Pipe Sizing Calculator
Determine the optimal pipe size for your water supply system based on flow rate and velocity limits.
🎯 What This Calculator Solves
Give it a peak flow in gallons per minute and it returns the smallest standard tube size that carries that flow without exceeding a velocity ceiling, plus the velocity that size really produces. Velocity is one of two constraints on a supply line; the other is pressure. Size for both and let the stricter one win.
Formula Reference
Each material is sized on its own internal diameters, which matters more than it sounds: PEX has a markedly smaller bore than copper at the same nominal size, so a nominal 1 inch PEX line carries appreciably less than a nominal 1 inch copper one. Choosing a material changes the bore table, the velocity ceiling and the friction coefficient together. Copper is Type L; PEX and CPVC are copper tube size and stop at 2 inch; Schedule 40 materials run to 4 inch with markedly larger bores than copper at the same nominal size.
The answer is the first standard size at or above the required bore. Steps are coarse: 3/4 to 1 inch adds 70% more area. The list stops at 4 inch. Past roughly 300 GPM — or about 190 at a 5 ft/s hot-line limit — nothing in it carries the flow, and the tool says so above the result rather than quietly handing back its largest size. A service that big is engineered work.
📏 Where the 8 ft/s Ceiling Comes From
Not from a code section. The Copper Development Association's Copper Tube Handbook states: "To avoid excessive system noise and the possibility of erosion-corrosion, the designer should not exceed flow velocities of 8 feet per second for cold water and 5 feet per second in hot water up to approximately 140°F. In systems where water temperatures routinely exceed 140°F, lower flow velocities such as 2 to 3 feet per second should not be exceeded."
That split matters, and this calculator does not make it — it applies 8 ft/s to copper, PEX, CPVC and PVC on hot and cold lines alike. On a hot line, judge the reported velocity against 5 ft/s, not 8. Recirculating hot water is the classic erosion-corrosion failure.
Whether any velocity limit is enforceable where you work is jurisdictional: the International Plumbing Code and the Uniform Plumbing Code are model documents, and what binds you is the edition your state or city adopted plus local amendments. The IPC sits behind ICC's paywall, so no figure here is attributed to it.
🧮 Worked Example: 10 GPM in Copper
- 10 GPM ÷ 448.831 = 0.02228 ft³/s
- Required bore at 8 ft/s: √(4 × 0.02228 ÷ (π × 8)) = 0.0596 ft = 0.715 in
- First standard size at or above it: 3/4 inch, Type L bore 0.785 in
- Velocity delivered: 6.63 ft/s — inside the 8 ft/s cold limit, past the 5 ft/s hot one
- One size down, 1/2 inch (0.545 in bore): 13.75 ft/s
📐 The Bores Behind the Nominal Sizes
Copper tube is named by a nominal size matching neither its outside nor its inside diameter. Outside is always nominal plus 1/8 inch; the bore depends on wall thickness, which is what the type letter denotes.
| Nominal | Outside dia. | Type L bore | Type K bore | Type M bore |
|---|---|---|---|---|
| 1/2" | 0.625" | 0.545" | 0.527" | 0.569" |
| 3/4" | 0.875" | 0.785" | 0.745" | 0.811" |
| 1" | 1.125" | 1.025" | 0.995" | 1.055" |
| 1-1/4" | 1.375" | 1.265" | 1.245" | 1.291" |
| 1-1/2" | 1.625" | 1.505" | 1.481" | 1.527" |
| 2" | 2.125" | 1.985" | 1.959" | 2.009" |
Every copper bore in this tool is the Type L dimension from that table, 3.905 in at 4 inch included. The other materials come from their own dimensional standards, not from this one.
⚠️ What Goes Wrong
Sizing every branch on whole-building demand. A segment carries only the fixtures downstream of it.
Skipping the pressure half. Velocity sizing knows nothing about developed length or elbow count. A 3/4-inch line at a comfortable 5 ft/s can still arrive upstairs with nothing left — subtract 0.433 psi per foot of rise on top of the friction loss.
Reading trade guidance as a code minimum. The 8 ft/s figure comes from a copper trade body and is specific to copper. Plastics carry their own manufacturer limits.
❓ Questions People Actually Ask
How do I calculate water pipe size?
Add up the fixture units on the branch, convert that to a peak flow in GPM, then pick the smallest tube whose bore carries that flow below your velocity ceiling — the Copper Development Association's limit for copper is 8 ft/s cold and 5 ft/s hot to about 140°F. Then check pressure over the developed length, because a size that passes on velocity can still fail on pressure. This calculator does the velocity half only. It is a planning estimate, not an engineered design, and it does not replace a licensed plumber or your adopted plumbing code.
What is the rule of thumb for pipe sizing?
The nearest thing to one is a velocity ceiling, since velocity drives noise and erosion-corrosion. The Copper Development Association tells designers not to exceed 8 feet per second cold and 5 feet per second hot to roughly 140°F, dropping to 2 to 3 feet per second above that. Those are trade-association design recommendations, not code minimums, and they say nothing about whether pressure survives to the last fixture. Any thumb rule that skips the fixture-unit count and the pressure check will eventually undersize something.
Is 3/4 inch pipe too small for a main water line to a house?
That cannot be answered from the size alone — it turns on peak demand and the length of the run. For scale: 3/4-inch Type L copper has a 0.785-inch bore, and 10 gallons per minute through it runs at 6.63 ft/s, inside the Copper Development Association's 8 ft/s cold limit but above their 5 ft/s hot figure. The same flow in 1/2-inch Type L would run at 13.75 ft/s. Count fixture units and check pressure loss over the real developed length before deciding, and have a licensed plumber confirm the service size.
A planning estimate against a velocity ceiling — not a code compliance check, not a pressure calculation, not an engineered design. It does not know your static pressure, developed length, fitting count, water temperature or chemistry, or which code your jurisdiction adopted. Do not buy material or submit for permit on this number alone.
The velocity limits for copper, the outside diameters and Type K, L and M bores, and the Hazen-Williams arrangement used for the friction figure all come from the Copper Development Association's Copper Tube Handbook (CDA publication A4015-26/24) — the copper industry's trade body, so design guidance rather than regulation. The 0.433 psi per foot of rise follows from the density of cold water, which the USGS Water Science School gives as 62.408 lb/ft³ at 50°F. Fixture-unit loads are on our fixture units calculator; friction loss with fittings and elevation is on the pressure drop calculator. The questions above came from Google's People Also Ask panel for "how to size water pipe", "pipe sizing chart plumbing" and "pipe sizing calculator" on 31 July 2026.