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Electric Underfloor Heating Mat Area Calculator

The floor area an electric heating mat can cover, room area less fixtures and the wall setback, and the largest standard mat size that fits it.

  • Answers as you type
  • Every formula cited
  • Calculated in your browser
SettingsSettings for this calculationUS
Market
Imperial · sales tax
Wall to wall, at floor level.

Measure at the floor rather than off a plan: a room with a boxed pipe run or a chimney breast is smaller on the floor than it is on the drawing, and the mat has to fit the floor.

The other dimension, also at floor level.

For an L-shaped room, work each rectangle separately and add the heatable areas — a single length and width cannot describe the shape and would overstate it at the corner that is not there.

The clear margin the cable may not enter.

Manufacturers commonly ask for 50 to 100 mm — about 2 to 4 inches — from every wall. It keeps the cable away from the skirting, from the silicone line at the floor junction, and from wherever somebody later drills a fixing. It applies at every wall, so it takes twice the setback off each dimension.

Everything standing on the floor: WC, pedestal, bath, units.

A close-coupled WC is roughly 0.3 m² and a basin pedestal about 0.1 m², which is 0.4 m² of a small bathroom before anything else. A bath is far larger — a standard 1700 mm tub is around 1.2 m² — and fitted furniture is whatever it is. Add them all: no mat goes under any of them, because heat under a sealed fixture has nowhere to go and the cable cooks.

Heatable floor area

49.9 sq ft

Medium confidence

77% of the room's floor is heatable once the setback and the fixtures come off. The largest standard mat that still fits is the one in the breakdown; makers' size lists differ by a size here and there, so check it against the list of the mat you are buying.

Room floor, gross
65 sq ft
After the setback at every wall
59.61 sq ft
Fixtures deducted
9.69 sq ft
Largest standard mat that fits
45 sq ft
Floor left unheated by that mat
4.92 sq ft
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • Heating mat is laid only on the free floor: never beneath a fixed fixture such as a WC pan, basin pedestal, bath or fitted furniture, where the heat it produces has no path out of the floor
  • A setback is kept from walls and fixtures so the cable clears skirtings, sealant lines and later fixings; manufacturers commonly specify between 50 and 100 mm
  • The mesh backing may be cut and turned to fit a shape, but the heating cable itself must never be cut or shortened — a mat is selected to fit the area rather than trimmed to it
  • SunTouch TapeMat installation manual, Table 2 - Mat Sizes: 120 VAC mats 2 ft wide in 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 and 100 sq ft — the whole-square-foot sizes the imperial breakdown chooses from

Inputs used

Room length
10 ft
Room width
6.5 ft
Setback from walls
2 in
Footprint of fixed fixtures
9.69 sq ft

Intermediate steps

Room floor, gross
65 sq ft
After the setback at every wall
59.61 sq ft
Fixtures deducted
9.69 sq ft
Largest standard mat that fits
45 sq ft
Floor left unheated by that mat
4.92 sq ft
Final result49.92 sq ft

Confidence note: 77% of the room's floor is heatable once the setback and the fixtures come off. The largest standard mat that still fits is the one in the breakdown; makers' size lists differ by a size here and there, so check it against the list of the mat you are buying.

What this calculation does not cover

  • NOTHING GOES UNDER A FIXED FIXTURE, and this is a safety point rather than an efficiency one. A cable beneath a WC pan, a bath or a run of units has no path for its heat out of the floor, and it overheats. That is the commonest way a mat fails in its first winter, and no area calculation catches it if the fixtures were not deducted.
  • THE CABLE CANNOT BE CUT. The mesh backing is cut and turned to work the mat around a shape; the heating cable inside it must never be shortened. A mat that does not fit is returned, not adjusted, which is why the answer here is matched against standard sizes rather than left as a bare area.
  • This is an AREA, not a heat output. Whether the wattage that area delivers will warm the room depends on the floor build-up, what is under it, the insulation below and how the room loses heat — a mat laid over an uninsulated concrete slab heats the slab first and the room eventually.
  • An L-shaped or stepped room is not one rectangle. Work each part separately and add the heatable areas; a single length and width overstates the floor by the corner that is not there.
  • Nothing here covers the thermostat, the floor sensor's position, the cold tail's route back, or whether the circuit and its RCD protection are adequate. Those are an electrician's, and in most markets the connection is notifiable work.
10 ft6.5 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Part of a bigger job

This trade is one line of a job takeoff. Run the whole job and every other trade comes back with it, off the same measurements.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-26 · v1.1.0

Regulatory standards & verification citations4
  1. Heating mat is laid only on the free floor: never beneath a fixed fixture such as a WC pan, basin pedestal, bath or fitted furniture, where the heat it produces has no path out of the floor
  2. A setback is kept from walls and fixtures so the cable clears skirtings, sealant lines and later fixings; manufacturers commonly specify between 50 and 100 mm
  3. The mesh backing may be cut and turned to fit a shape, but the heating cable itself must never be cut or shortened — a mat is selected to fit the area rather than trimmed to it
  4. SunTouch TapeMat installation manual, Table 2 - Mat Sizes: 120 VAC mats 2 ft wide in 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 and 100 sq ft — the whole-square-foot sizes the imperial breakdown chooses from
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Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

How to calculate electric underfloor heating mat area in 5 steps

  1. Room lengthWall to wall, at floor level.
  2. Room widthThe other dimension, also at floor level.
  3. Setback from wallsThe clear margin the cable may not enter.
  4. Footprint of fixed fixturesEverything standing on the floor: WC, pedestal, bath, units.
  5. Heatable floor areaThe tool computes the heatable floor area from those figures and shows the formula, its sources, and a confidence rating alongside it.

Frequently asked questions

Why can I not run the mat under the bath or the WC?
Because the heat has nowhere to go. A cable under an open floor gives its heat up through the tile into the room; a cable under a sealed fixture that sits flat on the floor cannot, so the temperature under that fixture climbs until something gives. It is the commonest way a mat fails in its first winter, and it is a safety point rather than an efficiency one. It is also why the deduction on this page is the whole calculation — a small bathroom can lose a third of its floor to a WC, a pedestal and a bath before anything else.
Can I cut the mat to fit?
You can cut the mesh. You must never cut the cable. The two are different things and the distinction matters: the mesh backing is there to hold the cable at the right spacing, and cutting between the runs so you can turn the mat around a corner is exactly how it is designed to be fitted. The heating cable itself is a fixed length with a fixed resistance, and shortening it changes both. A mat that does not fit the area is exchanged for one that does, which is why this page reports the largest standard size that fits rather than a bare number.
How much floor do the fixtures really take?
About 1.6 m² (17 sq ft) in a compact bathroom with a WC, a basin pedestal and a 1700 mm (67 in) bath, roughly a quarter of a 3 by 2 m (10 by 6.5 ft) room before the setback. That is more than people expect in a small room: a close-coupled WC is around 0.3 m², a basin pedestal about 0.1, and a standard 1700 mm bath about 1.2. It is the single reason people order a mat that then does not fit. Measure the footprints rather than estimating them, because the deduction is the answer.
Is a mat the right product for my room?
Not always, and the page says so when it is not. Below about a square metre (10 square feet) of heatable floor there is no standard mat small enough, and a room of an awkward shape wastes a lot of a rectangular mat in the offcuts you cannot use. Loose cable, laid at a spacing you choose, suits both of those better — it costs more in labour and less in wasted product, and it follows a shape a mat cannot. What this page gives you is the heatable area, which is the number that decides between them.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.