Plumbing Calculators

Choose a pipe with confidence

Pipe Size Calculator

Use this water pipe size calculator to choose a pipe for a new run from GPM or L/min, pipe material and preferred water speed, or check whether an existing pipe is large enough and has enough pressure for a long run.

Pipe sizing tool

Find or check a pipe size

Start with a common setupStarting values only. They do not calculate fixture demand, so review the material and every value before calculating.

1 Start with the basics

Tell us how much water the pipe needs to carry, what it is made of, and how fast you want the water to move.

Enter the most water this section needs to carry at one time, not your total daily water use.

Don’t know the water amount? Use the Water Flow Rate Calculator to estimate it from a simple measurement or the details you know about the system. If you only know the fixtures, start with the Plumbing Fixture Calculator.

Different pipe materials have different inside openings, even when the printed size is the same.

5 ft/s is a starting design preference, not a universal code limit. Choose the project criterion that applies; a lower preference usually selects a larger pipe.

2 Hot water or custom settingsOptional

This only changes the suggested starting water speed. Check that your chosen pipe is suitable for the water temperature and pressure.

3 Check pressure for a long runOptional

For a longer run, add the pipe length, pressure at the start, height change and the pressure you need at the end. Add fittings and equipment only when you know their details.

Measure along the pipe path from the start to the end of this run.

Use pressure while water is running if you can. A no-flow reading can look higher than the pressure you will have in use.

Enter 0 when the end is at the same level or lower.

psi

Use the pressure needed by the fixture, valve or appliance at the end of this run.

ft

Add the length allowance published for fittings and valves. Leave this at 0 if you do not have that information.

psi

Use the manufacturer’s loss number at your water flow, or leave this at 0 when there is no device.

Pressure at start−Height change−Pipe and fittings−Equipment=Pressure at end

Why the material matters

Nominal Pipe Size Is Not the Inside Diameter

Three pipe cutaways with different wall thicknesses and different openings for water, showing why pipes with the same printed size can carry water differently.
The inside opening is measured across the space where water flows, not across the outside of the pipe.

The size printed on a pipe is a label, not the size of the opening that water flows through. Pipes with the same printed size can have different wall thicknesses, so the opening can be different too. Less room inside means water has to move faster.

That is why the calculator asks you to choose the pipe family first. A 1 in copper pipe and a 1 in plastic pipe may not give water exactly the same amount of room. The calculator uses the opening for the material you select, rather than treating every pipe with the same printed size as identical.

When you compare a result with a pipe you plan to buy, match the pipe family as well as the printed size. If the product is a different type, check its manufacturer data or run the calculation again with the closest family shown here.

The same water in a different-size pipe

Why a Larger Pipe Can Help

The same water moving through small, medium and large pipes; water moves fastest through the narrow pipe and more slowly through the larger pipes.
Smaller pipeWater moves fasterRecommended sizeMatches your preferenceLarger pipeWater moves more slowly

The same amount of water has to move faster through a smaller opening. A larger pipe gives water more room to move, which can help leave more pressure at the end of the run.

This can be especially useful for a long route, a pipe feeding more than one outlet, or a run with several bends, valves, filters or other equipment. Slower-moving water usually loses less pressure along the way, so the fixture at the far end has a better chance of getting the flow you expect.

Bigger is not automatically better. It can cost more, take up more room and may not fit the connections already in place. Start with the recommended size, then use the optional long-run pressure check if you need to know whether the pipe will still leave enough pressure at the end.

A simple example

Pipe Size Example: 10 GPM Cold-Water Run

A pipe that needs to carry 10 GPM

For 10 GPM of cold water, a 3/4 in Copper Type L pipe makes the water move faster than the chosen 5 ft/s preference. Moving up to 1 in gives the water more room to flow and brings the speed down to about 3.9 ft/s.

  • Water to carry: 10 GPM
  • Pipe: Copper Type L
  • Water: Cold
  • Preferred water speed: 5 ft/s
  • Suggested pipe: 1 in; water moves at about 3.9 ft/s

What this example checks: water speed only. It does not include pressure, fittings, valves, meters or filters.

This example does not check pressure.For a long run, add the pipe length, starting pressure, height change, fittings and any equipment that water passes through.

