Heat Requirement Calculator

What Size Of Cast Iron Radiator Should You Choose?

With so much choice, it can require a lot of thought to choose the perfect cast iron radiator that looks great in your home. In addition, you will also need to make sure that it is capable of producing enough heat to keep you warm. Our cast iron radiators are built around your needs and thankfully there’s a standard way to calculate your room’s heat requirement. Based on the size, use and insulation of your room we can calculate the BTU (British Thermal Units) heat requirement that it requires.

To use our handy calculator, simply enter the dimensions of your room in metres, tell us the type of room and we’ll do the rest. We can then use this BTU number as a starting point to help pick you the perfect-sized radiators.

1

Room dimensions

Enter the length, width and height of the room in metres

2

Room type

3

Insulation level

4

External walls

How many walls in this room are external (outside-facing)? Each adds 10% heat loss.

1 wall selected
5

Windows & glazing

Your room needs

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BTU/hr

— m³ room volume
— heat loss multiplier

How we calculated this

Room volume × room type factor
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After insulation adjustment
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Heat loss multiplier applied
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Final BTU requirement
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Period property? Consider a heritage allowance

Older properties with solid walls or sash windows lose heat faster. A 10% uplift puts you at — BTU — use the for a full breakdown.

Browse our radiators

Enter your room dimensions and select a room type, insulation level, and window type to see your BTU requirement.

This tab gives owners of older or period properties an additional heating allowance on top of their standard BTU figure. Run the standard calculator first, then enter your result below.

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Property description

Select any that apply to your property

Suggested heritage allowance (+10%)

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BTU/hr

This is a guidance figure only — not automatically applied. We recommend erring on the higher side when sizing cast iron radiators for period homes, as they lose heat faster than modern buildings.

Why does this matter for cast iron radiators?
Cast iron radiators have excellent heat retention and radiate gently over time — ideal for period properties. However, older buildings lose heat faster, so choosing a radiator rated slightly above your base BTU figure gives you headroom on cold days without overworking your boiler.

Browse our radiators

Enter your BTU figure from the standard calculator and select at least one property description to see your heritage allowance.