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Flow Capacity Calculator | Flow from diameter & velocity

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Calculate how much liquid a pipe or channel can carry from its diameter and flow velocity, using Q = A × v with the cross-sectional area shown. Results in cubic feet per second and gallons per minute — essential for plumbing, irrigation and drainage design.

Last updated: July 2026 · Free · No sign-up required

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Results

Enter values above and click Calculate to see your result instantly.

About the Flow Capacity Calculator

Behind the inputs above sits one rule: Q = A × v, where A = π × (diameter ÷ 2)² and v is the flow velocity. The tool evaluates it instantly with the values you enter.

Scroll past the calculator for a quick-reference table, a step-by-step methodology with a fully worked example, and answers to the questions people most often ask about flow capacity calculator.

From our guides

Quick reference

DiameterVelocityFlow (GPM)
2 in5 ft/s≈ 98
4 in5 ft/s≈ 392
12 in3 ft/s≈ 1,058

info Flow Capacity Calculator

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info Educational

Includes the formula and step-by-step explanation so you understand the math, not just the answer.

Interpretation guide

OutputWhat it tells you
Flow rate (GPM)Volume the pipe carries per minute at that velocity.
Cubic feet/secondThe same flow in ft³/s before conversion.
Pipe areaCross-sectional area used in the calculation.

lightbulb Sizing a 4-inch irrigation main

A 4-inch inside-diameter pipe carries water at 3 ft/s. The radius is 2 inches = 1/6 ft, so the cross-sectional area is A = π × (1/6)² ≈ 0.08727 ft².

Result: Q = A × v = 0.08727 × 3 ≈ 0.2618 ft³/s. Convert: 0.2618 × 448.83 ≈ 117.5 gallons per minute.

What this means: 117.5 GPM is a healthy residential irrigation zone, or about two garden hoses at full blast. Note this is the ideal capacity — friction in long runs, elbows, and valves will reduce real flow, so designers apply a safety factor rather than running at the theoretical maximum.

Formula & methodology

Formula: Q = A × v, where A = π × (diameter ÷ 2)² and v is the flow velocity

How pipe flow capacity is calculated

Flow capacity is geometry times speed: Q = A × v, where the cross-sectional area of a round pipe is A = π × (diameter ÷ 2)² and v is the flow velocity. The page handles the unit juggling — diameter in inches becomes area in square feet — then converts the result to both ft³/s and gallons per minute (× 448.83).

  • Measure the pipe's inside diameter in inches.
  • Note the flow velocity in feet per second.
  • Read the area, the flow in ft³/s, and the flow in GPM.

This is the ideal capacity: friction, bends, fittings, and elevation all reduce real-world flow. Size the pipe for the velocity you want — typically 5–8 ft/s for water supply — and treat the GPM figure as the ceiling, not the promise.

Authoritative source: Wolfram MathWorld

Frequently asked questions

How do I calculate pipe flow capacity?
Enter the required values into the input fields and press the calculate button. The tool applies the formula Q = A × v, where A = π × (diameter ÷ 2)² and v is the flow velocity and returns the result with a short explanation of each step, so you can check the working yourself.
What velocity should I use?
The Flow Capacity Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Calculates flow capacity using Q = A × v, where A = π × (diameter ÷ 2)² and v is the flow velocity, right in your browser. There is no limit on usage, and it works on any device.
How do I convert ft³/s to GPM?
Multiply by 448.83: one cubic foot per second equals 448.83 US gallons per minute (60 seconds × 7.48052 gallons per cubic foot). So 0.5 ft³/s is about 224 gallons per minute. The calculator accepts both units and converts automatically, using the standard pipe-flow and cross-sectional area relationships.
Does this account for friction loss?
The basic calculator estimates ideal flow from pipe size and velocity; it does not model friction, bends, fittings, or elevation changes, which reduce real capacity. For design work, apply a friction-loss model like the Hazen-Williams equation or Darcy-Weisbach with your pipe material and length. Treat the calculator's result as the theoretical ceiling.
Is it free and private to use?
Yes — the Flow Capacity Calculator. There is no sign-up, no paywall, no trial, and no limit on how many calculations you can run. CalcaTools covers its costs with non-intrusive display advertising, so the calculator itself never asks for payment or restricts any feature.
How do I calculate water velocity in a pipe?
v = Q ÷ A: divide flow rate by the pipe's cross-sectional area. 50 GPM through a 2-inch pipe (A = 0.0218 sq ft) is 0.1114 ÷ 0.0218 = 5.1 ft/s. Design practice keeps supply lines under about 5–8 ft/s — faster invites water hammer and erosion, which is usually the reason to run this number at all.
What flow velocity should I design for?
Water supply practice typically keeps velocity between about 5 and 8 ft/s — fast enough to avoid sediment settling, slow enough to limit noise, pressure surge, and water hammer. Enter any velocity and the page converts it to GPM; staying in that band is a design choice, not a calculator limit.
Why does a small diameter increase change flow so much?
Because area grows with the square of the diameter: doubling the diameter quadruples the area, and flow is area × velocity. A 4-inch pipe carries four times what a 2-inch pipe does at the same velocity — which is why upsizing one pipe size is such a powerful fix for a starved zone.
Does the calculation include friction loss?
No — it computes the ideal Q = A × v from geometry and velocity alone. Real pipes lose capacity to wall friction, bends, fittings, valves, and elevation change. For design work, follow up with a Hazen-Williams or Darcy-Weisbach friction calculation and apply a safety margin.
How do I convert between ft³/s and GPM?
Multiply ft³/s by 448.83 — that's 60 seconds × 7.48052 gallons per cubic foot. So 0.2618 ft³/s ≈ 117.5 GPM. The page reports both units side by side, so you can sanity-check the conversion on every run.

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