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SZ-03Sizing · Plumbing

Water flow and velocity calculator

How fast the water moves for a given flow and pipe, checked against copper’s velocity guidance, with the friction loss, the pressure lost to the rise and the pressure left at the outlet.

Velocity and pressure loss

Example: 8 gpm through 120 ft of 3/4" Type L copper plus 30 ft for fittings, rising 30 ft, from 60 psi. Change any number.

Average velocity Example

5.30 ft/s in 3/4" Copper Type L; under the 8 ft/s guidance. Friction 6.49 psi per 100 feet; 37.28 psi left at the outlet.

8 gpm in 3/4" Copper Type L · ID 0.785 in

CDA’s guidance for copper: 8 ft/s for cold water. Under it by 2.70 ft/s.

Friction loss
6.49psi per 100 ft · C 150
Left at the outlet
37.28psi, flowing · 257 kPa
Change the numbers
Velocity against the limitDrawn to scale: the arrow is the water’s average speed
Water velocity in the pipe against the guidance limit, for the example, drawn to scalePipe with an inside diameter of 0.785 inches; the water's average velocity, 5.30 ft/s, drawn as an arrow to scale, under the 8 ft/s limit (CDA, cold water). Pressure: 60 psi at the start, 9.73 psi friction, 12.99 psi to the rise, 37.28 psi left.CDA, cold water8 ft/s5.30 ft/sPressure, start to outlet60 psi at the start37.28 psi leftEvery psi the same width · numbered parts below123
  1. Left at the outletstart − rise − friction − other37.28 psi
  2. FrictionHazen-Williams, pipe plus fitting allowance9.73 psi
  3. Lifting the water0.433 psi per foot of rise12.99 psi

Inside diameter: CDA Copper Tube Handbook Table 14.2b. Friction: Hazen-Williams with C = 150 — Copper Development Association, Copper Tube Handbook (A4015-26/24), Table 14.6 (C = 150).

Friction, run
9.73 psiover 150 ft
Lost to rise
12.99 psi30 ft up
Flow at limit
12.07gpm at 8 ft/s

0.4085 × 8 ÷ 0.785² = 5.30 ft/s
4.52 × 81.85 ÷ (1501.85 × 0.7854.87) × 100 = 6.49 psi per 100 ft

Estimate for clean water in full pipes. Fittings, meters, backflow preventers, filters and water heaters add losses you enter yourself.

Already showing the example.

What changes the answer

  • One size up (1") drops the velocity to 3.11 ft/s and friction to 1.77 psi per 100 ft.
  • At 8 ft/s this pipe carries 12.07 gpm.
  • If the pipe were rougher (C = 120, like old galvanized), friction would be 9.80 psi per 100 ft.

Change the numbers

Units: feet and gpm, or meters and L/min
Flow and pipe

The flow this line must carry.

ID 0.785 in

Water
The run

Straight pipe, measured.

Extra length for them, from the maker or CDA’s handbook.

Height of the outlet above the start; negative if it sits lower.

Pressure (optional)

Measured where the run begins.

Meter, backflow preventer, filters, heater: from their data.

Pipe roughness

Default for Copper Type L: 150. Higher is smoother; older pipe is rougher.

Velocity: 5.30 ft/s · friction 6.49 psi/100 ft · 37.28 psi left. Back to the result

How the math works

Velocity is flow divided by the pipe’s inside cross-section; friction comes from the Hazen-Williams formula in the form the Copper Development Association prints. Both use the real inside diameter of the material and size you pick.

