This water line size and GPM calculator turns flow into a pipe size. Enter a known flow in gallons per minute, or count your fixtures and let the IPC fixture-unit tables estimate peak demand. It then finds the smallest PEX, copper Type L or M, or CPVC size that stays under your velocity limit, using each material's real inside diameter. Three more modes estimate pressure loss with the Hazen-Williams formula, measure GPM with a bucket test, and work out the volume of water in a pipe.
Every result is preliminary. Water pipe sizing is set by your adopted plumbing code, and IPC Appendix E itself says it is not mandatory unless the local ordinance adopts it. Have a licensed plumber or designer confirm the final size.
What GPM means
GPM stands for US gallons per minute, the volume of water passing through a pipe, faucet or pump each minute. NIST Handbook 44 defines the US gallon as exactly 231 cubic inches (3.785411784 liters), and a cubic foot is 1,728 cubic inches, so one cubic foot holds 7.4805 gallons. Flow (GPM) and velocity (feet per second) are different things: the same 10 GPM moves slowly through a wide pipe and fast through a narrow one. Pipe sizing is about keeping that velocity, and the friction it causes, within limits.
How to use the calculator
- Pick a mode: pipe size, pressure loss, bucket test or pipe volume.
- For sizing, choose the material and enter either a flow in GPM or a count of each fixture. Add continuous flows such as hose bibbs or irrigation zones in GPM.
- Choose a velocity limit that matches the material and water temperature (see the table below).
- Read the smallest size that passes, its velocity and its friction loss per 100 ft. The chart shows every size against your limit.
- Switch to pressure loss to check the chosen size over your real run length and rise.
How to calculate gallons per minute with a bucket test
The simplest gallons per minute calculation needs a bucket and a stopwatch. Alabama Cooperative Extension describes the method: mark a known volume on the bucket, open the valve fully while starting the timer, and stop the timer when the water reaches the mark. Then GPM = gallons ÷ seconds × 60. Their example, 5 gallons in 22.33 seconds, gives 13.4 GPM. Flow is highest when nothing else on the system is running, so test with other taps closed when you want the capacity of a line, or with typical demand running when you want a realistic figure. Repeat two or three times and average.
GPM to FPS: flow velocity in a pipe
Velocity = flow ÷ cross-sectional area. With flow in GPM and inside diameter d in inches, that simplifies to ft/s = 0.4085 × GPM ÷ d², because one GPM is 1 ÷ 448.83 cubic feet per second. The factor below is the velocity each size gives per 1 GPM, so multiply it by your flow.
| Size | PEX (SDR9) | Copper Type L | Copper Type M | CPVC CTS (SDR11) |
|---|---|---|---|---|
| 1/2 in | 1.81 | 1.38 | 1.26 | 1.71 |
| 3/4 in | 0.91 | 0.66 | 0.62 | 0.80 |
| 1 in | 0.55 | 0.39 | 0.37 | 0.48 |
| 1 1/4 in | 0.37 | 0.26 | 0.25 | 0.32 |
| 1 1/2 in | 0.26 | 0.18 | 0.18 | 0.23 |
| 2 in | 0.15 | 0.10 | 0.10 | 0.14 |
Example: 6 GPM through 1/2 in PEX is 6 × 1.81 = 10.9 ft/s, above every common limit. The same flow in 1/2 in Type L copper runs at 8.3 ft/s. PEX has thicker walls, so its bore is about 85–87% of copper's at the same nominal size.
Velocity limits by pipe material
| Material and service | Maximum | Source |
|---|---|---|
| Copper, cold water | 8 ft/s | Copper Tube Handbook |
| Copper, hot water to about 140°F | 5 ft/s | Copper Tube Handbook |
| Copper, water routinely above 140°F | 2–3 ft/s | Copper Tube Handbook |
| PEX, water up to 80°F | 10 ft/s | PPI |
| PEX, 80°F to 140°F | 8 ft/s | PPI |
| PEX hot-water recirculation return | 2 ft/s | PPI citing IAPMO IS 31-2023 |
| Any material (general note) | 5–8 ft/s | IPC Appendix E |
High velocity causes noise, water hammer and, in copper, erosion-corrosion. PPI adds that PEX can survive more than 8 ft/s, but the pressure drop at those speeds makes it impractical for most designs. For CPVC we apply the general IPC 5–8 ft/s note.
