friction loss in psi, assuming the same flow, material, and size. Always use the actual developed length including offsets; some codes also add equivalent length for fittings and valves when sizing long or complex runs.

Can I use this calculator for hot water lines?▾

The Hazen–Williams method applies to both hot and cold water in typical plumbing temperature ranges. Hot water has slightly lower viscosity, which can reduce friction marginally, but the difference is small enough that most field sizing uses the same tables and C-factors. Verify final sizing against local code, fixture requirements, and manufacturer limits for PEX or CPVC temperature and pressure ratings.

Related Tools

Browse all Plumbing calculators · Full directory