The size of your room may be 5m x 4m (20sqm) but you may only need to heat 12sqm of the room.
How to measure
It is important to review the room itself. Where the contents of the room are placed, which features or contents are movable vs immovable, etc.
Using a kitchen for example. Let’s say your room is 7sqm in size. You will not be placing heating underneath the kitchen cupboards so this will reduce the area you need heated. So really, you are only potentially heating 2sqm of the room.
If you are looking at a Tiled Bathroom, then you would discount the vanity and bath space for example. You may decide to heat the inside of your shower, but perhaps cost for that separately and take in the costing factor of waterproofing and insulation as well.
Now that you know how much heating you really require, take a look at the Undertile DIY Sizing Guide in order to calculate exactly what you need, as well as start understanding what the running costs will be.