Pipe Sizing Calculator
Find the pipe size for a target flow rate. Enter the flow in GPM or L/min and the highest velocity you will accept, pick the pipe material, and the calculator returns the smallest standard pipe whose inside diameter is big enough, with its bore and the actual velocity.
Enter values to calculate
Pipe sizing resultFree download
Get the printable pipe flow chart
Schedule 40 pipe sizes, water flow in GPM and LPM at four design velocities, and recommended velocities by application. One PDF, made to pin above the bench.
How It Works
The Pipe Sizing Calculator uses the continuity equation to determine the minimum required pipe diameter based on flow rate and velocity constraints. It then recommends the smallest standard pipe of the selected material whose inside diameter is at least that large.
Continuity Equation
The minimum pipe diameter is calculated using:
D = √(4Q/πV)
Where:
- • D = Pipe diameter
- • Q = Flow rate
- • V = Flow velocity
- • π = Pi (3.14159...)
Standard Pipe Sizes and Inside Diameters
A pipe's nominal size is a name, not its bore. 2" Schedule 40 pipe has a 2.067" inside diameter and 1/2" Type L copper has 0.545", so the calculator compares the required diameter with each pipe's actual inside diameter, taken from the published standard:
- • Steel: ASME B36.10 Schedule 40, 1/2" to 24"
- • Copper: ASTM B88 Type L, 1/4" to 12"
- • PVC: ASTM D1785 Schedule 40, 1/2" to 24" (average bore: minimum wall + 6%, as manufacturers publish)
- • HDPE: iron pipe size (IPS) outside diameters with a DR 11 wall (minimum wall = outside diameter ÷ 11, average bore with + 6%), 1/2" to 24" except 2-1/2" and 3-1/2"
Pipe Capacity in GPM by Pipe Size
The flow each pipe carries at 5 ft/s and 8 ft/s, the usual range for water supply lines. Worked out from the same inside diameters the calculator uses. Schedule 40 PVC has a slightly smaller average bore than Schedule 40 steel (2.049 in against 2.067 in at 2"), so choose PVC in the calculator for its figures.
| Schedule 40 steel | Type L copper | |||||
|---|---|---|---|---|---|---|
| Nominal size | ID (in) | GPM at 5 ft/s | GPM at 8 ft/s | ID (in) | GPM at 5 ft/s | GPM at 8 ft/s |
| 1/2" | 0.622 | 4.7 | 7.6 | 0.545 | 3.6 | 5.8 |
| 3/4" | 0.824 | 8.3 | 13 | 0.785 | 7.5 | 12 |
| 1" | 1.049 | 13 | 22 | 1.025 | 13 | 21 |
| 1-1/4" | 1.380 | 23 | 37 | 1.265 | 20 | 31 |
| 1-1/2" | 1.610 | 32 | 51 | 1.505 | 28 | 44 |
| 2" | 2.067 | 52 | 84 | 1.985 | 48 | 77 |
| 2-1/2" | 2.469 | 75 | 119 | 2.465 | 74 | 119 |
| 3" | 3.068 | 115 | 184 | 2.945 | 106 | 170 |
| 4" | 4.026 | 198 | 317 | 3.905 | 187 | 299 |
| 6" | 6.065 | 450 | 720 | 5.845 | 418 | 669 |
FAQ
Here you will find the answers to the frequently asked questions about pipe sizing calculations.
Frequently Asked Questions
How do I choose the right maximum velocity?
It depends on the application. Typical design limits for water are 4-8 ft/s (1.2-2.4 m/s) for domestic cold water, 3-5 ft/s (0.9-1.5 m/s) for domestic hot water (2-3 ft/s above 140°F), up to 10 ft/s (3.0 m/s) for commercial water, and 2-5 ft/s (0.6-1.5 m/s) on a pump suction. Use the lower end where quiet operation matters; higher velocities increase noise, erosion and pressure drop.
What's the difference between minimum and recommended pipe size?
The minimum diameter is the theoretical size needed to meet your flow rate and velocity requirements. The recommended size is the next available standard pipe size that's equal to or larger than the minimum. Using a larger size reduces pressure drop and noise.
Why do different materials have different standard sizes?
Each material is made to its own standard, so the same nominal size has a different bore. Steel follows ASME B36.10, copper tube ASTM B88 and PVC ASTM D1785. For example, 1/2" Schedule 40 steel has a 0.622" bore, Schedule 40 PVC 0.609" on average and 1/2" Type L copper 0.545". That is why the calculator can recommend different nominal sizes for the same flow.
What if my calculated size isn't available?
If the calculated size isn't available, the calculator will recommend the next larger standard size. This is generally acceptable as it will result in lower velocity and pressure drop. However, significantly oversized pipes can increase costs and may require additional supports.
How accurate is this pipe sizing calculation?
The calculation is based on the continuity equation, which is a fundamental principle of fluid mechanics. For incompressible fluids like water, the accuracy is excellent. The main consideration is ensuring your input values (flow rate and velocity) are accurate for your specific application.
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