Measuring for Underfloor Heating: Why Accuracy is Everything

Planning an electric underfloor heating project is exciting, but it comes with one golden rule that every homeowner and installer must know: Heating cable CANNOT be cut.

Unlike water-based systems where you can simply trim a pipe, electric mats are manufactured to a specific resistance. If you buy a mat that is too large for your floor, you cannot trim it to fit. You’ll be stuck with a system that is unsafe to install and voids your warranty.

In this guide, we’ll show you how to calculate your heated area like a pro, accounting for margins and obstacles to ensure a perfect fit first time.

Step 1: The 100mm Perimeter Margin

The biggest mistake DIYers make is measuring from wall-to-wall. Professionals leave a 100mm (10cm) gap around the entire perimeter of the room. This gap is vital for two reasons:

• Thermal Expansion: It allows for the natural expansion of the floor as it heats up.
• Avoid Overheating: It prevents the cable from being buried too close to the wall where heat can’t escape.

Pro Tip: When calculating your room, subtract 200mm from both your length and width to account for the gaps on both sides.

Step 2: Mapping Your Obstacles

You should never install underfloor heating under fixed furniture or appliances. This can cause “thermal blocking,” leading to the cable overheating and failing. You must accurately measure and subtract the following:

• Kitchen islands and floor-standing units.
• Bath tubs and shower trays.
• Toilet pans and vanity units.
• Large appliances like fridges or washing machines.

Step 3: Calculating the Total Heated Area

Once you have your wall-to-wall area and have subtracted your margins and obstacles, you are left with your Total Heated Area.

Industry standard is to aim for roughly 90% coverage of that final area. It is much easier to “free-weave” a slightly smaller mat to fill a gap than it is to deal with excess cable.