Pipe Sizing Calculator
How the formula works
Pipe sizing by the velocity method is the quickest way to choose a pipe for a given water flow. Water flowing in a full pipe obeys the continuity equation: flow equals velocity multiplied by the flow area of the pipe. If you know two of these values, you can find the third.
1. Required internal diameter
d = square root of (4 x Q / (pi x v))
2. Velocity in a known pipe
v = Q / A = 4 x Q / (pi x d^2)
3. Flow in a known pipe
Q = v x A = v x pi x d^2 / 4
4. Flow area
A = pi x d^2 / 4
Where
- Q = flow in m3/s (1 L/s = 0.001 m3/s, 1 m3/h = 1/3600 m3/s, 1 gpm = 0.0000631 m3/s)
- v = velocity in m/s, d = internal diameter in metres, A = flow area in m2
- 1 L/s = 3.6 m3/h = 15.85 gpm (US)
Typical design velocities (common practice)
- Pump suction line: about 1.0 m/s (3 ft/s), usually in the range 0.6 to 1.5 m/s
- Hot water line: about 1.2 m/s (4 ft/s), kept low to avoid erosion and noise
- Cold water distribution: about 1.5 m/s (5 ft/s)
- Pump discharge line and riser: about 2.0 m/s (7 ft/s), usually up to 2.4 m/s
- Maximum limit used in this calculator: 3.0 m/s (10 ft/s)
How to use the calculator
- Select the calculation, the unit system and the flow unit.
- For pipe size: enter the flow, select the pipe type and the design velocity.
- For velocity: enter the flow, then select the pipe type and the nominal size, or type your own internal diameter.
- For flow: select the pipe type and size, then the design velocity.
- Click Calculate, then Download PDF Result if you need a record.
Important notes
- Typical values only. The local plumbing code, the project specification and the pipe supplier data are final.
- Internal diameters used: Schedule 40 steel / PVC and Copper Type L. Other pipe classes such as HDPE, uPVC pressure pipe and ductile iron have different internal diameters. Use the Custom option for them.
- Velocity sizing does not check pressure loss. For long pipes, also check the friction loss and the available pump head.
Worked example
Example 1 – Cold water pipe, metric
Flow 10 L/s (0.01 m3/s), design velocity 1.5 m/s, Schedule 40 steel / PVC pipe.
d = square root of (4 x 0.01 / (3.1416 x 1.5)) = 0.0921 m = 92.1 mm
The smallest standard pipe with an internal diameter of at least 92.1 mm is DN 100 (4 in), internal diameter 102.3 mm.
Actual velocity = 4 x 0.01 / (3.1416 x 0.10226^2) = 1.22 m/s
Example 2 – Pump suction line, imperial
Flow 100 gpm (0.006309 m3/s), design velocity 3 ft/s (0.9144 m/s), Copper Type L.
d = square root of (4 x 0.006309 / (3.1416 x 0.9144)) = 0.0937 m = 3.69 in
The smallest Copper Type L pipe that fits is DN 100 (4 in), internal diameter 3.905 in.
Actual velocity = 2.68 ft/s
Example 3 – Checking an existing pipe
Flow 25 m3/h (0.006944 m3/s) in a DN 50 (2 in) Schedule 40 pipe, internal diameter 52.5 mm.
Flow area = 3.1416 x 0.0525^2 / 4 = 21.65 cm2
Velocity = 0.006944 / 0.002165 = 3.21 m/s, which is above the 3 m/s limit. Use DN 65 or larger.
Example 4 – Flow from a known pipe
DN 100 (4 in) Schedule 40 pipe (internal diameter 4.026 in), design velocity 7 ft/s (2.134 m/s).
Flow area = 12.73 in2 (0.008213 m2)
Q = 2.134 x 0.008213 = 0.01752 m3/s = 17.52 L/s = 63.08 m3/h = 277.8 gpm.
Common mistakes
- Using the nominal size as the internal diameter. A DN 100 pipe is not 100 mm inside, and the real internal diameter depends on the pipe type and schedule.
- Choosing a pipe by velocity only. A pipe that meets the velocity limit can still have a high pressure loss over a long run.
- Using the same velocity for every service. Suction lines need a lower velocity than discharge lines to avoid cavitation, and hot water lines need a lower velocity to avoid erosion.
- Rounding the diameter down. You must always select the next larger standard size.
- Mixing flow units. Check whether your flow is in L/s, L/min, m3/h or gpm before entering it.
- Ignoring future demand. Pipes are hard to replace, so allow some spare capacity if the building may expand.
- Using peak flow for the whole building without a diversity factor, or average flow for a peak demand. Use the right flow for the pipe you are sizing.
- Using a very low velocity to be safe. Very large pipes cost more and can cause stagnant water in domestic systems.
- Using this calculator for gravity drains or partly full pipes. It is for full pipes under pressure only.
Frequently asked questions
How do I calculate pipe size from flow and velocity?
Use d = square root of (4 x Q / (pi x v)), with Q in m3/s and v in m/s. The answer is the internal diameter in metres. Then choose the next larger standard pipe size.
What is a good velocity for water pipes?
As common practice, cold water distribution lines are designed at about 1.5 m/s (5 ft/s), hot water lines at about 1.2 m/s, pump suction at about 1.0 m/s and pump discharge at about 2.0 m/s. Always check the project specification.
What is the maximum velocity in a water pipe?
Many designers keep it below about 3 m/s (10 ft/s) to limit noise, erosion and water hammer. Some specifications set lower limits, especially for copper and hot water pipes.
Why should pump suction velocity be low?
High velocity on the suction side lowers the pressure and can cause cavitation, which damages the pump. A typical suction velocity is about 0.6 to 1.5 m/s.
Is the nominal pipe size the same as the internal diameter?
No. The nominal size (DN or inch size) is only a name. The real internal diameter depends on the wall thickness, such as Schedule 40 or Copper Type L.
How do I convert L/s to m3/h and gpm?
1 L/s = 3.6 m3/h = 15.85 gpm (US). The calculator shows the converted values in the result.
Does this calculator include pressure loss?
No. It only uses the velocity method. For friction loss in a long pipe, use the Fire Pipe Pressure Loss Calculator with the Hazen-Williams formula.
What if the velocity is too high for my pipe?
Choose the next larger pipe size, or split the flow into two parallel pipes. The calculator shows a red message when the velocity is above the typical limit.
Can I use it for HDPE or uPVC pressure pipe?
Yes. Choose Custom as the pipe type and type the internal diameter from the manufacturer data for the pressure class you use.
Can I use it for hot water pipes?
Yes. Select the hot water velocity or a custom value. Keep the velocity lower, especially for copper, to avoid erosion and noise.
Can I use it for drainage or sewer pipes?
No. Drain pipes run partly full under gravity, so they need a different method. This calculator is for full pipes under pressure.
Is this a replacement for a plumbing design?
No. It gives a quick preliminary size. Final design needs the demand calculation, pressure loss, the local code and the approval of the engineer in charge.