Watch the video on YouTube that shows you in more detail How to Use A Free Heat Output Calculator for Radiator Sizing

Central Heating - How to Use a free Heat Output Calculator for Radiator Sizing
Introduction
Choosing the correct size radiator for any room in your home can be a daunting task if you don't know what to look out for. Make the wrong choice and you could be wasting money or facing an uncomfortable room through the Winter.

Oversizing or Undersizing a Radiator
Oversizing a radiator is not normally seen as a major problem and is often done as a matter of course, to ensure the room gets enough heat on the coldest of days. However, this can have an effect on the overall cost and efficiency of your central heating system. Specifying an oversized radiator means that you will generally pay more to buy the radiator - bigger radiators normally cost more than smaller ones - and if you have a whole house to fit-out, can make a significant difference to the budget. As well as this, larger radiators need more water to fill them. Having more water in the central heating system requires more energy to heat it and consequently more energy means more money to burn.

Undersizing
Undersizing a radiator means that the radiator will not be able to give-out enough heat to warm the room. This is because the room will lose more heat than the radiator can deliver due to the heat-loss of the fabric of the room (the construction materials). This will be most noticeable during those cold spells over winter, and will make the room very uncomfortable to be in.

One way to ensure you size the radiator correctly is to use a Heat Output Calculator. You just need a few basic measurements of your room to begin:

Tools Required - Tape Measure 0 to 8m will normally suffice
All measurements need to be in metres
First measure the height of the room from the floor to the ceiling
Then measure the room width, that's along the shortest wall
Then measure the length of the room along the longest wall
Finally you need to determine the total window area as follows: Width x Height = Area in square metres. Repeat this for all the windows in the room and add the areas together to get a total area. When measuring the width and height include the visible part of the window frame.
Note all four measurements down for each room.

Go and Get a Free Heat Output Calculator
You can easily find a Free Heat Output Calculator by searching in Google for Free Heat Output Calculator. There are plenty about, the one I use can be found here: Heat Output Calculator for Radiator Sizing

You enter the four measurements from your room. An example is given below:
Room Height 2.4
Room Width 5.0
Room Length 6.0
Window Area(m2) 1.5

Then you will need to make a few selections from some drop-down menus to tell the calculator some basic information about your room such as the type of room, what type of windows you have, the type of outside walls etc.

Results
For convenience the results are given as Watts and the equivalent BTU (British Thermal Units). Note both down, then you can go and select a radiator for your room. You can then choose a suitable size of radiator, taking into consideration things such as any windows above the proposed location or any length restrictions.

Find Out More
Watch the video on YouTube that shows you in more detail How to Use A Free Heat Output Calculator for Radiator Sizing