# What is the flow rate?

Flow rate calculator: the volume flow rate through a pipe from its inside diameter and the flow speed (Q = Av), the speed from the flow rate, or the flow from a volume and a time, in gpm, L/min, m³/h and cfm.

- Page: https://www.acalculator.org/physics/flow-rate-calculator
- JSON spec: https://www.acalculator.org/physics/flow-rate-calculator.json
- Version: 772e27591d59

## Default answer

Example with the default inputs (Find Flow in a pipe, Pipe inside diameter 2 in, Flow speed 5 ft/s): The answer is Q = 48.9599 gal/min (185.333 L/min).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| find | Find | Work out the flow from a pipe and speed, the speed from a flow, or the flow from a volume and a time. |
| D | Pipe inside diameter | The inside diameter of the pipe, which runs full. |
| v | Flow speed | The average speed of the fluid through the pipe. |
| Q | Flow rate | The volume of fluid per unit of time, in the unit chosen below. |
| qu | Flow unit | The unit of the flow rate you type. |
| V | Volume | The volume that flowed, such as a bucket filled. |
| t | Time | How long that volume took to flow. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| result | Answer | The flow rate or the speed, to 6 significant figures. |
| gpm | Flow (gal/min) | The flow rate in US gallons per minute. |
| lpm | Flow (L/min) | The flow rate in litres per minute. |
| lps | Flow (L/s) | The flow rate in litres per second. |
| m3h | Flow (m³/h) | The flow rate in cubic metres per hour. |
| cfm | Flow (ft³/min) | The flow rate in cubic feet per minute. |
| m3s | Flow (m³/s) | The flow rate in cubic metres per second. |
| mps | Speed (m/s) | The average flow speed in metres per second. |
| fps | Speed (ft/s) | The average flow speed in feet per second. |
| area | Pipe area (in²) | The inside cross-section area of the pipe, π × D² ÷ 4, in square inches. |

## Method

Pipe: A = π × D² ÷ 4 and Q = A × v, or v = Q ÷ A. Volume and time: Q = V ÷ t.

## Assumptions

- The pipe is round and runs full; v is the average speed over the cross-section.
- Gallons are US gallons (231 in³ = 3.785411784 L).

## Worked examples

1. find = flow, D = 0.0508, v = 1.524 gives gpm = 48.959886, area = 3.141593. Source: OpenStax, University Physics Volume 1, §14.5 Fluid Dynamics (Q = Av), https://openstax.org/books/university-physics-volume-1/pages/14-5-fluid-dynamics (retrieved 2026-10-05); NIST SP 811, Appendix B.8 (1 gal = 3.785411784 L; 1 ft = 0.3048 m; 1 in = 0.0254 m), https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-05).
2. find = volume, V = 0.018927, t = 30 gives gpm = 10, lpm = 37.854118, result = Q = 10 gal/min (37.8541 L/min). Source: NIST SP 811, Appendix B.8 (1 gal = 3.785411784 L; 1 ft = 0.3048 m; 1 in = 0.0254 m), https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-05).
3. find = speed, D = 0.1, Q = 1, qu = lps gives mps = 0.127324. Source: OpenStax, University Physics Volume 1, §14.5 Fluid Dynamics (Q = Av), https://openstax.org/books/university-physics-volume-1/pages/14-5-fluid-dynamics (retrieved 2026-10-05).

## FAQ

### How do you calculate flow rate?

Multiply the pipe’s cross-section area by the flow speed: Q = A × v, with A = π × D² ÷ 4. A 2 in pipe has A = 3.1416 in²; at 5 ft/s (60 in/s) it carries 188.5 in³/s, or 48.96 gal/min.

### How do I measure flow rate with a bucket?

Time how long it takes to fill a bucket of known volume, then divide: Q = V ÷ t. A 5 gallon bucket filled in 30 seconds is 5 ÷ 0.5 = 10 gal/min.

### How do I find the flow speed from the flow rate?

Divide the flow by the area: v = Q ÷ A. 1 L/s (0.001 m³/s) through a 100 mm pipe (0.007854 m²) is 0.127 m/s.

### Why does a smaller pipe make the water faster?

The same flow must pass through every section of a full pipe, so A × v stays the same. Halving the diameter quarters the area, so the speed is four times as high.

### How many liters per minute is 1 gpm?

1 US gallon per minute is 3.785411784 L/min, because 1 US gallon is exactly 3.785411784 L.

### Should I use the inside or outside diameter?

The inside diameter, because only the bore carries fluid. For a pipe of a given nominal size, look up its inside diameter for its wall thickness (schedule).

## Sources

- OpenStax, University Physics Volume 1, §14.5 Fluid Dynamics: flow rate Q = Av, retrieved 2026-10-05. https://openstax.org/books/university-physics-volume-1/pages/14-5-fluid-dynamics
- NIST Special Publication 811 (2008), Appendix B.8: 1 gal (US) = 3.785411784 L, 1 ft = 0.3048 m, 1 in = 0.0254 m, retrieved 2026-10-05. https://www.nist.gov/pml/special-publication-811