Flow Rate Calculator
What the flow rate calculator does
The flow rate calculator divides a volume of fluid by the time it took to pass, giving volumetric flow rate. Enter the volume in litres and the time in seconds, press Calculate, and the result is shown in litres per second. It is the “bucket and stopwatch” method that plumbers, irrigation installers and lab technicians use to measure a real flow, and the first number needed for pipe sizing, pump selection and tank filling times.
The formula
Q = V / t
where Q is flow rate, V is volume and t is time. With litres and seconds the result is L/s. Multiply by 60 for L/min, by 3,600 for L/h, or by 3.6 for m³/h.
Unit conversions
| L/s | L/min | m³/h | US gal/min |
|---|---|---|---|
| 0.1 | 6 | 0.36 | 1.59 |
| 0.25 | 15 | 0.9 | 3.96 |
| 1 | 60 | 3.6 | 15.85 |
| 5 | 300 | 18 | 79.3 |
| 25 | 1,500 | 90 | 396 |
The flow rate converter handles cubic feet per minute and other units.
Worked examples
Kitchen tap. A 10 L bucket fills in 48 s. Q = 10 ÷ 48 = 0.208 L/s, or 12.5 L/min, a normal figure for a mains-fed tap.
Shower. 9 L collected in 60 s gives 0.15 L/s (9 L/min). Water-efficient showers are rated at 6 to 9 L/min; older ones can exceed 15.
Swimming pool. A pump moves 50,000 L through the filter in 6 hours (21,600 s). Q = 50,000 ÷ 21,600 = 2.31 L/s, or 8.3 m³/h. The pool’s full volume turns over four times a day.
Laboratory. A peristaltic pump delivers 250 mL (0.25 L) in 300 s: 0.000833 L/s, better expressed as 50 mL/min.
From flow rate to velocity
Flow rate in a pipe equals the cross-sectional area times the average velocity: Q = A × v. For a 25 mm internal diameter pipe, A = π × 0.0125² = 0.000491 m². At 0.208 L/s (0.000208 m³/s) the water moves at 0.000208 ÷ 0.000491 = 0.42 m/s. Domestic pipes are usually sized for 1 to 2 m/s; above about 3 m/s noise and erosion become problems. Velocity is the input the Reynolds number calculator needs to classify the flow, and the Darcy-Weisbach calculator uses it to find pressure loss along the pipe.
How long to fill or empty a tank
Rearranging the formula gives t = V / Q. A 1,000 L rainwater tank fed by a 0.15 L/s downpipe fills in 6,667 s, about 1 h 51 min. A 200 L water heater at 0.2 L/s drains in 1,000 s, or 17 minutes. For tanks that drain under gravity the rate falls as the level drops, so the calculated time is a lower bound; the orifice flow rate calculator covers that case.
Measuring accurately
- Use a container of known volume and a stopwatch; start both at the same moment.
- Time a fill of at least 30 seconds so reaction-time errors stay below a few percent.
- Repeat three times and average.
- For hot water, let the tap run until the temperature is steady; the flow changes as the mixer settles.
Common mistakes
- Entering minutes as seconds. 2 minutes is 120 s.
- Entering millilitres as litres. 500 mL is 0.5 L.
- Confusing flow rate with pressure. Pressure (bar, psi) is the push; flow rate is what actually arrives. High pressure through a narrow pipe can still give a low flow.
- Using nominal pipe size for velocity. A “25 mm” plastic pipe may have a 20 mm bore. Use the internal diameter.
Related tools
The pump horsepower calculator turns flow and head into motor power. The Bernoulli equation calculator links flow, pressure and elevation, and the water hammer calculator estimates the surge when a fast-closing valve stops the flow you have measured. Pressure units are handled by the pressure unit converter.
External references
Frequently asked questions
What is a normal flow rate for a house?
A single tap delivers 0.1 to 0.25 L/s. Whole-house peak demand with several outlets open is typically 0.5 to 1 L/s, which is what a 20 to 25 mm supply pipe is designed for.
How do I convert L/s to gallons per minute?
Multiply by 15.85 for US gallons or 13.2 for imperial gallons.
Can I use this for gas flow?
Yes for the arithmetic, but gas volume depends on pressure and temperature, so state the conditions (usually standard litres) when quoting the result.
Does the calculator give mass flow?
No. Multiply the volumetric result by the fluid density: 1 L/s of water is about 1 kg/s; 1 L/s of diesel is about 0.85 kg/s.
Why does my measured flow drop when a second tap is opened?
The supply pipe has a fixed capacity at a given pressure. Opening more outlets shares that capacity and increases friction loss, so each outlet gets less.