Appliances & Utilities calculator
What Radiator Size Do I Need?
Use calculated room heat loss, sizing margin, candidate radiator output at system delta-t to find radiator output requirement and candidate check.
Enter your measurements
What Radiator Size Do I Need uses calculated room heat loss, sizing margin, candidate radiator output at system delta-t and the other configured measurements to return radiator output requirement and candidate check with a formula written specifically for this tool.
- Calculated room heat loss — Required
- Sizing margin — Required
- Candidate radiator output at system delta-T — Required
Radiator output requirement and candidate check
Enter the measurements above to calculate radiator output requirement and candidate check.
How the calculation works
V9 calculation: requiredOutput=roomHeatLoss×(1+designMargin); outputPass=radiatorRatedOutput≥requiredOutput; outputMargin=radiatorRatedOutput-requiredOutput. The result is reported as radiator output requirement and candidate check.
Worked example
Example: Calculated room heat loss = 1500 W; Sizing margin = 20%; Candidate radiator output at system delta-T = 2000 W. Result: Fits; Output Margin = 20000 cm; Required Output = 1800.
Before you decide
Enter the dimensions that will exist in real use, not an approximate catalogue category. This page calculates the stated geometry or quantity only; product-specific construction or installation details can still affect the final choice. For what radiator size do i need, the limiting result on this page is radiator output requirement.
Direct measurement/geometry calculation. No external reference is required for the arithmetic itself; the result is derived only from the user-entered measurements and the formula stored in calculationLogicJson.
Frequently asked questions
What does the Radiator Size calculator tell me?
It returns radiator output requirement from the measurements entered on this page. The calculation is specific to radiator size, so the result should be compared with the exact product or specification you are considering.
Which measurements should I take for radiator size?
Measure calculated room heat loss, sizing margin, candidate radiator output at system delta-t. Use clear, usable dimensions rather than nominal room or product labels.
Should I round the radiator output requirement?
Keep the calculated value for comparison, then choose a real product that meets or stays within the returned radiator output requirement while allowing for the clearances, tolerances or ratings shown by its manufacturer.