Flow Rate Calculator
Calculate pipe flow rate from diameter and velocity or volume over time, with mass flow from fluid density. Convert L/s, L/min and m³/h.
Water ≈ 1,000 kg/m³. Enter 0 to skip the mass flow output.
Flow rate
3.93 L/s
Litres per minute
235.62 L/min
Litres per hour
14,148 L/h
Cubic metres per second
0 m³/s
Cubic metres per hour
14.14 m³/h
Mass flow rate
3.93 kg/s
Volumetric flow × fluid density
Mass per hour
14,137.17 kg/h
Pipe flow rate = cross-section area × velocity (Q = A × v). Mass flow rate = volumetric flow × density (ṁ = ρ × Q).
Last updated: January 2026
How this calculator is verified
Checked by True Calculator automated test suite on
- Formula verified against a published worked example in the automated test suite
- Edge cases (zero, negative, boundary and unit-mismatch inputs) covered by unit tests
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Flow rate decides the size of pumps, pipes and tanks — from a garden hose filling a water butt to irrigation channels and industrial cooling circuits. Plumbers need it to check whether a pump matches a building's daily demand, and engineers use it to design water-supply and process piping. The volume-over-time mode needs no instruments: time how long it takes to fill a known container and you know the pump's real output, which often differs from the nameplate. When density is known, the mass flow output converts directly between litres per minute and kilograms per hour. Whether you think in L/min, m³/h or L/s, this calculator converts between them and tells you whether the infrastructure can actually deliver.
How to Use This Calculator
- Step 1: Choose 'Pipe flow' for Q = A × v, or 'Volume / time' for a bucket-and-stopwatch measurement.
- Step 2: For pipe flow, enter the pipe diameter in mm and the water velocity in m/s.
- Step 3: Optionally enter the fluid density (water ≈ 1,000 kg/m³) to also get the mass flow rate.
- Step 4: For volume over time, enter the volume in litres and the time in seconds, then read the flow in L/s, L/min and m³/h.
Worked Example
A 50 mm pipe delivering water at 2 m/s has a cross-section of π × (0.025)² = 0.00196 m², so the flow rate is 0.00196 × 2 = 0.00393 m³/s — shown as 3.93 L/s or 235.62 L/min. With a density of 1,000 kg/m³ the mass flow is 3.93 kg/s, useful for sizing pumps and checking pipe capacity. Using the other mode, collecting 500 litres from a pump in 60 seconds gives 8.33 L/s, which converts to exactly 30 m³/h — the number printed on pump nameplates.
Tips and Common Mistakes
- •Tip 1: Compare the result with the pump's rated L/min on its nameplate to check for a clogged or undersized line.
- •Tip 2: For canals and channels, use volume ÷ time with a known container rather than estimating pipe diameter.
- •Tip 3: Flow rate through a pipe rises with the square of the diameter — a 75 mm pipe carries 2.25× more than a 50 mm one at the same velocity.
- ✗Mistake 1: Mixing millimetres and metres — the diameter must be in mm and velocity in m/s.
- ✗Mistake 2: Measuring time in minutes and entering it as seconds; keep everything in seconds.
Sources and References
- •ISO 4064 / CPHEEO manual on pipe hydraulics and flow measurement
- •Darcy-Weisbach equation and ISO 5167 flow measurement standards
Official sources are linked so you can confirm the current rate yourself. Check the linked page for the latest notification before relying on these figures.
Frequently Asked Questions
How is pipe flow rate calculated?
Q = A × v, where A is the cross-section area of the pipe (π × r²) and v is the flow velocity. A 50 mm pipe at 2 m/s delivers about 3.93 L/s or 236 L/min.
What is the difference between L/s and L/min?
They are the same flow expressed in different time units. 1 L/s equals 60 L/min, since one minute has 60 seconds. The calculator shows both.
Can I measure flow from a bucket and a stopwatch?
Yes. Use the 'Volume / time' mode: fill a known-volume container, time how long it takes, and enter both. 500 litres collected in 60 seconds is 8.33 L/s.
Where is flow rate used in daily life?
Sizing water pumps for overhead tanks, irrigation pipes, filter pumps and industrial pipelines all start from the required flow rate, so this calculation avoids buying an undersized pump.
Compare these tools
- Density Calculator — Find density in kg/m³, g/cm³ and lb/ft³ from mass and volume, with specific gravity vs water
- Half-Life Calculator — Solve any half-life problem: remaining or initial amount, elapsed time or half-life itself, plus the decay constant and mean lifetime of exponential decay
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