Flow Rate Calculator

A flow rate calculator returns a pipe's flow in gallons per minute (GPM) from its inner diameter and velocity. It multiplies the cross-sectional area by velocity to get cubic feet per second, then converts to GPM using 7.48052 gallons per cubic foot and 60 seconds per minute. A 2 in. inner diameter pipe at 5 ft/s carries 48.96 GPM.

Last reviewed September 2026Method & sources Report an error

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Flow Rate

48.96 GPM

Breakdown
Flow Rate	48.96 GPM
Flow Rate	0.109 cu ft/s

How to Calculate Flow Rate

This applies the continuity equation from fluid dynamics: flow rate equals the pipe's cross-sectional area times the fluid's velocity (`Q = A × V`). The pipe's cross-sectional area comes from its inner diameter (`π × radius²`). Multiplying by velocity in ft/s gives flow rate in cubic feet per second. That converts to gallons per minute (GPM) using the exact 7.48052 gallons-per-cubic-foot factor, times 60 seconds per minute.

Example: 2 in. inner diameter pipe, water moving at 5 ft/s

A 2 in. inner diameter pipe, water moving at 5 ft/s: 1. Radius: 1 in., which is 1 ÷ 12 = 0.0833 ft 2. Cross-sectional area: π × (1 ÷ 12)² = π ÷ 144 = 0.021817 sq ft 3. Flow rate: 0.021817 × 5 = 0.109085 cu ft/s 4. GPM: 0.109085 × 7.48052 × 60 = 48.96 GPM

What Is Flow Rate?

A flow rate calculator converts a pipe's inner diameter and the water's velocity into its flow rate in GPM (gallons per minute). It's useful for checking whether a pipe size can deliver a needed flow rate. It also shows how velocity and pipe size trade off against each other for the same flow.

Flow Rate: Pros and Cons

Pros

Applies the standard fluid dynamics continuity equation (Q = A × V) directly, the same relationship engineers use for pipe sizing
Uses the exact gallons-per-cubic-foot conversion rather than an approximation
Works for any pipe diameter and velocity combination, useful for exploring how changing pipe size affects flow at a given velocity

Cons

Doesn't account for friction losses, fittings, or pipe roughness that reduce actual flow in a real system compared to this ideal calculation
Assumes a pipe flowing completely full. A partially full gravity-drain pipe follows different hydraulics
Doesn't determine what velocity is appropriate for your application. See the Water Velocity calculator's guidance on typical design velocity ranges for plumbing supply lines

Flow Rate: What a Pro Checks

Don't size purely by flow rate. Check the Water Velocity calculator's guidance on typical design velocity ranges too, since the same GPM through a smaller pipe means noticeably higher velocity, noise, and erosion risk.
Remember this is an ideal, friction-free figure. Actual flow in a real system runs lower once fittings, pipe roughness, and length are factored in.
This only applies to a pipe flowing completely full. A partially full gravity-drain line follows different hydraulics than this pressurized-flow calculation.

Frequently Asked Questions

How do I calculate GPM from pipe size and velocity?

Multiply the pipe's cross-sectional area (from its inner diameter) by the water's velocity to get flow rate in cubic feet per second. Then convert to GPM by multiplying by 7.48052 (gallons per cubic foot) and 60 (seconds per minute). A 2 in. pipe at 5 ft/s delivers about 48.96 GPM.

What is the continuity equation?

The continuity equation (Q = A × V) states that flow rate equals cross-sectional area times velocity, for an incompressible fluid like water flowing through a pipe. It's the fundamental relationship behind sizing pipes for a needed flow rate.

Does a bigger pipe always mean more flow?

Not necessarily at the same velocity. For the same flow rate, though, a bigger pipe carries that flow at a lower velocity, since area increases with the square of diameter. Lower velocity is often desirable, because it generally means less noise, less erosion, and less pressure loss from friction.

What is a typical water velocity in a pipe?

It varies by application. Plumbing supply lines are commonly designed for a moderate velocity range to balance adequate flow against noise and erosion concerns. Check the Water Velocity calculator's guidance for more detail on typical ranges.

Does this account for pressure loss from pipe friction?

No. This calculates ideal flow rate from area and velocity alone. Real-world flow in a piping system is also affected by pipe length, roughness, fittings, and elevation changes. Those create friction losses that this simple calculation doesn't model.

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