Pipe Size Calculator

What size water pipe do I need?

gpm
Water

Use at least

3/4inch Type L copper

Velocity limits for copper from the copper industry design guide. Pressure loss and fixture-unit tables in your plumbing code can require a larger pipe. For gas or drain piping, use the code tables and a licensed plumber.

0.785 in inside12 gpm at 8 fpsLimit 8 fps
Minimum inside diameter0.783 in
1/2 in Type L16.5 fps
3/4 in Type L8 fps
1 in Type L4.7 fps
1-1/4 in Type L3.1 fps
1-1/2 in Type L2.2 fps
2 in Type L1.2 fps

Find the smallest Type L copper that keeps water velocity inside published limits for your flow.

How it works

Velocity in feet per second equals 0.4085 × gpm ÷ inside diameter squared. This picks the first size that stays at or under the limit you chose.

Before you order

  • Fast water is noisy and wears copper, especially hot water near elbows.
  • Long runs often need a larger size because of pressure loss even when velocity looks fine.
  • Your plumbing code fixture-unit tables decide legal sizing, so check them before you buy.

Common questions

Does this size gas pipe?

No. Gas piping uses code tables by length and pressure, and a licensed plumber should size and pressure-test it.

What about PEX or CPVC?

They have different inside diameters and velocity limits, so use the maker's tables.