Radiator Size Calculator – Disclaimer

​

This tool is designed to provide an approximate radiator output requirement (in BTU/hr) based on the following factors:

• Room volume (Length × Width × Height)

• Insulation quality (affects heat retention and therefore radiator size)

• Heating system type (e.g. heat pumps require larger radiators due to lower flow temperatures)

• Room use (e.g. bathrooms need more heat)

• Glazing and orientation (north-facing or heavily glazed rooms lose more heat)

 

Calculation Method:

We use an industry-recognised baseline of BTUs per cubic metre:

• Poor insulation → 70 BTU/m³

• Average insulation → 50 BTU/m³

• Good insulation → 40 BTU/m³

​

Adjustments are then made based on:

• +40% for low-temperature heating systems (e.g. air source heat pumps)

• +10% for bathrooms

• +5% if north-facing

• +10% if glazing is above average

 

Important Notes:

• This tool provides a guideline only and is not a substitute for a detailed heating design or a site-specific assessment.

• It does not factor in ceiling materials, wall construction types, airtightness, or ventilation heat losses.

• Final radiator sizes should always be verified by a qualified heating engineer or plumber, especially for systems using heat pumps or underfloor heating.

​

Houz Design accepts no liability for decisions made based on this tool. Use it to inform discussions and understand general requirements, not as a guaranteed specification.