The short answer: measure the floor, take away everything that is fixed to it (bath, shower tray, toilet, vanity unit, kitchen units, island) and choose the largest heating mat that is no bigger than the free area that is left. Termofol heating mats are 50 cm wide, deliver 150 W/m² and come in 18 sizes from 0.5 m² to 20 m², in half-metre steps up to 5 m² and one-metre steps up to 10 m².
Why the largest mat that fits, and not the one closest to your room size? The heating cable inside a mat must never be cut or shortened. A mat that is too big cannot be made smaller, and the spare length cannot be tucked under a bath or a run of cabinets. A mat that is slightly smaller than the free area simply leaves a narrow strip without heating, usually along a wall or beside a fitting.
This guide walks you through the measuring, shows two worked examples, a bathroom and a kitchen, and lists every mat size with its length and total output.
Step 1: Measure the room
Measure the length and width of the room in metres and multiply them. For an L-shaped room, split the floor into rectangles, work out each one and add them together. Write the figures down on a simple sketch of the room, you will need the sketch again in step 4.
This first number is the gross floor area. It is almost never the size of mat you should order.
Step 2: Take away everything fixed to the floor
A heating mat must not go under anything that sits permanently on the floor and traps heat. Measure each of these and subtract it from the gross area:
- Bath and shower tray, heating mats must never be laid under a bath or a shower tray.
- Toilet and pedestal basin, the footprint of the pan or pedestal, plus a little room to fix it.
- Vanity units and kitchen cabinets standing on a plinth.
- Kitchen island and any other built-in units.
- Furniture without legs, if a piece of furniture will stay in one place, it needs legs at least 3 cm high to stand over a heated floor.
What is left is the free floor area, the part of the room you actually walk and stand on. You do not have to heat every last corner: the mat goes where the floor is open.
Step 3: Round down to the nearest mat size
Compare your free floor area with the table below and pick the largest size that does not exceed it. If your free area is 3.4 m², the right mat is 3.0 m², not 3.5 m². If it is 9.7 m², the right mat is 9.0 m², not 10 m².
Each mat is 50 cm wide, so its length is twice its area: a 1 m² mat is 2 m long, a 10 m² mat is 20 m long. Total output is the area multiplied by 150 W/m².
| Mat size | Choose it when your free area is | Mat length (50 cm wide) | Total output |
|---|---|---|---|
| 0.5 m² | 0.5 – 0.9 m² | 1 m | 75 W |
| 1.0 m² | 1.0 – 1.4 m² | 2 m | 150 W |
| 1.5 m² | 1.5 – 1.9 m² | 3 m | 225 W |
| 2.0 m² | 2.0 – 2.4 m² | 4 m | 300 W |
| 2.5 m² | 2.5 – 2.9 m² | 5 m | 375 W |
| 3.0 m² | 3.0 – 3.4 m² | 6 m | 450 W |
| 3.5 m² | 3.5 – 3.9 m² | 7 m | 525 W |
| 4.0 m² | 4.0 – 4.4 m² | 8 m | 600 W |
| 4.5 m² | 4.5 – 4.9 m² | 9 m | 675 W |
| 5.0 m² | 5.0 – 5.9 m² | 10 m | 750 W |
| 6.0 m² | 6.0 – 6.9 m² | 12 m | 900 W |
| 7.0 m² | 7.0 – 7.9 m² | 14 m | 1,050 W |
| 8.0 m² | 8.0 – 8.9 m² | 16 m | 1,200 W |
| 9.0 m² | 9.0 – 9.9 m² | 18 m | 1,350 W |
| 10 m² | 10 – 11.9 m² | 20 m | 1,500 W |
| 12 m² | 12 – 14.9 m² | 24 m | 1,800 W |
| 15 m² | 15 – 19.9 m² | 30 m | 2,250 W |
| 20 m² | 20 m² or more | 40 m | 3,000 W |
All sizes are listed in our electric underfloor heating mats category, each available on its own or with a thermostat.
