How do I calculate the heat output I need from my cast iron radiators in a specific room?

We have a simple BTU Calculator for cast iron radiators on our website to provide you with an indicative guide for the heat required in a specific room. However, it is wise to ask for a second opinion from your plumber or heating engineer as they can make a specific assessment of your property’s requirements and note any elements that might potentially impact your heating need.

Issues such as age, location, insulation, heating system may require more complex methods available for calculating the precise heat requirement and in these situations we advise you seek out the advice of a heating consultant.

Heat Calculations

The formula below is for all basic heat calculations. You can also use our heat calculator to do the below for you!

Step 1. – Calculate the Volume of your room

Calculate the volume of the room in cubic metres (length x width x height)

Step 2. – Room Type

Take the room volume and multiply it be the relevant Room Type Multiple figure below.

  • Lounges and Dining Rooms – multiply by 50.
  • Bedrooms – multiply by 40.
  • Hallways, Landings and Kitchens – multiply by 30.
  • Bathrooms – multiply by 90.
  • Conservatories – see below.

Step 3. Any Adjustments

Take into consideration any relevant factors that may later the amount of heat required from your cast iron radiators.

  • For rooms facing north + 15%.
  • For French Windows or large windows + 20%.
  • For double glazing + 10%.
  • For very exposed sites, or very cold weather + 10%
  • For new builds (which have much better insulation) – 20%

Total = heat output in watts needed from one or more radiators to heat the room.

Every radiator in every size on our site has its output shown in watts and BTUs (British Thermal Units) in Delta 30, Delta 50 and Delta 60 figures. To adjust BTUs to Watts, multiply your BTU figure by 0.293071. Therefore, 1000BTUs is 293 Watts

The vast majority of heating systems should use either Delta 50 or Delta 60 calculations. Electric or solid fuel heating should use Delta 50 calculations, Gas fired heating system should use delta 60 calculations. For ground source heating, air source or solar thermal heating system you should use Delta 30 calculations.

To understand more about delta calculations, read our blog article on delta calculations.

If you are uncertain about your cast iron radiators requirements, please do not hesitate to contact us on 01723 321 000.