acalculator

What is the flow rate?

Pick what to find. The flow rate calculator gives the flow through a full round pipe from its inside diameter and the flow speed, the speed for a given flow, or the flow from a volume and the time it took, in gpm, L/min, L/s, m³/h and cfm.

Your numbers

Units
Find
Answer
Q = 48.9599 gal/min (185.333 L/min)

The answer is Q = 48.9599 gal/min (185.333 L/min).

Flow (gal/min)
48.9599
Flow (L/min)
185.333
Flow (L/s)
3.08889
Flow (m³/h)
11.12
Flow (ft³/min)
6.54498
Flow (m³/s)
0.00308889
Speed (m/s)
1.524
Speed (ft/s)
5
Pipe area (in²)
3.14159

Answer: Q = 48.9599 gal/min (185.333 L/min). The answer is Q = 48.9599 gal/min (185.333 L/min).

How to calculate

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.

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).

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

  • 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).

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Find Flow in a pipe, Pipe inside diameter 2 in, Flow speed 5 ft/s gives Flow (gal/min) 48.959886, Pipe area (in²) 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 ÷ time, Volume 5 gal, Time 30 s gives Flow (gal/min) 10, Flow (L/min) 37.854118, Answer 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 in a pipe, Pipe inside diameter 3.937 in, Flow rate 1, Flow unit L/s gives Speed (m/s) 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)

How it works

  • Pipe area A = π × D² ÷ 4, with D the inside diameter.
  • Flow in a pipe Q = A × v, with v the average flow speed.
  • Speed in a pipe v = Q ÷ A.
  • Volume ÷ time Q = V ÷ t.

Unit sizes (exact, in m³/s): 1 gal/min = 0.003785411784 ÷ 60; 1 gal/h = 0.003785411784 ÷ 3,600; 1 L/min = 1 ÷ 60,000; 1 L/s = 0.001; 1 m³/h = 1 ÷ 3,600; 1 ft³/min = 0.028316846592 ÷ 60; 1 ft³/s = 0.028316846592. Speeds: 1 ft/s = 0.3048 m/s, 1 ft/min = 0.3048 ÷ 60 m/s, 1 mph = 0.44704 m/s, 1 km/h = 5/18 m/s.

Rules

  • Diameter from 10⁻⁶ m to 100 m; speed from 10⁻⁹ to 100,000 m/s; a typed flow from 10⁻⁹ to 10⁹ in its unit; volume from 10⁻⁹ to 10⁹ m³; time from 10⁻⁶ to 10⁹ s.
  • No answer when the flow is over 10⁶ m³/s, a found speed is over 100,000 m/s, or the flow is too small to show as a number.

Output format

The answer line shows the flow in gal/min and L/min, or the speed in ft/s and m/s, to 6 significant figures. The answer line writes each number as a plain decimal from 10⁻⁶ up to 10²¹ (else d.ddde+N), rounded half up from its exact value, with the true minus sign (−) for a negative value. The list gives the flow in gal/min, L/min, L/s, m³/h, ft³/min and m³/s, the speed in m/s and ft/s, and the pipe area in in², each to 6 significant figures. Volume ÷ time is exact until it is rounded once; the pipe modes use π as a float.

Worked examples by hand

A 2 in pipe at 5 ft/s. A = π × 2² ÷ 4 = 3.14159 in². Q = 3.14159 × 60 in/s = 188.496 in³/s = 11,309.7 in³/min ÷ 231 = 48.9599 gal/min.

A bucket. 5 gal in 30 s = 5 ÷ 0.5 min = 10 gal/min = 37.8541 L/min.

Speed from flow. 1 L/s = 0.001 m³/s through a 100 mm pipe: A = π × 0.1² ÷ 4 = 0.00785398 m², so v = 0.001 ÷ 0.00785398 = 0.127324 m/s.

Other questions people ask

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).