Quick answer: For most residential water supply lines, use ¾" (20mm) pipe for single fixtures and 1" (25mm) for main supply lines carrying 10–20 GPM. Enter your exact flow rate above for a precise recommendation.
Related Calculators
What pipe size do I need for a 3-bedroom house?
A 3-bedroom house typically needs a 1" (DN25) main supply line to handle 55–75 L/min peak demand. Individual branch lines can be ½" (DN15) for toilets and basins, ¾" (DN20) for showers and washing machines.
What is the difference between nominal pipe size and actual diameter?
Nominal pipe size (NPS or DN) is a standardised label, not the actual dimension. A 1" nominal pipe has an actual outside diameter of 1.315" (33.4mm). The internal diameter varies by schedule or type — a copper Type L 1" pipe has an ID of 0.985".
How do I calculate pipe size for irrigation?
For irrigation, calculate the total flow rate of all zones running simultaneously in GPM or L/min. Use PVC (C=150) and keep velocity under 5 ft/sec (1.5 m/s) for lateral lines, under 7 ft/sec (2.1 m/s) for main lines. Enter your total flow and run length above.
What pipe material is best for drinking water?
Copper Type L is the gold standard — durable, antimicrobial and approved everywhere. PEX is the modern favourite for new construction — flexible, freeze-resistant and fast to install. PVC is fine for cold water only. Never use galvanised steel for new drinking water installations.
Can I use the same pipe size for hot and cold water?
Yes — pipe sizing is based on flow rate and pressure requirements, which are the same for hot and cold. However, PVC cannot be used for hot water (use CPVC or copper). PEX is suitable for both. Check the manufacturer's temperature rating before selecting materials.
What causes low water pressure in a house?
The most common causes are: undersized supply pipe, excessive pipe length creating pressure drop, partially closed valves, scale or corrosion buildup inside old pipes, or insufficient municipal pressure. Use this calculator to check if your pipe diameter is causing the pressure drop.
Pipe Size Calculator for Plumbers & Contractors
Free tool · Works offline · PVC, Copper, Steel, PEX · Metric & imperial
How to size a pipe for any flow rate
Choosing the wrong pipe diameter is one of the most expensive mistakes in plumbing. Too small and you get excessive pressure drop, noisy flow and premature wear on fittings. Too large and you waste money on materials and end up with low velocity that allows sediment to settle. This calculator uses the Hazen-Williams equation — the industry standard for water supply systems — to find the minimum internal diameter and the nearest nominal pipe size available off the shelf. Enter your flow rate, pipe material and run length and it does the rest.
What pipe size do I need for a house?
For a typical 3-bedroom house the main supply line should be at least 1" (25mm) to handle peak simultaneous demand — typically 15–20 GPM (55–75 L/min) when multiple fixtures run at once. Individual branch lines feeding single fixtures can be ½" (15mm) for toilets and basins, ¾" (20mm) for showers and washing machines. Always size for the peak flow, not the average — a supply line that's fine 95% of the time but starves your shower when someone runs the kitchen tap is the wrong size.
What is GPM and why does it matter?
GPM (gallons per minute) or L/min in metric measures how much water moves through the pipe per unit time. Every fixture has a design flow rate set by code: shower 2.5 GPM (9.5 L/min) · toilet 2 GPM (7.5 L/min) · kitchen sink 2.2 GPM (8.3 L/min) · hose bib 5 GPM (19 L/min) · washing machine 5 GPM (19 L/min). To correctly size a supply branch, add up the peak simultaneous demand of every fixture it serves — not all fixtures run at once, so use a diversity factor for large systems.
Pipe size by material — what changes?
Each pipe material has a different internal roughness, captured by the Hazen-Williams C coefficient. A higher C means smoother bore and less friction loss, so you can use a smaller pipe for the same flow. PVC (C=150) is the smoothest — ideal for cold water and irrigation. Copper Type L (C=130) is the gold standard for hot and cold supply. PEX (C=140) is increasingly popular for its flexibility and freeze resistance. Galvanized steel (C=120) has higher friction and is mainly found in legacy installations. Cast iron (C=100) has the roughest bore and needs the largest diameter for a given flow.
Velocity limits — the most important check
After sizing for flow, always check velocity. The ideal range for water supply is 2–4 ft/sec (0.6–1.2 m/s). Below 1 ft/sec (0.3 m/s) and sediment settles in horizontal runs. Above 8 ft/sec (2.4 m/s) and you risk noise, erosion of pipe walls, and water hammer — the bang you hear when a tap closes quickly. The velocity tab in this calculator checks any combination of pipe size and flow rate instantly.
How does pipe length affect sizing?
Pressure drop increases with length — double the run, double the friction loss. For short runs under 30 m (100 ft) the nominal size from the flow calculation is usually fine. For longer runs, always enter the actual length in the calculator. A ¾" pipe that handles 10 GPM perfectly over 20 ft may drop below acceptable pressure over 100 ft — you'd need to go up to 1" to compensate. This is especially important for garden irrigation, fire suppression lines and any run that feeds multiple fixture groups.
Frequently asked questions
What pipe size for 50 L/min (13 GPM)?
For 50 L/min (13 GPM) in copper, a DN32 (1¼") pipe keeps velocity around 1.1 m/s (3.6 ft/sec) — ideal. For runs over 60 m (200 ft), consider DN40 (1½") to reduce pressure drop below 4 psi/100ft.
What pipe size for a shower?
A single shower uses about 9.5 L/min (2.5 GPM). A ½" (15mm) pipe handles this easily. If the branch serves multiple bathrooms, calculate for the combined peak demand.
Does pipe length affect the diameter needed?
Yes. For runs over 30 m (100 ft), enter the actual length — you may need to go up one nominal size to keep pressure drop under 30 kPa/100m (4 psi/100ft).
What is the maximum water velocity in a pipe?
Most codes recommend a maximum of 2.4 m/s (8 ft/s) for water supply. Above this you risk noise, erosion and water hammer. Keep to 0.6–1.2 m/s (2–4 ft/s) for long-term reliability.