How your result is chosen

How We Choose a Pipe Size

We start with the amount of water you need to move and the water speed you prefer. Then we choose the first listed pipe with an inside opening that is big enough. The pipe data and sources used are shown below.

Pipe-family data and sources Reviewed September 2026
Pipe familyDimensional basis usedFriction screenSource
Copper Type LASTM B88 Type L drawn tube dimensionsC = 140Copper Development Association
Copper Type MASTM B88 Type M drawn tube dimensionsC = 140Copper Development Association
PEX-A, ASTM F876 SDR 9Uponor AquaPEX SDR 9 product dimensionsC = 150Uponor AquaPEX dimensions
CPVC CTS SDR 11CTS SDR 11 dimensional basisC = 150Charlotte Pipe Technical Manual
PVC Schedule 40 / 80ASTM D1785 schedule dimensionsC = 150Charlotte Pipe Technical Manual
Steel Schedule 40ASME B36.10 Schedule 40 dimensionsC = 100ASME B36.10 dimension reference
Data and methodologyReviewed September 2026

Calculation basis

Velocity sizing uses continuity: flow divided by selected maximum velocity establishes a theoretical minimum bore. The result then rounds up to a listed nominal size with a sufficient documented inside diameter. The optional pressure screen uses Hazen–Williams friction, elevation and entered device loss.

Review approach

Inputs and result language are editorially reviewed for clarity and consistency. The page’s scope and local-code boundary are explained in “What This Calculator Can and Cannot Do” below.

Review record

Last reviewed: September 26, 2026
Review status: Editorially reviewed.
Source record: ASTM B88, ASTM F876 SDR 9, ASTM D1785 and ASME B36.10 dimensional bases; linked source pages rechecked on the review date.

Corrections and change log

Report a data, dimensional or calculation issue through Contact. We review the source, record material corrections and update the review date when a dataset or method changes.

v1.1 · September 2026: Added existing-pipe checks, fitting equivalent length, device loss, shareable inputs and result-specific warnings.

Before you install

What This Calculator Can and Cannot Do

This tool helps you choose or check a water pipe that stays full while water is moving through it. It does not work out drain-pipe size, decide how much water a group of fixtures needs, or replace your local plumbing rules. Before installing anything, check the pipe’s product instructions, the conditions at your site and the rules that apply where you live. For permits or unusual jobs, have the design checked by the right qualified professional.

Read IPC 2024 Chapter 6: Water Supply and Distribution

Pipe-size questions

Pipe Size Calculator FAQs

How do I check a pipe that is already installed?

Choose “Check my existing pipe,” then select the material and printed size that match the pipe. The result shows its inside opening and water speed for the flow you entered. If the run is long, open the pressure check and add the route details before deciding whether to keep it.

Why did the calculator suggest a larger size than the pipe I expected?

The suggestion is based on the room inside the pipe, not its printed label. Your chosen water speed, pipe material and wall thickness can all lead to a larger size. A larger pipe slows the water down and can preserve more pressure over a long distance.

Should I include elbows, valves, filters and a water meter?

For a quick starting size, you can leave them out. When checking a long run, add the extra path caused by bends and valves as fitting length. Add a known pressure loss for equipment such as a filter, backflow device or meter. Use product information when it gives a pressure-loss value.

What length should I enter for a long-run pressure check?

Enter the distance water travels along the pipe, including horizontal and vertical sections. Do not use a straight line from the start to the fixture. Enter the height change separately, because a fixture above the supply point needs some pressure just to lift water to that level.

What if I do not know how much water the pipe needs to carry?

Start with the Water Flow Rate Calculator if you can measure water filling a container or know details about the system. If you are planning a group of fixtures and need to estimate demand first, use the Plumbing Fixture Calculator, then return here with that flow.

Can I use this calculator for hot water?

Yes. Choose the hot-water setting, then confirm that the pipe product is rated for your water temperature and pressure. The setting adjusts the starting water-speed guidance only; it does not confirm whether a specific pipe, fitting or connection is approved for the job.

Can I use this to size a drain or sewer pipe?

No. Drain pipes work differently because they are designed to carry wastewater with air space inside, usually by gravity. Use the Drain Pipe Slope Calculator for the fall of a drain run, and follow the plumbing rules that apply to the installation.