Flow Capacity Calculator
Last updated: July 2026 · Free · No sign-up required
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.
Quick reference
| Diameter | Velocity | Flow (GPM) |
|---|---|---|
| 2 in | 5 ft/s | ≈ 98 |
| 4 in | 5 ft/s | ≈ 392 |
| 12 in | 3 ft/s | ≈ 1,058 |
functions Shows the working, not just the answer
For students and teachers, the final number is only half the value. Every math tool here exposes the formula it applied, the intermediate steps, and the rounding rule, so you can follow along, check your homework, or use the answer in a proof or report with confidence.
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Calculations use 64-bit floating point with sensible rounding for the domain (currency to 2 decimals, percentages to 4 decimals, algebra to 6 significant figures). Where exact rational arithmetic matters — fractions, factorials, simplification — we use a dedicated BigNumber path so 1/3 + 1/6 returns ½, not 0.49999.
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Educators are welcome to link to any math calculator on CalcaTools from a class site, Google Classroom, or worksheet. The pages are mobile-friendly, free, ad-supported (so we can keep them free) and have no sign-up wall — students just click and use them in class or at home.
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Below the calculator we link a small set of plain-English explainer pages — "What is a percentage?", "Why does PEMDAS matter?", and so on. They cover the underlying concept in 4–6 short paragraphs. Read those before the calculator if the topic is new, or after if you want the extra context.
Interpretation guide
| Output | What it tells you |
|---|---|
| Flow rate (GPM) | Volume the pipe carries per minute at that velocity. |
| Cubic feet/second | The same flow in ft³/s before conversion. |
| Pipe area | Cross-sectional area used in the calculation. |
Formula & methodology
Formula: Q = A × v, where A = π × (diameter ÷ 2)² and v is the flow velocity
Pipe flow capacity equals the pipe's cross-sectional area times the fluid velocity. This tool converts the result to gallons per minute (GPM).
- Enter the pipe's inside diameter in inches.
- Enter the flow velocity in feet per second.
- The calculator finds the area A = π(d/2)² in square feet.
- It multiplies area × velocity and converts ft³/s to GPM (× 448.83).
Worked example: A 12 in pipe at 3 ft/s carries about 1,058 GPM.