Pipe Size Calculator

A pipe size calculator returns the inner diameter a pipe needs to carry a target flow rate at a chosen velocity. It converts GPM to cubic feet per second, divides by velocity to get the required area, then solves for the diameter. A 49 GPM flow at 5 ft/s needs an inner diameter of about 2.00 in.

Last reviewed September 2026Method & sources Report an error

Required Inner Diameter

2.001 in

Breakdown
Required Inner Diameter	2.001 in

How to Calculate Pipe Size

Starting from the continuity equation (`flow rate = cross-sectional area × velocity`), this solves for area first (`area = flow rate ÷ velocity`). It then works backward from a circle's area formula to find the radius (`radius = √(area ÷ π)`) and doubles it for the diameter. This is the third way to solve the same relationship. The Flow Rate calculator solves for flow, and the Water Velocity calculator solves for velocity.

Example: Required flow rate of 49 GPM, targeting 5 ft/s velocity

A required flow rate of 49 GPM, targeting 5 ft/s velocity: 1. Flow rate in cu ft/s: 49 ÷ (7.48052 × 60) = 0.10917 cu ft/s 2. Required cross-sectional area: 0.10917 ÷ 5 = 0.021834 sq ft 3. Radius: √(0.021834 ÷ π) = √0.00695 = 0.08337 ft 4. Diameter: 0.08337 × 2 × 12 = 2.00 in. This confirms the Flow Rate and Water Velocity calculators' own worked examples: a 2 in. pipe at 5 ft/s carries almost exactly 49 GPM.

What Is Pipe Size?

A pipe size calculator converts a required flow rate and a target velocity into the pipe inner diameter needed to deliver that flow without exceeding the velocity you want. It's useful when sizing a new supply line. It also helps when you're checking whether an existing pipe can handle a needed flow at a reasonable velocity.

Pipe Size: Pros and Cons

Pros

Solves the same fluid dynamics relationship as the Flow Rate and Water Velocity calculators, completing all three directions of the same equation
Useful for sizing a new pipe run from a known flow requirement, rather than checking an already-chosen pipe size
Gives an exact theoretical diameter, which you then round up to the nearest available standard pipe size

Cons

Gives a theoretical exact diameter, not a standard commercially available size. Round up to the nearest stock pipe size. A calculated 1.8 in. would mean choosing 2 in. pipe, not a custom 1.8 in. size
Doesn't determine what velocity is appropriate for your application. See the Water Velocity calculator's guidance on typical design velocity ranges
Doesn't account for friction losses, fittings, or elevation changes that affect a real system's actual performance beyond this ideal calculation

Pipe Size: What a Pro Checks

Round the calculated diameter up to the nearest standard stock pipe size, never down. Rounding down forces the same flow through less cross-sectional area, pushing velocity above your target.
Remember that doubling a pipe's diameter cuts the velocity needed for the same flow to a quarter. Cross-sectional area grows with the square of diameter, so this is a common way to trade material cost for less noise and pressure loss.

Frequently Asked Questions

How do I calculate the pipe size I need for a given flow rate?

Divide your target flow rate, converted to cubic feet per second, by your target velocity to get the required cross-sectional area. Then work backward to find the diameter. A 49 GPM flow at a 5 ft/s target velocity needs about a 2 in. diameter pipe.

What velocity should I target when sizing a pipe?

It depends on the application and pipe material. Many plumbing design references suggest a moderate velocity to balance adequate flow against noise, water hammer, and erosion concerns. See the Water Velocity calculator's FAQ for more detail.

Do I round the calculated diameter up or down?

Round up to the nearest standard available pipe size. Rounding down would mean the pipe carries the target flow at a higher velocity than intended. A smaller pipe forces the same flow through less cross-sectional area.

How is this different from the Flow Rate calculator?

Flow Rate starts from a known pipe diameter and velocity to find GPM. This calculator starts from a known, required flow rate and a target velocity to find the diameter you need. It's useful when you're choosing a pipe size rather than checking one you already have.

Does a larger pipe always mean lower velocity for the same flow?

Yes. A pipe's cross-sectional area grows with the square of its diameter, so doubling the diameter quarters the velocity needed to carry the same flow rate. That's why oversizing a pipe is a common way to reduce noise and pressure loss, at the cost of more material and larger fittings.

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