How to Use the Pipe Size Calculator
- Pick your units. USA mode uses gallons per minute (GPM), feet and inches. UK mode uses litres per minute (L/min), metres and millimetres.
- Enter the peak flow rate. This is the most water the pipe needs to carry at once, not the daily total. If you don't know it, use the fixture flow table below or the Pipe Flow Rate Calculator to measure what your supply delivers today.
- Choose the pipe material. Copper Type L, Copper Type M, PEX, PVC Schedule 40, or UK copper and MDPE. Each one has a different internal diameter for the same nominal size, which changes the result.
- Set the maximum velocity. The default is 8 ft/s (2.4 m/s) for cold water and 5 ft/s (1.5 m/s) for hot water. Lower it if you want a quieter system.
- Add the pipe length (optional). With a length, the calculator also shows the pressure you'll lose through friction on the way to the fixture.
The result shows the minimum internal diameter, the smallest standard pipe size that meets it, the actual velocity in that pipe, and the pressure drop.
How Pipe Sizing Works: Flow, Velocity and Diameter
Every water pipe sizing calculation starts with one relationship: flow rate = velocity × internal cross-section area. For a fixed flow, a smaller pipe forces water to move faster. Faster water means more noise, more friction loss, more erosion inside copper fittings, and a harder bang when a valve snaps shut (water hammer).
Rearranged to find the diameter you need:
- USA: minimum internal diameter (inches) = √(0.4085 × GPM ÷ velocity in ft/s)
- UK / metric: minimum internal diameter (mm) = √(4 × Q ÷ (π × v)) × 1,000, with Q in m³/s and v in m/s
The calculator then rounds up to the next standard size whose real internal diameter is at least that big. This matters because nominal sizes are labels, not measurements. A ¾-inch Type L copper pipe has an internal diameter of 0.785 inches, while ¾-inch PEX is only about 0.671 inches. The same "¾-inch" label carries noticeably less water in PEX.
Recommended maximum velocities
| Application | USA (ft/s) | Metric (m/s) | Why |
|---|---|---|---|
| Cold water supply, copper or plastic | 8 | 2.4 | Limits noise and water hammer |
| Hot water up to 140°F / 60°C, copper | 5 | 1.5 | Hot water erodes copper faster |
| Hot water recirculation loops | 2–4 | 0.6–1.2 | Constant flow, erosion risk |
| Quiet residential design target | 4–6 | 1.2–1.8 | Comfortable balance of size and noise |
The copper limits follow Copper Development Association guidance, and most designers apply similar limits to plastic pipe because noise and hammer don't depend on material.
Friction Loss: Why Pipe Length Matters
Velocity tells you whether a pipe is fast. Friction loss tells you whether enough pressure reaches the far end. Water loses pressure as it rubs along the pipe wall, and the loss grows with roughly the square of the flow (strictly the 1.85 power). The calculator uses the Hazen-Williams equation, the standard method for water in building and fire-sprinkler pipework:
- USA: pressure loss (psi per foot) = 4.52 × Q^1.85 ÷ (C^1.85 × d^4.87), with Q in GPM and d in inches
- Metric: head loss (m per m) = 10.67 × Q^1.852 ÷ (C^1.852 × d^4.87), with Q in m³/s and d in metres
C is the roughness coefficient. New copper is about 140, PEX and PVC about 150, and old galvanised steel can drop to 100 or lower as it scales up. That last figure explains why many older homes with galvanised lines have weak showers.
Worked example (USA)
A house needs a peak of 10 GPM through a 100-foot run of copper Type L.
- Minimum diameter at 8 ft/s: √(0.4085 × 10 ÷ 8) = 0.715 in. A ¾-inch Type L pipe (0.785 in) passes.
- Velocity in ¾-inch Type L: 6.6 ft/s. That's fine for cold water but too fast for hot.
- Friction loss in ¾-inch: about 11 psi per 100 ft. Over 100 feet that eats 11 psi of a typical 50–60 psi supply.