Sizing from water supply fixture units
When you don't know the peak flow, IPC Appendix E estimates it. Each fixture gets a load in water supply fixture units (wsfu), and Table E103.3(3) converts the total into GPM. The table is not linear, because not every fixture runs at once: 1 wsfu is 3 GPM, but 100 wsfu is only 43.5 GPM.
| Fixture | Total wsfu |
|---|---|
| Bathroom group (flush tank) | 3.6 |
| Bathtub | 1.4 |
| Shower head | 1.4 |
| Lavatory | 0.7 |
| Water closet (flush tank) | 2.2 |
| Kitchen sink | 1.4 |
| Dishwasher | 1.4 |
| Washing machine (8 lb) | 1.4 |
| Laundry tray | 1.4 |
| Bidet | 2 |
| Load (wsfu) | Demand (GPM) |
|---|---|
| 1 | 3 |
| 2 | 5 |
| 3 | 6.5 |
| 5 | 9.4 |
| 8 | 12.8 |
| 10 | 14.6 |
| 12 | 16 |
| 15 | 17.5 |
| 20 | 19.6 |
| 25 | 21.5 |
| 30 | 23.3 |
| 40 | 26.3 |
| 50 | 29.1 |
| 60 | 32 |
| 80 | 38 |
| 100 | 43.5 |
A bathroom group already includes its toilet, lavatory and tub or shower, so don't count those again. Fixtures that run continuously, such as hose bibbs, sprinklers and cooling equipment, can't be converted with fixture units: IPC says to add them in GPM. Between table rows the calculator interpolates in a straight line; your code official may require you to read the next higher row instead.
Worked example: a two-bath house
Two bathroom groups (2 × 3.6), a kitchen sink, a dishwasher and a washing machine (3 × 1.4) total 11.4 wsfu, which the table turns into about 15.6 GPM. Add one hose bibb at 5 GPM, the value used in the IPC sample problem, and the design flow is 20.6 GPM. At an 8 ft/s limit that needs 1 1/4 in PEX (7.6 ft/s) or 1 1/4 in Type L copper (5.3 ft/s).
Inside diameters: PEX, copper Type L and M, CPVC
Velocity and friction depend on the bore, so every calculation here uses the actual inside diameter. The PEX values are PPI's averages under ASTM F876. The copper values are the nominal dimensions in the Copper Tube Handbook. The CPVC values are the average outside diameter minus twice the minimum wall from an ASTM D2846 submittal, and they match the internal volumes in IPC Table E105.1.
| Size | PEX (SDR9) | Copper Type L | Copper Type M | CPVC CTS (SDR11) |
|---|---|---|---|---|
| 1/2 in | 0.475 · 4.4 gpm | 0.545 · 5.8 gpm | 0.569 · 6.3 gpm | 0.489 · 4.7 gpm |
| 3/4 in | 0.671 · 8.8 gpm | 0.785 · 12.1 gpm | 0.811 · 12.9 gpm | 0.715 · 10.0 gpm |
| 1 in | 0.862 · 14.6 gpm | 1.025 · 20.6 gpm | 1.055 · 21.8 gpm | 0.921 · 16.6 gpm |
| 1 1/4 in | 1.054 · 21.8 gpm | 1.265 · 31.3 gpm | 1.291 · 32.6 gpm | 1.125 · 24.8 gpm |
| 1 1/2 in | 1.244 · 30.3 gpm | 1.505 · 44.4 gpm | 1.527 · 45.7 gpm | 1.329 · 34.6 gpm |
| 2 in | 1.629 · 52.0 gpm | 1.985 · 77.2 gpm | 2.009 · 79.0 gpm | 1.739 · 59.2 gpm |
Type M copper has thinner walls than Type L, so it carries slightly more at the same velocity. Check which type your code allows for the job.
Pressure loss with the Hazen-Williams formula
Friction loss in feet of head is hf = 10.434 × L × Q^1.85 ÷ (C^1.85 × d^4.8655), with L the length in feet, Q the flow in GPM and d the inside diameter in inches. This is the form in an Indiana Department of Health worksheet, and it matches the University of Florida IFAS version for C = 150. Multiply feet of head by 0.433 to get psi. IPC Appendix E bases its fitting allowances on C = 150, and UF IFAS uses 150 for plastic pipe, which is why that is the default.
The IPC method is a pressure budget. Start from the minimum static pressure at the main. Subtract the pressure the highest fixture needs (at least 8 psi flowing for flush tanks and 15 psi for flushometer valves), 0.433 psi per foot of rise, and meter, backflow preventer and filter losses. What is left can be spent on pipe friction. For a first trial size, IPC adds 50% to the developed length to allow for fittings.
Worked example: 12 GPM through 3/4 in Type L copper
With 80 ft of pipe plus 50% for fittings (120 ft equivalent) and C = 150, friction is 16.46 psi. A 15 ft rise costs 6.50 psi more. From 55 psi at the source, about 32.0 psi is left before meter and equipment losses. That comfortably clears 8 psi, and the velocity of 8.0 ft/s is under the 8 ft/s cold-water limit.
