The GPM to Pipe Size Calculator is a useful tool in the plumbing and engineering category. It helps users find the correct pipe diameter required for a specific flow rate in gallons per minute (GPM). Choosing the right pipe size is very important because undersized pipes create high pressure losses, while oversized pipes waste material and increase cost. This calculator makes it easier for people to balance efficiency, safety, and cost by providing accurate pipe sizing quickly.
Formula
Primary Formula: Calculating Required Pipe Inside Diameter
This is the main formula used to determine the minimum internal pipe diameter required for a specific flow rate and a target velocity.
Inside Diameter = √((0.4085 * GPM) / Velocity)
Where:
- Inside Diameter = Internal diameter of the pipe (inches)
- GPM = Flow rate in gallons per minute
- Velocity = Fluid velocity in feet per second
This formula ensures that fluid velocity stays within recommended limits, usually between 3 to 8 feet per second for water systems.
Helpful Reference Table
The following table shows approximate pipe sizes for water flow at a velocity of 5 feet per second. This gives a quick reference so users don’t always need to calculate.
| Flow Rate (GPM) | Recommended Pipe Size (inches) |
|---|---|
| 10 | 1/2 |
| 20 | 3/4 |
| 40 | 1 |
| 80 | 1-1/4 |
| 160 | 2 |
| 320 | 3 |
| 500 | 4 |
| 800 | 6 |
| 1600 | 8 |
This table is a general guide. For exact sizing, the formula should always be applied considering the desired velocity.
Example
Suppose we want to calculate the pipe size for 100 GPM water flow at 5 ft/s velocity.
Inside Diameter = √((0.4085 * 100) / 5)
Inside Diameter = √(40.85 / 5)
Inside Diameter = √8.17
Inside Diameter ≈ 2.86 inches
So, a 3-inch pipe would be recommended.
Most Common FAQs
For most water systems, a velocity of 3 to 8 feet per second is recommended. Lower velocity reduces noise and pressure loss, while higher velocity may be used in compact systems where saving space is important.
Correct pipe sizing prevents pressure loss, ensures efficient flow, and reduces energy consumption. Oversized pipes increase cost, while undersized pipes cause operational problems.
Yes, the same formula works for other liquids, but velocity recommendations may change depending on the type of fluid and application.