- Step up to 1-inch Type L: velocity drops to 3.9 ft/s and friction loss to about 3 psi per 100 ft.
So ¾-inch "works", but 1-inch is the better main line if the run is long or the fixtures are upstairs. The same flow through ¾-inch PEX hits 9.1 ft/s and about 24 psi per 100 ft, which is why PEX mains are often upsized one step.
Worked example (UK)
A combi boiler is rated for 12 L/min and you want to know whether the 15mm cold main feeding it is enough.
- 15mm copper (13.6mm bore) carries about 13 L/min at 1.5 m/s and about 17 L/min at 2 m/s.
- 22mm copper (20.2mm bore) carries about 29 L/min at 1.5 m/s.
- At 20 L/min, 15mm loses about 5 kPa (0.05 bar) per metre, while 22mm loses about 0.7 kPa per metre.
A 15mm feed can just about serve a small combi on a short run; for anything larger or longer, 22mm is safer. If you're replacing a boiler, the Boiler Size Calculator shows what flow rate each size of combi actually needs.
Pipe Size Chart: Flow Capacity by Size
Approximate maximum flow at 8 ft/s (cold) and 5 ft/s (hot), using actual internal diameters:
| Nominal size | Copper Type L ID | Cold max (8 ft/s) | Hot max (5 ft/s) | PEX ID | PEX cold max |
|---|---|---|---|---|---|
| ½ in | 0.545 in | 5.8 GPM | 3.6 GPM | 0.475 in | 4.4 GPM |
| ¾ in | 0.785 in | 12.1 GPM | 7.5 GPM | 0.671 in | 8.8 GPM |
| 1 in | 1.025 in | 20.6 GPM | 12.9 GPM | 0.862 in | 14.6 GPM |
| 1¼ in | 1.265 in | 31.3 GPM | 19.6 GPM | 1.054 in | 21.8 GPM |
| 1½ in | 1.505 in | 44.4 GPM | 27.7 GPM | 1.244 in | 30.3 GPM |
| 2 in | 1.985 in | 77.2 GPM | 48.2 GPM | 1.629 in | 52.0 GPM |
UK copper (EN 1057) at 1.5 m/s: 15mm ≈ 13 L/min, 22mm ≈ 29 L/min, 28mm ≈ 48 L/min, 35mm ≈ 75 L/min.
These are velocity limits only. On a long run, friction loss may push you up a size even when velocity is fine.
Typical Fixture Flow Rates
If you don't know your peak demand, add up the fixtures likely to run at the same time. Don't add every fixture in the house, because they rarely all run together.
| Fixture | USA typical flow | UK typical flow |
|---|---|---|
| Shower (standard head) | 1.8–2.5 GPM | 8–12 L/min |
| Bathroom basin tap | 1.0–1.5 GPM | 4–6 L/min |
| Kitchen sink tap | 1.5–2.2 GPM | 6–8 L/min |
| Bath tap filling | 4–6 GPM | 12–20 L/min |
| Toilet cistern refilling | 2–3 GPM | 6–9 L/min |
| Washing machine / dishwasher | 1.5–2 GPM | 6–8 L/min |
| Outdoor hose bib / garden tap | 4–5 GPM | 12–18 L/min |
US federal rules cap new showerheads at 2.5 GPM at 80 psi, and WaterSense-labelled heads use 2.0 GPM or less. A family home with two showers and a kitchen tap running together peaks around 6–8 GPM (25–30 L/min).
How Plumbing Codes Size Pipes (Fixture Units)
The calculator sizes by flow and velocity, which is how engineers check a design. Plumbing codes add a second method built around fixture units:
- USA: The Uniform Plumbing Code (UPC) and International Plumbing Code (IPC) give each fixture a water supply fixture unit (WSFU) value. A private-bathroom lavatory is about 1 WSFU and a bathtub about 4. You total the WSFU, convert to a probable peak flow using a demand table (based on Hunter's curve), then size the pipe from available pressure and the developed length to the farthest fixture.
