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BTU calculator - How to work out what Radiator you need

Before installing new radiators, you’ll need to establish what level of heating output will satisfy the demands of each room in your home. The heating output of a radiator is measured in BTUs (British Thermo Units) per hour. The larger a room is, naturally the higher the demand for heat will be in comparison to smaller rooms. As a general rule this also means that larger radiators tend to have larger BTU outputs, although the panel style of a radiator may also affect this too. Other factors such as flooring, insulation and wall space may also impact the heating output needed from a radiator.

Calculating the number of BTUs required is straight forward with our BTU calculator below. Our BTU calculator should be used for guidance purposes only and you may wish to therefore check with a qualified heating engineer for a more accurate estimate on the BTU output required for rooms in your home, as other external factors will play a part in this.

1: How Big Is Your Room?

The first thing you need to do is establish the volume of the room that you will be adding your new radiator to. This can be calculated using the below formula:

Length of room (m) x Width of room (m) x Height of room (m)

This will then give you the volume of your property (in m3) and essentially dictates the amount of space that needs to be heated within that room by your radiator(s).

2: BTU Calculator

Having successfully established the volume of the room you want to install your new radiator(s) in, the next step is to work out the standard level of heating output (in BTU’s) required to heat a room of that size.

This can be calculated using the following formula:

Room volume (m3) x 153

Having carried out task 2 you should now know the BTU output required for your desired room size. Unfortunately there are a number of other influential factors can play a part in increasing or decreasing the required BTU output for the room. For example, if you do not have any loft insulation in your property, then you can expect to require a radiator with a little more BTU output than previously specified.

In order to take these factors into account we recommend that you work out if you need to take or add to the BTU output number you established in task 2.

Here are some of the main factors that may influence this BTU figure:

Factor Instruction

For solid floor

-10%

For un-insulated cavity walls

+10%

For foam filled cavity walls

-20%

For upstairs bedrooms

-25%

For double glazing

-5%

For two outside wall

+15%

For three outside walls

+40%

For northern aspects

+10%

For no loft insulation

+15%

For high ceiling - 3m

+20%

3: What Size Radiator Do You Need?

The following table gives you a rough idea of the size of radiator needed to reach your desired BTU output.

Obviously the larger the radiator, the larger the heating output (BTU). Sadly it is not as easy as just choosing the radiator size that you need – the amount of available space is also a large consideration. It is therefore worth considering the different panel style options, with single panel single convector, double panel single convector and double panel double convector radiators all available and offering different levels of heat output despite holding the same radiator dimensions.

The table below is a good guide to follow to gain an understanding about the type of radiator you should be looking for. If you need any further information about the radiator that is best suited for your room then we recommend contacting a qualified heating engineer.

Single panel single convector

Double panel double convector

Height (mm)

Width (mm)

BTU

Height (mm)

Width (mm)

BTU

400

600

1356

400

900

2033

400

1200

2711

500

400

1066

500

400

2078

500

500

1333

500

500

2597

500

600

1599

500

600

3117

500

700

1866

500

700

3636

500

800

2132

500

800

4156

500

900

2399

500

900

4675

500

1000

2666

500

1000

5195

500

1100

2932

500

1100

5714

500

1200

3199

500

1200

6234

500

1400

3732

500

1400

7272

500

1600

4265

500

1600

8311

600

400

1297

600

400

2497

600

500

1584

600

500

3121

600

600

1900

600

600

3745

600

700

2217

600

700

4370

600

800

2534

600

800

4994

600

900

2851

600

900

5618

600

1000

3167

600

1000

6242

600

1100

3484

600

1100

6867

600

1200

3801

600

1200

7491

600

1400

4434

600

1400

8739

600

1600

5068

600

1600

9988

700

400

1421

700

400

2635

700

500

1776

700

500

3294

700

600

2132

700

600

3952

700

700

2487

700

700

4611

700

800

3842

700

800

5270

700

900

3198

700

900

5928

700

1000

3553

700

1000

6587

700

1100

3908

700

1100

7246

700

1200

4264

700

1200

7905

700

1400

4974

700

1400

9222

700

1600

5685

700

1600

10539

 

Direct Heating Supplies stocks a wide selection of radiators in an array of BTU outputs, styles, colours and sizes. Use our BTU calculator above to find what you need and explore our range of radiators and towel rails here.