  1. Velocitygpm × 231 ÷ 60 ÷ (π ÷ 4 × ID²) ÷ 12 = 0.4085 × gpm ÷ ID² ft/s231 cubic inches make one U.S. gallon (NIST Handbook 44).
  2. Flow at a limitft/s × 12 × π ÷ 4 × ID² × 60 ÷ 231
  3. Friction per foot4.52 × gpm^1.85 ÷ (C^1.85 × ID^4.87) psiCDA Copper Tube Handbook, Table 14.6 note. EPA’s EPANET states the same formula in feet and cubic feet per second; converted, it agrees within 0.3%.
  4. Friction for the runpsi per foot × (length + fitting allowance)Fitting allowances are yours to enter; the calculator does not reproduce anyone’s equivalent-length table.
  5. Risefeet of rise × 0.433 psiCDA uses 0.434. A negative rise, an outlet below the start, adds pressure.
  6. Left at the outletstart − rise − friction − other lossesThe pressure while the water flows, before the fixture itself.
Where every number comes from 23 values, each with its source
Velocity, copper, cold
8 ft/sCopper Development Association, Copper Tube Handbook (A4015-26/24), "Water Velocity Limitations", p. 15
Velocity, copper, hot
5 ft/sCopper Development Association, Copper Tube Handbook (A4015-26/24), p. 15 (hot water up to about 140 °F)
Copper above 140 °F
2–3 ft/sCopper Development Association, Copper Tube Handbook (A4015-26/24), p. 15 (water routinely above 140 °F: 2 to 3 ft/s)
Other materials
8 ft/sWisconsin Adm. Code SPS 382.40(7)(e) (one state's rule, shown as a common design limit)
Hazen-Williams
4.52 × Q^1.85 ÷ (C^1.85 × d^4.87)Copper Development Association, Copper Tube Handbook (A4015-26/24), Table 14.6 note 4 (Hazen-Williams)
C, copper
150Copper Development Association, Copper Tube Handbook (A4015-26/24), Table 14.6 (C = 150)
C, PEX
150Site choice: U.S. EPA, EPANET 2.2 Users Manual, Ch. 3 gives 140–150 for new plastic; PPI PEX Design Guide Table 7.13 reproduces with 150
C, PVC
150Site choice: U.S. EPA, EPANET 2.2 Users Manual, Ch. 3 gives 140–150 for new plastic
C, CPVC
150Site choice: U.S. EPA, EPANET 2.2 Users Manual, Ch. 3 gives 140–150 for new plastic
C, steel
120Site choice: U.S. EPA, EPANET 2.2 Users Manual, Ch. 3 gives 140–150 for new steel and 120 for galvanized iron. 120 is a site default; new steel 140–150, old pipe lower (EPANET).
Pressure per foot of water
0.433 psiSite arithmetic: USGS Water Density, 62.366 lb/ft³ at 60 °F ÷ 144 = 0.43310. CDA uses 0.434.
Pressure-reducing valve
above 80 psi static2018 IPC model text (§604.8); your state may have adopted a different edition or amendments
Low-velocity note
below 1 ft/sSite choice (no source gives a laminar threshold in ft/s; the note is qualitative)
ID, Copper Type K
1/4" to 12"CDA Copper Tube Handbook Table 14.2a
ID, Copper Type L
1/4" to 12"CDA Copper Tube Handbook Table 14.2b
ID, Copper Type M
3/8" to 12"CDA Copper Tube Handbook Table 14.2c
ID, PEX (CTS, SDR 9)
3/8" to 2"PPI PEX Design Guide Table 7.11; Uponor submittal (1/2"–2")
ID, PVC Schedule 40
1/4" to 12"Spears Dimensions & Pressure Ratings (avg I.D.); GF Harvel Clear PVC
ID, PVC Schedule 80
1/4" to 12"Spears (avg I.D.); GF Harvel PVC 80 specification
ID, CPVC Schedule 40
1/4" to 12"Spears CPVC Sch 40 (avg I.D., same as PVC Sch 40)
ID, CPVC Schedule 80
1/4" to 12"Spears CPVC Sch 80 (avg I.D., same as PVC Sch 80)
ID, CPVC CTS (SDR 11)
1/2" to 2"Spears SDR 11 CTS (avg I.D.); Charlotte FlowGuard Gold walls
ID, Steel Schedule 40
1/2" to 10"Wheatland Tube Standard Pipe (ASTM A53 / ASME B36.10M). Unconfirmed: Galvanized pipe: no source read says whether the zinc coating changes the inside diameter; the same IDs are used for black and galvanized Schedule 40.

Before you use the number

  • Clean water in full pipes. Estimate for clean water in full pipes. Fittings, meters, backflow preventers, filters and water heaters add losses you enter yourself. Hazen-Williams was developed for turbulent flow (EPANET); below about 1 ft/s treat the friction figure as rough.
  • C changes with age and material. New copper and plastic sit near 150; old steel can be much rougher (EPANET), and CDA says materials other than copper need allowances for corrosion and scale.
  • Velocity limits are guidance. CDA’s 8 ft/s cold and 5 ft/s hot apply to copper, and CDA suggests lower velocities for 1/2-inch and smaller tube. Wisconsin writes 8 ft/s into its code. The model IPC sets no number but requires velocity to be controlled to reduce water hammer. In PPI’s erosion test PEX tolerated more than 12 ft/s; valves and fittings set its limit.
  • Code sizing is different. Plumbing codes size supply piping with their own fixture-unit tables and methods; this tool does not apply them and does not say a size passes. Check the code your jurisdiction has adopted — the IPC or UPC, in the edition and with the amendments your state uses.
  • Static pressure over 80 psi. IPC §604.8 (IPC 2018 text; check the edition your jurisdiction enforces) requires a pressure-reducing valve where static pressure exceeds 80 psi.

Sources

  1. Copper Tube Handbook (A4015-26/24): water velocity limitations, Table 14.6 (Hazen-Williams), pressure per foot of waterCopper Development Association
  2. EPANET 2.2 Users Manual, Chapter 3: The Network Model (Hazen-Williams, roughness by material)U.S. EPA (USEPA/EPANET2.2 on GitHub)
  3. Design Guide: Residential PEX Water Supply Plumbing Systems, 2nd ed., Chapter 3 (erosion) and Chapter 7 (tables)Plastics Pipe Institute
  4. Wisconsin Administrative Code SPS 382.40 (water distribution design, 8 ft/s)Wisconsin Legislature
  5. Dimensions & Pressure Ratings (average I.D.)Spears Manufacturing
  6. Standard Pipe brochure (Schedule 40, ASTM A53)Wheatland Tube
  7. Water DensityU.S. Geological Survey
  8. NIST Guide to the SI, Appendix B.8National Institute of Standards and Technology
  9. Handbook 44 (2026), Appendix CNational Institute of Standards and Technology
  10. 2024 International Plumbing Code, Chapter 6: Water Supply and DistributionInternational Code Council, Digital Codes (free to read)
  11. Read IAPMO Codes Online (Uniform Plumbing Code)IAPMO

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