- UK: BS EN 806-3 uses loading units (LU) in the same way. Each outlet gets an LU value, the total is converted to a design flow, and pipes are sized so velocity stays within limits. The Water Supply (Water Fittings) Regulations 1999 cover what you can connect.
Fixture-unit methods assume not everything runs at once, which is why a code-sized main for a 3-bathroom house is usually ¾ or 1 inch. If you're pulling a permit, your inspector will expect the code method, so use this calculator to check velocity and friction, and let your plumber produce the code calculation.
Common Pipe Sizes in a Home
USA
- Service line from the meter: ¾ inch for most homes; 1 inch for larger homes, long runs, or irrigation.
- Main distribution trunk: ¾ inch, stepping up to 1 inch in larger houses.
- Branches to fixtures: ½ inch. PEX "home-run" manifold systems commonly use ½ inch, or ⅜ inch to basins.
UK
- Incoming MDPE main: 25mm (about 20mm bore) is the modern standard; older homes often have ½-inch lead or 20mm MDPE.
- Rising main and combi feed: 15mm is common, 22mm is better.
- Bath taps: 22mm.
- Basin, sink and toilet: 15mm, often 10mm "microbore" for short tails.
Signs Your Pipes Are Undersized
- Shower pressure drops when someone flushes a toilet or runs the kitchen tap.
- Bath takes much longer to fill than it should.
- Banging or hammering when taps close quickly.
- Whistling or hissing in the walls when water runs.
- A combi boiler cuts out or runs lukewarm because flow falls below its minimum activation rate.
Low pressure can also come from the supply, a partly closed stop valve, or scaled galvanised pipe. Measure flow with a bucket and timer before replacing pipework.
Frequently Asked Questions
What size pipe do I need for 10 GPM?
For cold water in copper Type L, ¾ inch is the minimum (about 6.6 ft/s). For long runs, hot water, or PEX, use 1 inch to keep velocity and pressure loss down.
Is a ¾-inch or 1-inch water line better for a house?
A ¾-inch line suits most homes with one or two bathrooms and short runs. A 1-inch line is better for three or more bathrooms, long service lines, upstairs fixtures, irrigation, or low street pressure, because it loses roughly a third of the pressure for the same flow.
How many litres per minute can a 15mm pipe carry?
About 13 L/min at a comfortable 1.5 m/s and about 17 L/min at 2 m/s. You can push more through it, but noise and pressure loss rise quickly.
Should I use 15mm or 22mm pipe for a combi boiler?
22mm is recommended for the cold main feeding most combis, especially models over 30 kW or on runs longer than a few metres. 15mm can work for small combis on short runs with good mains pressure.
Does PEX need to be a bigger size than copper?
Often, yes. PEX has thicker walls, so its internal diameter is smaller at the same nominal size. ¾-inch PEX carries roughly 25–30% less water than ¾-inch Type L copper at the same velocity.
What is the maximum velocity for water in a pipe?
A common limit is 8 ft/s (2.4 m/s) for cold water and 5 ft/s (1.5 m/s) for hot water in copper. Many residential designers aim lower, around 4–6 ft/s, for quiet operation.
How do I size the main water line from the meter?
Total the fixture units (USA) or loading units (UK), convert to a design flow, then check velocity and friction loss over the full length from the meter, including elevation and any water meter or backflow device losses. Most homes end up with ¾ to 1 inch (USA) or 25mm MDPE (UK).
Can a pipe be too big?
Yes. Oversized hot water pipes hold more water, so you wait longer for hot water at the tap. Size for the flow you need, not the biggest pipe that fits.
Get a licensed plumber to confirm your pipe sizes
Code sizing depends on your street pressure, pipe run and local rules. Get local quotes before you buy pipe or fittings.