Step 4: Check the layout on your sketch
Before you order, draw the mat onto your sketch as 50 cm wide strips running back and forth across the free area. This quick check catches most problems:
- Turning the mat: at the end of each run you cut the fibreglass mesh, never the cable, and turn the mat back on itself.
- Spacing: heating cables must never overlap or cross, and runs must stay at least 8 cm apart.
- Start and finish: mark where the mat’s connection lead will run to the thermostat on the wall, and where the floor temperature sensor will sit between two runs of cable.
If the strips do not fit comfortably, drop to the next size down. Step-by-step installation guidance is in our underfloor heating installation guide.
Worked example: a family bathroom
A bathroom measures 2.4 m × 2.0 m, so the gross floor area is 4.8 m². Now take away the fixed items:
- Bath, 1.7 m × 0.7 m: 1.19 m²
- Toilet and vanity unit together: about 0.6 m²
Free floor area: 4.8 − 1.19 − 0.6 = 3.01 m². The right choice is the 3.0 m² mat, 6 m long, 450 W. The 3.5 m² mat would be half a square metre too big, with nowhere to put the extra cable.
Worked example: a kitchen with an island
A kitchen measures 4.5 m × 3.5 m, so the gross floor area is 15.75 m². The fixed items:
- Cabinets along two walls, 4.5 m and 2.9 m long, 0.6 m deep: 4.44 m²
- Island, 1.8 m × 0.9 m: 1.62 m²
Free floor area: 15.75 − 4.44 − 1.62 = 9.69 m². The right choice is the 9.0 m² mat, 18 m long, 1,350 W. It is tempting to round up to the 10 m² mat, but that would leave 0.3 m² of cable with nowhere to go except under the units.
For large open-plan kitchens and living areas, the 15 m² mat and 20 m² mat follow exactly the same rule.
Before you order
- Thermostat and floor sensor. Every mat is available on its own, if you already have a suitable underfloor heating thermostat, or with one of five Termofol thermostats: TF-H5 Standard, TF-H1 Touch Sensitive, TF-H6, TF-H6 Wi-Fi and TF Wi-Fi. A floor temperature sensor is included with every thermostat option. Compare them in smart thermostats and controls.
- Tiles and adhesive. The mat is laid directly in a flexible tile adhesive suitable for underfloor heating, under ceramic tiles or stone.
- Insulation. Insulation under the mat reduces heat loss into the subfloor, our Insulation Tile Backer Board is designed for this.
- Electrician. We recommend having the connection to the mains made by a qualified electrician, who will also check the total output of your mat against the thermostat and the circuit.
- Delivery. Delivery is free to the Republic of Ireland and Northern Ireland, usually within 1–3 working days. We recommend not booking your tiler or electrician until the mat has arrived.
Frequently asked questions
Can I cut a heating mat to fit my room?
You can cut the fibreglass mesh to turn the mat and follow the shape of the room. You must never cut or shorten the heating cable itself. That is why the mat size has to be chosen before you order, not adjusted on site.
My free floor area falls between two sizes. Which should I choose?
Choose the smaller one, the largest mat that still fits the free floor area without going under fixed units. If you are unsure, send us your room measurements and we will help you pick a size.
Can the mat go under a bath, shower tray or kitchen units?
No. Heating mats must not be laid under a bath or shower tray, under built-in units, or under furniture without legs. Furniture that stays in place over the heated floor needs legs at least 3 cm high.
Can I use a heating mat under laminate or wooden flooring?
No. The mats are designed to be laid in tile adhesive under tiles or stone. For laminate, vinyl or engineered wood, choose one of our underfloor heating film kits instead.
Not sure? Send us your measurements
If your room has an unusual shape, several fixed units or a free area above 20 m², contact Termofol Ireland with a sketch and the measurements before you order. Once you know your size, you will find every mat, with or without a thermostat, in our electric underfloor heating mats range.