Plumbers
Pipe Size Calculator
Determine the right pipe diameter for your water supply line based on flow rate and velocity.
Minimum pipe ID required: 0.825″ at 6 ft/s. Nearest standard size: 1" (ID: 1.049″). Applies to copper, PVC, CPVC, and PEX. Local codes and pressure calculations may require a larger size.
How to size supply pipes
Pipe sizing comes down to flow rate and pressure. Every fixture in a house has a flow rate measured in gallons per minute. Add up the fixtures on a line, apply a diversity factor because not everything runs at once, and size the pipe to deliver that flow without losing too much pressure. Undersized pipes mean weak flow at the fixtures. This calculator helps you pick the right pipe diameter for the job.
Fixture unit method explained
Plumbers use fixture units instead of raw GPM because it simplifies the math. A bathroom sink is 1 fixture unit. A toilet is about 2.5. A shower is 2. You add up all the fixture units on a line and look up the pipe size in a table. More fixture units means a bigger pipe. This is the method most plumbing codes reference and what this calculator is based on.
Common residential pipe sizes
The main water line coming into a house is usually 3/4 inch or 1 inch. Branch lines to individual fixtures are typically 1/2 inch. Showers and bathtubs often get 3/4 inch lines for better flow. The hot water heater feed and output are usually 3/4 inch. If you have a lot of fixtures or long runs, upsizing the main line to 1 inch makes a big difference in overall pressure.
Common Questions
Can I use PEX instead of copper pipe?
Yes. PEX is cheaper, faster to install, and handles freezing better than copper. It is approved in most areas for residential water supply. Some places still require copper for the first 18 inches from the water heater.
What size pipe for a garden hose bib?
A standard outdoor hose bib gets a 1/2 inch supply line. If you are running a long distance from the main line, bump up to 3/4 inch so you do not lose pressure.
Does pipe size affect water pressure?
Pipe size affects flow rate, not pressure directly. But if the pipe is too small for the demand, the pressure at the fixture drops because the water cannot flow fast enough. Bigger pipe means better flow.
How long does copper pipe last?
Copper supply pipes last 50 to 70 years in most conditions. Acidic water or high chlorine can shorten that. If you see green stains at joints, the copper is starting to corrode.
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