Show the formula and working
This free pipe flow rate calculator works out how many litres per minute flow through a water pipe, either from a simple bucket test at a tap or from the water speed, and it also works backwards from a flow rate to the speed in the pipe. It is built for homeowners, renters and DIYers in the UK, Ireland and Europe checking mains flow before choosing a combi boiler, shower or tap, and it covers 15, 22 and 28 mm copper and common MDPE supply pipes.
Quick answer: A pipe flow rate calculator divides the litres collected by the seconds taken and multiplies by 60. A 10 litre bucket filled in 50 seconds means 12 L/min. Through a 15 mm copper pipe with a 13.6 mm bore, that flow moves at about 1.38 m/s.
What Is a Pipe Flow Rate Calculator?
A pipe flow rate calculator is a tool that finds the volumetric flow rate of water in a pipe, usually in litres per minute, and the speed the water travels at.
The formula in plain words: flow rate equals water speed times the pipe’s inside area. The inside area is pi times the bore squared, divided by four. To turn cubic metres per second into litres per minute, multiply by 60,000. For a bucket test, divide the litres collected by the seconds taken and multiply by 60. The copper bores used here come from the standard wall thicknesses for UK half hard copper tube: 0.7 mm for 15 mm pipe and 0.9 mm for 22 and 28 mm pipe.
How Do You Use the Pipe Flow Rate Calculator?
- Choose what to Work out: “Flow rate from a bucket test”, “Flow rate from water speed” or “Water speed from flow rate”.
- Choose the Pipe. For any other pipe, choose “Other: enter the bore” and fill in Inside diameter (bore).
- For a bucket test, enter the Bucket volume and the Seconds to fill with the tap fully open.
- Otherwise enter the Water speed in m/s or the Flow rate in L/min.
- Read the result and open “Show the formula and working” to see the pipe area, flow rate, speed and other units.
How Do You Measure Water Flow Rate at Home?
Open the cold kitchen tap fully, time how many seconds it takes to fill a bucket of known size, then divide litres by seconds and multiply by 60. Use the tap nearest the stopcock and make sure no other taps, showers or appliances are running. Repeat the test two or three times and use the average. A 10 litre bucket in 50 seconds is 12 L/min.
How Much Water Flows Through a 15 mm Pipe?
A 15 mm copper pipe carries about 8.7 L/min at 1 m/s and about 13.1 L/min at 1.5 m/s, because its bore is only 13.6 mm. A 22 mm pipe carries more than twice as much at the same speed. The table shows common home pipes.
| Pipe | Bore | Area | Flow at 1.0 m/s | Flow at 1.5 m/s | Speed at 12 L/min |
|---|---|---|---|---|---|
| 15 mm copper | 13.6 mm | 145.3 mm² | 8.7 L/min | 13.1 L/min | 1.38 m/s |
| 22 mm copper | 20.2 mm | 320.5 mm² | 19.2 L/min | 28.8 L/min | 0.62 m/s |
| 28 mm copper | 26.2 mm | 539.1 mm² | 32.3 L/min | 48.5 L/min | 0.37 m/s |
| 20 mm MDPE | about 16 mm | 201.1 mm² | 12.1 L/min | 18.1 L/min | 0.99 m/s |
| 25 mm MDPE | about 20.4 mm | 326.9 mm² | 19.6 L/min | 29.4 L/min | 0.61 m/s |
What Is a Good Water Flow Rate for a House?
Check the flow rate your appliance maker asks for, because combi boilers, mixer showers and unvented cylinders each state a minimum flow and pressure in their installation manuals. As a rule of thumb, a household with more than one bathroom benefits from a higher flow than a single bathroom flat. Flow and pressure are different things: pressure in bar is the push, flow in L/min is how much arrives, and a narrow or long supply pipe can cut flow even when pressure is good.
What Water Speed Is Too Fast in a Pipe?
Many designers keep water in home pipework below about 1.5 m/s, because faster water can be noisy and wears fittings over time. The calculator warns above that speed. If a flow you need gives a high speed in 15 mm pipe, the next pipe size up carries it at under half the speed. Water fittings must also meet the Water Supply (Water Fittings) Regulations 1999, so get a qualified plumber for changes to the supply.
Worked Example: A Bucket Test at the Kitchen Tap
Example: a 10 litre bucket fills in 50 seconds from the kitchen cold tap, which is fed by 15 mm copper pipe. These are the tool’s default inputs.
- Flow rate: 10 L / 50 s x 60 = 12 L/min, or 0.2 L/s.
- Pipe area: pi x 0.0136² / 4 = 0.0001453 m², which is 145.3 mm².
- Water speed: 12 / 60,000 / 0.0001453 = 1.38 m/s.
- Other units: 0.72 m³/h, 2.64 UK gallons or 3.17 US gallons per minute. An 80 litre bath would take about 6.7 minutes.
To convert the flow into other units, the litres to gallons converter helps, and the electric shower cost calculator shows what hot water at a given flow costs.
Pipe Flow Rate Calculator FAQs
How do I calculate flow rate from pipe size?
Multiply the water speed by the inside area of the pipe. A 15 mm copper pipe with a 13.6 mm bore has an area of 145.3 mm², so water at 1 m/s gives about 8.7 litres per minute. Pipe size alone does not fix the flow.
How do I convert litres per minute to litres per second?
Divide litres per minute by 60. A flow of 12 L/min is 0.2 L/s. To get cubic metres per hour, multiply litres per minute by 0.06, so 12 L/min is 0.72 m³/h.
What is the bore of 15 mm copper pipe?
Standard UK 15 mm half hard copper has a 0.7 mm wall, so the bore is 13.6 mm. 22 mm and 28 mm pipe have a 0.9 mm wall, giving bores of 20.2 mm and 26.2 mm.
Why is my flow rate low but pressure fine?
A partly closed stopcock, a narrow or long supply pipe, a blocked filter or an old lead or iron main can restrict flow even when static pressure looks good. Test with the stopcock fully open, then ask a plumber or your water company if flow stays low.
How do I convert litres per minute to gallons per minute?
Divide by 4.54609 for UK gallons or by 3.78541 for US gallons. 12 L/min is 2.64 UK gallons or 3.17 US gallons per minute. The calculator shows both.
Does the bucket test measure the whole house supply?
It measures the flow at that tap, which includes losses in the pipes leading to it. The kitchen cold tap is usually closest to the incoming main, so it gives the best picture of the supply.
Checked October 2026. Sources: Wikipedia, Volumetric flow rate, legislation.gov.uk, Water Supply (Water Fittings) Regulations 1999, Wikipedia, Gallon, EN 1057 copper tube wall thicknesses.