Quick answer
Flow in gallons per minute = π ÷ 4 × (diameter in feet)² × velocity (ft/s) × 448.83. Water moving at 5 ft/s in a 2 inch pipe flows at 48.96 GPM (185.3 L/min). The second mode divides a volume by the minutes taken.
Choose a mode, then enter the pipe diameter and velocity or a volume and time. The calculator returns GPM and litres per minute.
Related tools: Friction Loss Calculator: Pipe friction head loss with the Hazen–Williams equation. Pipe Pressure Drop Calculator: Pressure drop in a straight pipe from the Darcy–Weisbach equation.
How to use the Gallons per Minute Calculator
- Choose use: Pipe diameter and velocity and Volume and time.
- Optionally, enter inside diameter (in) (above 0).
- Optionally, enter velocity (ft/s) (above 0).
- Optionally, enter volume (gal) (above 0).
- Optionally, enter time (min) (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Use | Choose one: Pipe diameter and velocity; Volume and time | Starts at Pipe diameter and velocity |
| Inside diameter (optional) | A number, in in | > 0 |
| Velocity (optional) | A number, in ft/s | > 0 |
| Volume (optional) | A number, in gal | > 0 |
| Time (optional) | A number, in min | > 0 |
Formula and method
Formula
GPM = π/4 × (d ÷ 12)² × v × 448.83; GPM = volume ÷ minutes
IEEE-754 double precision; shown to the decimals indicated.
1 ft³ = 7.480519 US gallons (1728 ÷ 231).
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Use = Pipe diameter and velocity; Inside diameter = 2; Velocity = 5.
| Result | Value |
|---|---|
| Flow | 48.96 GPM |
| Liters per minute | 185.333 |
Example 2
Inputs: Use = Volume and time; Volume = 55; Time = 4.
| Result | Value |
|---|---|
| Flow | 13.75 GPM |
| Liters per minute | 52.049 |
Example 3
Inputs: Use = Pipe diameter and velocity; Inside diameter = 1.5; Velocity = 8.
| Result | Value |
|---|---|
| Flow | 44.064 GPM |
| Liters per minute | 166.8 |
The first line is the headline result (flow). Below it the gallons per Minute Calculator lists liters per minute, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The 448.83 factor
One cubic foot per second equals 448.83 US gallons per minute. A 2 inch pipe has a cross-sectional area of 0.0218 ft², so 0.0218 × 5 ft/s = 0.109 ft³/s, which is 48.96 GPM.
Measuring flow
To measure a flow, time how long it takes to fill a bucket of known volume: a 5 gallon bucket filled in 30 seconds is 10 GPM. Velocity in pipes is often kept under 5 to 8 ft/s to limit noise and wear.
How the result changes with inside diameter
Holding every other input at the first worked example, this table changes only inside diameter. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Inside diameter | Flow | Liters per minute |
|---|---|---|
| 1 | 12.24 GPM | 46.333 |
| 2 | 48.96 GPM | 185.333 |
| 4 | 195.84 GPM | 741.333 |
| 8 | 783.358 GPM | 2,965.333 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Inside diameter: left empty shows "Enter the diameter." beside that field and no result.
- Time: left empty shows "Enter the time." beside that field and no result.
Choosing between the Gallons per Minute Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Friction Loss Calculator](/engineering/friction-loss/) | flow, inside diameter and pipe length | Head loss |
| [Hydraulic Cylinder Force Calculator](/engineering/hydraulic-cylinder-force-calculator/) | units, pressure and bore diameter | Extend force |
| [Pipe Pressure Drop Calculator](/engineering/pipe-pressure-drop-calculator/) | flow, inside diameter and length | Pressure drop |
| [Pump Flow Calculator](/engineering/pump-flow-calculator/) | displacement, speed and volumetric efficiency | Flow |
| [Drops per Minute Calculator](/science/drops-per-minute/) | volume, time and drop factor | Drip rate |
Background: fluid flow
Volumetric flow rate is Q = A × v (cross-sectional area times average velocity). Bernoulli's equation, P + ½ρv² + ρgh = constant, holds along a streamline for steady, incompressible, frictionless flow, so real pipes need friction corrections. The Reynolds number Re = ρvL/μ compares inertia with viscosity.
Laminar flow in a pipe is usually quoted below Re of about 2,100 to 2,300 and fully turbulent above about 4,000, with an uncertain transition between; treat the thresholds as approximate.
Assumptions and limitations
- Only the fields for the chosen mode are used
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for inside diameter and time to confirm this.
Frequently asked questions
How many litres per minute is 1 GPM?
3.785 litres per minute.
What velocity is typical?
About 3 to 7 ft/s in water pipes.
Does pipe diameter mean inside diameter?
Yes, use the inside diameter.
What does the Gallons per Minute Calculator calculate?
Flow in gallons per minute from pipe size and velocity, or volume and time. It returns Flow and Liters per minute.
What formula does the Gallons per Minute Calculator use?
GPM = π/4 × (d ÷ 12)² × v × 448.83; GPM = volume ÷ minutes. 1 ft³ = 7.480519 US gallons (1728 ÷ 231).
What do I need to enter in the gallons per Minute Calculator?
You enter Use. Optional: Inside diameter (in), Velocity (ft/s), Volume (gal) and Time (min).
What does the “Use” option change?
It sets how the gallons per Minute is worked out. The choices are Pipe diameter and velocity and Volume and time.
What is an example of a gallons per Minute result?
With use Pipe diameter and velocity, inside diameter 2 and velocity 5, the flow is 48.96 GPM.
Where does the method come from?
The method follows: OpenStax, University Physics Volume 1 (Rice University): mechanics; 1 US gallon = 231 in³ (exact; NIST Handbook 44, Appendix C).
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Only the fields for the chosen mode are used.
Related calculators
- Friction Loss Calculator — Pipe friction head loss with the Hazen–Williams equation.
- Pipe Pressure Drop Calculator — Pressure drop in a straight pipe from the Darcy–Weisbach equation.
- Pump Flow Calculator — Pump output from displacement, speed and volumetric efficiency.
- Drops per Minute Calculator — IV drip rate in drops per minute from volume, time and drop factor.
- Words per Minute Calculator — Typing or reading speed in words per minute.
Sources
- OpenStax, University Physics Volume 1 (Rice University): mechanics
- 1 US gallon = 231 in³ (exact; NIST Handbook 44, Appendix C)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.