Measure the space
Length, width and height establish the floor area, exposed surface estimates and room volume.
Free room heat-output calculator
What size radiator do I need?
Describe the room and its exposed surfaces to estimate the heat output to look for. The result shows watts, BTU/h and how lower heating-system temperatures change the catalogue rating you need.
Step 1 of 8Room type
Room type
How the estimate works
A simplified room heat-loss calculation
Estimate fabric loss
Broad U-value assumptions represent heat passing through walls, glazing, floor and ceiling.
Allow for air changes
The model adds ventilation and assumed air leakage before applying a modest uncertainty allowance.
The underlying method follows the same physical structure used in formal heat-load work—fabric transmission plus ventilation—but simplifies construction details into plain-language bands. The MCS Heat Load Calculator explains why professional BS EN 12831 calculations use postcode-specific outdoor conditions and element U-values. Current and improved U-value benchmarks can be checked in Approved Document L.
Guides and advice
Understand radiator output before choosing
Before choosing
Radiator sizing questions
What radiator output does this calculator estimate?
It estimates the room heat needed on a cold day, then gives a practical radiator-output range in watts and BTU/h. It is not a whole-home heating-system design.
Why does the calculator ask about rooms above and below?
A room beneath a roof or above the ground loses more heat than the same room between heated spaces. The calculator includes that difference instead of treating every floor and ceiling as exposed.
What does ΔT50 mean on a radiator specification?
ΔT describes the difference between the radiator’s mean water temperature and the room temperature. UK catalogue outputs are commonly quoted at ΔT50. A lower-temperature system delivers less heat from the same radiator, so the published output must be corrected.
Can I add two radiator outputs together?
Yes. Two radiators can meet the target together if their corrected outputs under the same operating conditions add up to the required room output.
Should I buy directly from this result?
Use the result to compare documented output ratings, dimensions and system-temperature performance. A competent heating professional should confirm sizing, valves, pipework, flow temperature and installation before purchase.