Show the formula and working
This combi boiler flow rate calculator works out how many litres of hot water a minute a combi boiler can deliver at a chosen temperature rise, checks it against your mains water flow from a simple bucket test, and shows how many showers or taps it can run at once. It is built for homes in the UK and Europe with metric units and 230 V gas combis, and it also works backwards to show the boiler output your hot water demand needs.
Quick answer: Multiply the boiler’s hot water output in kW by 60, then divide by 4.18 and by the temperature rise. A 30 kW combi gives about 12.3 litres a minute at a 35 °C rise, or 10.8 at 40 °C. The combi boiler flow rate calculator also checks your mains flow, which can limit any combi.
What Is a Combi Boiler Flow Rate Calculator?
A combi boiler flow rate calculator is a tool that converts a combination boiler’s hot water output into the flow of hot water it can deliver. A combi heats mains water as it flows through, so its flow rate depends on how much power it has and how much it has to raise the water temperature. The formula in plain words: flow in litres per minute = kW x 60 / (4.18 x temperature rise in °C). Water needs about 4.18 kJ to warm one litre by 1 °C, a figure from its specific heat capacity.
The usable flow is the lower of the boiler’s flow and the flow your mains supply can provide, because a combi cannot deliver more water than comes into the house.
How Do You Use the Combi Boiler Flow Rate Calculator?
- Enter the Boiler hot water output in kW from the data sheet.
- Set the Temperature rise from your cold mains to the hot water you want.
- For a mains check, fill a bucket from the cold kitchen tap and enter the Bucket test: litres collected and Bucket test: seconds to fill.
- Enter the Showers or taps running at once and the Flow per shower or tap.
- Read the hot water flow rate and outlets it can run, then open “Show the formula and working” to see each step.
What Flow Rate Does a Combi Boiler Give?
A 24 kW combi gives about 9.8 litres a minute at a 35 °C rise, a 30 kW combi about 12.3 litres and a 35 kW combi about 14.4 litres. At a 40 °C rise each figure is about an eighth lower.
| Hot water output | Flow at 35 °C rise | Flow at 40 °C rise |
|---|---|---|
| 24 kW | 9.8 L/min | 8.6 L/min |
| 25 kW | 10.3 L/min | 9.0 L/min |
| 28 kW | 11.5 L/min | 10.0 L/min |
| 30 kW | 12.3 L/min | 10.8 L/min |
| 35 kW | 14.4 L/min | 12.6 L/min |
| 40 kW | 16.4 L/min | 14.4 L/min |
These figures match makers’ data closely. Worcester Bosch quotes 9 litres a minute at a 40 °C rise for its Greenstar 4000 25kW combi, and the formula gives 9.0. Always check the rise your boiler’s figure is quoted at.
How Do I Check My Mains Water Flow Rate?
Open the cold kitchen tap fully, time how long it takes to fill a bucket of known size, then divide the litres by the seconds and multiply by 60. A 10 litre bucket that fills in 40 seconds means 15 litres a minute. If the mains flow is lower than the boiler’s flow, a bigger boiler will not help, and a plumber may need to check the supply pipe or stopcock. This is a rough test; an installer measures flow and pressure with gauges.
What Size Combi Boiler Do I Need for Two Showers?
Two mixer showers at about 9 litres a minute each need about 18 litres a minute, which at a 35 °C rise is about 44 kW, more than most domestic combis supply. That is why a combi usually runs one shower well at a time. Homes with two or more bathrooms used together often suit a system or regular boiler with a hot water cylinder. Our boiler size calculator looks at heating and hot water together, and the electric shower cost calculator covers showers that heat their own water.
Why Is My Combi Hot Water Weak in Winter?
Cold mains water is colder in winter, so the boiler has to raise the temperature further and the flow at the same temperature drops. Going from a 35 °C rise to a 40 °C rise cuts a 30 kW combi from about 12.3 to 10.8 litres a minute. Turning the tap down slightly brings the temperature back up. Boiler choice, installation and any work on gas appliances must be done by a Gas Safe registered engineer.
Worked Example: 30 kW Combi With One Shower
Example: a home has a 30 kW combi and wants hot water at a 35 °C rise, for instance from 10 °C mains to 45 °C at the shower. A bucket test fills 10 litres in 40 seconds, and the household runs one shower at an example 9 litres a minute. These are the calculator’s default inputs.
- Boiler flow: 30 kW x 60 / (4.18 x 35) = 12.3 litres a minute.
- Mains flow: 10 litres / 40 seconds x 60 = 15 litres a minute.
- Usable flow: the lower figure, 12.3 litres a minute, limited by the boiler.
- Demand: 1 shower x 9 litres a minute needs 9 x 4.18 x 35 / 60 = 21.9 kW.
The boiler runs one shower comfortably with about 3 litres a minute to spare, but not two at once.
Combi Boiler Flow Rate Calculator FAQs
How many litres per minute does a 30 kW combi produce?
About 12.3 litres a minute at a 35 °C temperature rise, or about 10.8 litres a minute at a 40 °C rise. The real figure depends on the boiler’s hot water output, which is on the data sheet, and on your mains flow.
What temperature rise should I use?
Use the difference between your cold mains temperature and the temperature you want at the tap or shower. Makers quote flow at a stated rise, often 35 °C or 40 °C, so use the same rise when comparing boilers. Winter mains water needs a bigger rise.
Is a higher kW combi always better?
Not always. A bigger combi only gives more hot water if your mains supply can provide the extra flow. If the bucket test shows a low mains flow, a larger boiler will not improve your showers, and the extra output may be wasted.
Can a combi run two showers at once?
Most domestic combis struggle to run two showers at full flow, because two showers need over 40 kW at a 35 °C rise. You may get two weak showers or a drop in temperature. A cylinder based system suits homes that use two showers together.
What is a good mains flow rate for a combi?
Makers state a minimum working flow and pressure for each boiler, and a mains flow of at least the boiler’s rated hot water flow lets it perform as designed. Flows under about 10 litres a minute can limit any combi, so ask a plumber to test the supply.
Who should fit or replace a combi boiler?
In the UK, installing, replacing or moving a gas boiler must be done by a Gas Safe registered engineer. They will check the gas supply, flue, water flow and pressure before recommending a size. The calculator is for planning and comparing only.
Checked October 2026. Sources: Worcester Bosch: Greenstar 4000 (9 litres a minute at a 40 °C rise, 25kW model), Wikipedia: Specific heat capacity (water about 4.18 kJ per kg per °C).