Flow Rate Calculator
Find the volumetric flow rate from velocity and pipe size, from a volume and a time, from a channel's width and depth, or from mass flow and density.
How to use the Flow Rate Calculator
- Under Calculate flow rate from, choose a mode. Pipe mode uses velocity and pipe inside diameter. Volume mode uses a collected volume and the time it took. Channel mode uses velocity with width and depth, for rectangular ducts or open channels. Mass mode uses mass flow and density.
- Enter your values in each field. Every quantity has its own unit menu, so you can mix metric and US customary units.
- In pipe and channel modes, check the flow area shown with the result. It appears in cm2 below 0.1 m2 and in m2 above.
- Under Show answer in, pick the unit for the main answer: gpm, L/min, L/s, m3/h, cfm, cfs or m3/s.
- Read the table under the result, which lists the same flow rate in all seven units.
- Open “How it was calculated” to see the formula, the values substituted and the answer with units.
- Read any warning. One appears when velocity is above 100 m/s, and another when the fill time in volume mode is under 5 seconds.
The result updates as you type, so you can test different values. Preset buttons above the form load typical cases, and Reset restores the starting example. Less common inputs are under More options.
What is the flow rate formula?
The basic flow rate formula is Q = A × v. Q is volumetric flow rate, A is the cross-sectional area the fluid passes through, and v is the average velocity across that area. It works for any fluid, including air and other gases, as long as the density stays about constant along the flow.
For a pipe, the area comes from the inside diameter D: A = π/4 × D². The pipe mode assumes the pipe runs full and that the velocity you enter is the average. For a rectangular duct or open channel, the area is width × flow depth.
Example: a pipe with a 2 in inside diameter and water at 5 ft/s has a flow area of 20.27 cm². It carries 48.96 gpm, which is 185.3 L/min or 11.12 m3/h. A 200 mm duct at 10 m/s moves 1,131 m3/h, which is 665.7 cfm.
What is the formula for volume flow rate?
To calculate flow rate, pick the formula that matches your data. Use Q = V / t when you know the volume and time, or Q = ṁ / ρ when you know the mass flow and density. Pipe and channel modes use Q = A × v instead.
Volume and time give the volume flow formula directly: divide the volume collected by the time it took. If 5 gal is collected in 30 s, the flow is 37.85 L/min, or 10 gpm.
Mass flow divides by density, because density is mass per unit volume. For example, 3,600 kg/h of water at the default density of 998.2 kg/m3 (water at 20 C) is 3.606 m3/h. Enter a different density if your fluid is not water at 20 C.
Sources
- Continuity equation Q = A v and Q = V/t (White, Fluid Mechanics, ch. 3)
- Unit factors: NIST SP 811 (1 US gallon = 3.785411784 L)
- Water density 998.2 kg/m³ at 20 °C: NIST Chemistry WebBook
Results are for estimation; verify critical designs against the applicable standard.
Worked examples
| Input | Result |
|---|---|
| v = 5 ft/s, pipe ID = 2 in | Q = 48.96 gpm |
| 5 gal collected in 30 s | Q = 37.85 L/min |
| v = 1.2 m/s, channel 0.5 m wide, 0.2 m deep | Q = 432 m³/h |
| v = 10 m/s, pipe ID = 200 mm | Q = 665.7 cfm |
| ṁ = 3600 kg/h, ρ = 998.2 kg/m³ | Q = 3.606 m³/h |
Calculated by the tool above with these values.
Frequently asked questions
Can I use the results in commercial projects or client reports?
Yes. Results may be used in commercial work, such as quotes, design notes, reports and client projects, without attribution.
Is the calculator free, and is anything I type uploaded?
The calculator is free and needs no account or sign-up. Everything is calculated in your browser, and nothing you type is uploaded.
Can I print or share a calculation?
Yes, in two ways. Print / save as PDF creates a one-page record with the inputs, results, calculation steps and date. Copy link copies the page address, which stores every input, so the same calculation can be reopened later or sent to a colleague.
How do I convert gpm to L/min?
Multiply gpm by 3.785, because one US gallon is 3.785411784 L. For example, 10 gpm is 37.85 L/min.
Can I calculate flow rate from pressure?
Not with this tool, because pressure alone does not set a flow. The pipe flow calculator finds the flow a pipe carries for an available pressure drop, and the orifice flow calculator does the same for an orifice or nozzle.
Does the result meet code requirements?
No. The tool does not certify compliance with any code. The result is an estimate, so verify critical designs against the applicable standard.
Related tools
- Mass Flow Rate Formula and Calculator: use it when you have a mass flow and need the formula behind the conversion.
- Orifice Flow Calculator: use it to work out flow through an orifice opening.
- Pipe Flow Calculator: use it to work out flow in a pipe run.
- PVC Pipe Sizing Calculator and Chart: use it to choose a pipe size for PVC, copper, PEX or steel.
- Compressed Air Pipe Sizing Calculator: use it to size pipe for compressed air lines.
- Cv Calculator (Valve Flow Coefficient): use it when you work with a valve’s flow coefficient.
- Gas Pipe Sizing Calculator (Natural Gas and Propane): use it to size gas pipe for natural gas or propane.
- All pipe, pump and valve calculators: use it to browse the full set of tools.