Appliances & Utilities calculator
What Boiler Size Do I Need?
Use design space-heating load, domestic hot-water load if simultaneous, system margin to find boiler output requirement.
Enter your measurements
What Boiler Size Do I Need uses design space-heating load, domestic hot-water load if simultaneous, system margin and the other configured measurements to return boiler output requirement with a formula written specifically for this tool.
- Design space-heating load — Required
- Domestic hot-water load if simultaneous — Optional
- System margin — Required
Boiler output requirement
Enter the measurements above to calculate boiler output requirement.
How the calculation works
V9 calculation: requiredBoilerOutput=(spaceHeatingLoad+dhwPriorityLoad)×(1+systemMargin). The result is reported as boiler output requirement.
Worked example
Example: Design space-heating load = 10000 W; Domestic hot-water load if simultaneous = 3000 W; System margin = 20%. Result: Required Boiler Output = 15600 W.
Before you decide
Use the finished, usable dimensions after trim, handles, skirting, packaging or other projections where relevant. Check the exact product documentation for ratings, clearances, installation and safety requirements before relying on the result. For what boiler size do i need, the limiting result on this page is boiler output requirement.
Manufacturer or safety specification required for the final decision. The CMS formula handles the stated measurement calculation only; installation clearances, ratings, certification and safety instructions must come from the exact product documentation.
Frequently asked questions
What does the Boiler Size calculator tell me?
It returns boiler output requirement from the measurements entered on this page. The calculation is specific to boiler size, so the result should be compared with the exact product or specification you are considering.
Which measurements should I take for boiler size?
Measure design space-heating load, domestic hot-water load if simultaneous, system margin. Use clear, usable dimensions rather than nominal room or product labels.
Should I round the boiler output requirement?
Keep the calculated value for comparison, then choose a real product that meets or stays within the returned boiler output requirement while allowing for the clearances, tolerances or ratings shown by its manufacturer.