Water & Containers
Flow Rate Calculator
Convert a measured volume and time, or a pipe diameter and velocity, into flow rate.
Enter your measurements
Results update as you type.
About this calculator
Two ways to measure flow: time a known volume, or compute from internal pipe diameter and mean velocity. The result is reported in litres per minute, cubic metres per hour and both US and imperial gallons per minute — with the gallon definitions labelled explicitly so they are never mixed.
How to measure
- For timed volume, collect a measured amount and record the elapsed time using the selected time unit.
- For pipe flow, enter internal diameter and mean velocity in metres per second.
- A nominal pipe size is not necessarily its internal diameter.
How to use this calculator
- Choose the timed-volume or pipe method.
- For a bucket test, record the volume and the time it took.
- For a pipe, measure the internal diameter and estimate mean velocity.
- Calculate; results appear in L/min, m³/h and US/imperial GPM.
Formula
Timed volume: flow = volume ÷ elapsed time
Pipe: cross-section = π × diameter² ÷ 4; flow = cross-section × mean velocity
1 US gallon = 3.785411784 L; 1 imperial gallon = 4.54609 L
Assumptions
- The timed volume is collected at a steady rate.
- Pipe flow uses the full internal cross-section and a mean velocity.
- Gallon figures are labelled US or imperial explicitly.
Common mistakes
- Mixing seconds and minutes in a timed collection.
- Using pipe diameter without a measured average velocity.
- Treating a single fluctuating sample as a reliable flow measurement.
Worked example
Try it: press “Load example” above the form.
120 L collected in 2 minutes is 60 L/min, which is 3.6 m³/h.
Limitations
No friction loss, pump curve, pressure or pipe-sizing design. GPM is an alias inside this calculator, not a separate tool.
Frequently asked questions
How do I measure flow with a container?
Collect a known volume and time it. Divide volume by elapsed time, keeping seconds and minutes consistent. Repeat the measurement if the supply fluctuates.
Is pipe diameter enough to find flow?
No. The diameter gives cross-sectional area. The pipe mode also requires average fluid velocity. Pressure, roughness and fittings need a separate hydraulic model.
What does the worked example show?
120 L collected in 2 minutes is 60 L/min, which is 3.6 m³/h.
Which assumptions should I check before using the result?
No friction loss, pump curve, pressure or pipe-sizing design. GPM is an alias inside this calculator, not a separate tool.
Sources
Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.
- Omni: flow rate
Q = volume/time and area × mean velocity. ACH is airflow per hour divided by room volume; no required ventilation rate is recommended.
Calculation documentation · Updated 2026-11-08 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.
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