Water Pipe Size Calculator — Velocity & Pressure
Sizing a supply line means satisfying two separate rules, and the larger answer wins. First velocity: water moving too fast is noisy and erodes the pipe from the inside. Second pressure: after the lift to the top fixture, the fixture's own requirement and the meter have taken their cut, only so much psi is left to spend on friction. This calculator runs both and tells you which one governed.
Size your supply line
From the Water Supply Fixture Unit Calculator, or a measured figure if you have one.
Measured run plus fitting equivalents.
Static street or well pressure at the meter.
Costs 0.4331 psi per foot, regardless of flow.
8–15 psi typical; a flushometer wants about 25.
Plus any softener or backflow device.
ASPE and tube-manufacturer design practice, not a numeric IPC figure.
Required nominal size
Both rules land on this size — the next size down fails the velocity cap and the friction budget at the same time.
Velocity
7.00 ft/s
Loss per 100 ft
8.98 psi
Over the pressure budget
The chosen size satisfies velocity but spends more pressure than the system has. Shorten the run, reduce fittings, or raise the supply pressure.
See the breakdown
Planning estimate. Simultaneous-use diversity beyond the demand figure you enter, water quality, and existing piping are not modelled. A licensed plumber and your AHJ have final say.
The formula, explained in plain English
Build a pressure budget, then find the smallest pipe that lives inside it without moving the water too fast.
Why d to the power of 4.87
Friction is brutally sensitive to bore. Going up one size from 3/4 to 1 inch copper widens the bore by 31% and cuts the loss by roughly 73% at the same flow. This is why upsizing beats almost every other fix.
Nominal size is not the bore
Nominal 1 inch means 1.025 in of bore in Type L copper, 1.055 in Type M, 1.049 in Schedule 40 — and only 0.875 in PEX. Every calculation here uses the real inside diameter, computed from the outside diameter and wall thickness.
The lift is free of flow
Elevation costs 0.4331 psi per foot whether you are drawing 2 gpm or 200. It is the one loss in the system you cannot design away — and the reason top-floor fixtures are always the worst case.
Hot water gets a lower cap
Erosion-corrosion in copper accelerates with temperature, so hot lines are held to about 5 ft/s against 8 for cold. A recirculating loop that runs constantly is often designed lower still.
Worked examples
The defaults, the same job in PEX, and the long run where friction takes over from velocity.
Two-bathroom house main — 18 gpm, 120 ft
Type L copper · 60 psi supply · 20 ft to the highest fixture · 15 psi at the fixture · 8 psi meter · 8 ft/s cap. These are the calculator's defaults.
available = 60 − 8.66 − 15 − 8 = 28.34 psi
allowable = 28.34 ÷ 120 × 100 = 23.62 psi/100 ft
3/4": v = 11.93 ft/s ✗, loss = 32.91 psi/100 ft ✗
1": v = 7.00 ft/s ✓, loss = 8.98 psi/100 ft ✓
→ 1 inch — both rules agree
Result: 1 inch, using 10.77 psi of a 28.34 psi budget. The 3/4-inch pipe most people reach for fails on velocity by 49% and on friction by 39% simultaneously.
The same job in PEX — one size up
Identical inputs, material changed to PEX.
1" PEX: v = 0.4085 × 18 ÷ 0.875² = 9.60 ft/s ✗ over 8
1-1/4" PEX bore 1.069: v = 6.43 ft/s ✓
→ 1-1/4 inch PEX
Result: PEX needs a size up despite having the better roughness coefficient — 150 against copper's 140. Outside-diameter-controlled tubing trades bore for wall thickness, and the friction term cares about bore far more.
Long run to an outbuilding — 12 gpm, 400 ft
Type L copper · same 60 psi and 28.34 psi budget · developed length stretched to 400 ft.
3/4": v = 7.96 ft/s ✓ velocity, loss = 15.53 psi/100 ft ✗ friction
1": v = 4.67 ft/s, loss = 4.24 psi/100 ft ✓
→ 1 inch — friction governs alone
Result: at 120 ft this flow rides 3/4 inch comfortably; at 400 ft the same flow needs 1 inch on friction alone, while velocity still says 3/4 inch is fine. Distance, not flow, moved the answer.