Pipeline volume: how to calculate the volume in a pipe
Volume = π ÷ 4 × d² × length. With d in inches and length in feet, gallons = 0.7854 × d² × (feet × 12) ÷ 231. Knowing the volume tells you how much water to flush when you disinfect a new line, how much antifreeze to add, and how long you wait for hot water: 50 ft of 3/4 in PEX holds 0.92 gallons, so at 1.5 GPM it takes about 37 seconds to flush.
| Size | PEX (SDR9) | Copper Type L | Copper Type M | CPVC CTS (SDR11) |
|---|---|---|---|---|
| 1/2 in | 0.92 | 1.21 | 1.32 | 0.98 |
| 3/4 in | 1.84 | 2.51 | 2.68 | 2.09 |
| 1 in | 3.03 | 4.29 | 4.54 | 3.46 |
| 1 1/4 in | 4.53 | 6.53 | 6.80 | 5.16 |
| 1 1/2 in | 6.31 | 9.24 | 9.51 | 7.21 |
| 2 in | 10.83 | 16.08 | 16.47 | 12.34 |
Common sizing mistakes
- Using the nominal size as the bore. 3/4 in PEX is 0.671 in inside. Plugging in 0.75 in understates velocity by about 20%.
- Adding GPM for every fixture. Peak demand is estimated from total fixture units, not the sum of each fixture's flow.
- Forgetting equipment losses. Meters, backflow preventers, softeners and tankless heaters can take far more pressure than the pipe, as IPC Appendix E warns.
- Sizing the service below 3/4 in. IPC E104.1 sets 3/4 in as the minimum water service pipe.
For related work, convert pressure readings with the water column to psi converter, find the weight of a tank of water with the weight per gallon calculator, or size electrical feeders with the separate wire size calculator.
Frequently asked questions
What does GPM mean?
GPM means US gallons per minute, the volume of water that passes a point each minute. One US gallon is exactly 231 cubic inches (NIST Handbook 44), so 1 GPM is 231 cubic inches or about 3.785 liters every minute.
How do I calculate gallons per minute?
Time how long a container of known size takes to fill, then GPM = gallons ÷ seconds × 60. A 5-gallon bucket that fills in 22.33 seconds gives 13.4 GPM, the worked example Alabama Cooperative Extension uses.
How do I convert GPM to FPS?
Velocity in ft/s = GPM ÷ (448.83 × inside-diameter area in square feet), or, with the inside diameter d in inches, ft/s = 0.4085 × GPM ÷ d². Always use the real inside diameter, not the nominal size.
What size water line for 10 GPM?
At 8 ft/s, 3/4 in PEX runs 10 GPM at 9.1 ft/s, too fast, so PEX needs 1 in (5.5 ft/s). Type L copper passes at 3/4 in (6.6 ft/s). A long run or low street pressure may still need the next size up.
What is the maximum water velocity in a pipe?
The Copper Tube Handbook says not to exceed 8 ft/s for cold water and 5 ft/s for hot water up to about 140°F in copper. PPI recommends at most 10 ft/s for PEX with water up to 80°F and 8 ft/s from 80°F to 140°F. IPC Appendix E notes velocities over 5 to 8 ft/s are not usually recommended.
How many fixture units is a bathroom?
IPC Table E103.3(2) assigns a private bathroom group with a flush-tank toilet 3.6 water supply fixture units in total (2.7 cold, 1.5 hot). A private kitchen sink, dishwasher, bathtub, shower and 8 lb washing machine are 1.4 each, and a lavatory 0.7.
How do I calculate the volume of water in a pipe?
Volume in gallons = π ÷ 4 × d² × length in inches ÷ 231, with d the inside diameter in inches. For 100 ft of 3/4 in PEX (0.671 in ID) that is about 1.84 gallons.
What is the minimum water service pipe size?
IPC Section E104.1 says the minimum size water service pipe is 3/4 inch. Your local code may differ, and the meter and service size also depend on developed length and pressure.
How much pressure is lost per foot of rise?
IPC Appendix E uses 0.433 psi per foot of elevation. Lifting water 20 ft to the highest fixture costs about 8.66 psi before any friction losses.
Can I use this calculator to size a whole house?
Use it for a first estimate only. A design also needs meter, backflow preventer and equipment losses, fitting counts and the minimum pressure required at each fixture. Your local plumbing code and inspector have the final say.
Sources & method
- International Code Council — 2024 IPC Appendix E: Sizing of water piping system (Tables E103.3(2), E103.3(3), E104.1, E105.1)
- Plastics Pipe Institute — Designing PEX Plumbing Systems (dimensions per ASTM F876, velocity limits)
- Copper Development Association — The Copper Tube Handbook (Tables 2b, 2c; velocity limits), university-hosted copy
- Charlotte Pipe — FlowGuard Gold CPVC CTS SDR 11 submittal (ASTM D2846 OD and wall)
- University of Florida IFAS Extension — Hydraulic considerations (Hazen-Williams, C = 150)
- Indiana Department of Health (Miami County) — Hazen-Williams equation worksheet
- Alabama Cooperative Extension System — Measuring water flow rate (bucket test)
- NIST Handbook 44 (2026), Appendix C — gallon = 231 cubic inches
Results are estimates for general information. Found an error? It helps everyone — see our methodology.