Pipe size by demand
Type L copper over a 120-foot developed run on the default 28.34 psi friction budget — computed by the same code that runs the calculator above.
| Demand | Size | Velocity | Loss / 100 ft | Governed by |
|---|---|---|---|---|
| 5 gpm | 1/2" | 6.88 ft/s | 18.15 psi | both |
| 8 gpm | 3/4" | 5.30 ft/s | 7.33 psi | both |
| 12 gpm | 3/4" | 7.95 ft/s | 15.53 psi | both |
| 18 gpm | 1" | 7.00 ft/s | 8.98 psi | both |
| 25 gpm | 1-1/4" | 6.38 ft/s | 5.92 psi | velocity |
| 40 gpm | 1-1/2" | 7.21 ft/s | 6.07 psi | velocity |
The same 18 gpm in every material
Nominal size is not bore. This is why a material swap can quietly change the size you need.
| Material | Size | Actual bore | Velocity | Loss / 100 ft |
|---|---|---|---|---|
| Copper, Type L | 1" | 1.025 in | 7.00 ft/s | 8.98 psi |
| Copper, Type M | 1" | 1.055 in | 6.61 ft/s | 7.80 psi |
| PEX (SDR-9) | 1-1/4" size up | 1.069 in | 6.43 ft/s | 6.42 psi |
| CPVC, Schedule 40 | 1" | 1.049 in | 6.68 ft/s | 7.06 psi |
| PVC, Schedule 40 | 1" | 1.049 in | 6.68 ft/s | 7.06 psi |
| Galvanized steel, Schedule 40 | 1" | 1.049 in | 6.68 ft/s | 10.67 psi |
Sources & standards: IPC 2021 Appendix E — Sizing of Water Piping System; IPC 604.3 maximum fixture flow rates; pipe dimensions from ASTM B88 (copper), F876 (PEX) and D1785 (Schedule 40), with every inside diameter computed from outside diameter and wall thickness; velocity limits and the Hazen-Williams method per ASPE and the Copper Development Association. Appendix E applies only where adopted; UPC jurisdictions size differently. Local amendments override the model code.
Frequently asked questions
Common questions about water pipe sizing, velocity limits, and pressure budgets.
What size water line do I need for a house?
For a typical two-bathroom house drawing about 18 gpm over a 120-foot developed run on 60 psi, the answer is 1 inch Type L copper — 7.00 ft/s and 8.98 psi per 100 ft. Those are the defaults above. The 3/4-inch pipe most people guess at would run 11.93 ft/s and lose 32.91 psi per 100 ft, failing both tests at once.
Why are there two rules instead of one?
Because a pipe can pass one and fail the other, and you have to satisfy both — so the larger size wins. Velocity is a durability and noise problem: fast water is loud and erodes copper from the inside. Friction is a performance problem: a pipe that is thermally and acoustically fine can still fail to deliver pressure at the far fixture. Short runs are usually velocity-governed; long runs are friction-governed. The calculator names which one decided.
Does PEX need a bigger size than copper?
Often yes, by one size — and it catches people out. PEX is outside-diameter controlled, so nominal 1-inch PEX has a 0.875-inch bore against Type L copper's 1.025 inch. At 18 gpm copper needs 1 inch and PEX needs 1-1/4 inch. PEX has the better roughness coefficient (150 against copper's 140) and still loses, because bore beats smoothness — the friction term goes as diameter to the power of 4.87.
What is developed length, and why not just measure the pipe?
Developed length is the measured run plus the equivalent length of every fitting and valve, because an elbow costs pressure the same way a length of pipe does. On 3/4-inch copper a 90° elbow is worth about 2 feet of pipe and a tee taken through the branch about 4 feet. The Friction Loss Calculator adds them up for you.
What pressure does a fixture actually need?
It varies by fixture and it is a real design input, not a rounding error. Most ordinary fixtures want 8 to 15 psi at the inlet; a flushometer valve wants closer to 25 psi, and some tankless water heaters and body-spray showers specify more. The default here is 15 psi, which suits a normal residential mix. Check the fixture spec on anything unusual — it comes straight off the top of your budget.
Why does the meter get its own loss figure?
Because it is often the single largest fixed loss in a residential system, and it is easy to forget. A 5/8-inch meter at 20 gpm can cost 8 to 12 psi on its own. The default 8 psi is typical for a residential meter at moderate flow; get the real curve from the utility if the budget is tight, because the difference can be a whole pipe size.
Can I just size the pipe on velocity and ignore pressure?
Only on short runs. Velocity does not care how long the pipe is, but friction accumulates with every foot — so the two rules swap places as the run grows. At 18 gpm on this page's defaults, both land on 1 inch; stretch the developed length to 400 feet and the friction budget alone forces 1-1/4 inch while velocity still says 1 inch is fine.
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