Understanding your result
The headline is the heated area, with the watts, amps and circuit beneath it. The stats repeat the heater size, the current and the circuit; the table gives the room, the heated area and its share of the room, the heater size, the current, the circuit and the approximate length of a mat of the width you entered.
Heated area is the room less the margin at the walls and less the area under fixtures. The margin is taken off all four walls; the fixtures are taken off in total. The result is the open floor, the part people actually stand on, and it is the figure to match against a kit: choose the kit whose heated area is the size just below this figure, never above, because cable cannot be cut.
Heater size is that area times the output per square foot, and the current is the watts divided by the voltage. The circuit applies the 125% continuous-load rule: the circuit must carry 1.25 times the heater current, so a 15 A circuit takes up to 12 A of heater. For what the heat costs to run, use the radiant floor running cost calculator with the watts from here; for the tile that goes on top, the floor tile calculator, and for the mortar over the mat, the thinset calculator, since mats embedded in thinset use more of it.
How we calculate this
The continuous-load rule is from the National Electrical Code (an electric space-heating load is treated as continuous and its branch circuit sized at 125%), and the GFCI requirement for floor heating cables is in NEC 424.44. Output figures come from the makers of mats and membrane systems; a membrane system at standard spacing gives about 12.7 W per sq ft. The guide to planning electric radiant heat walks through layout and thermostats, and the shower tiling guide covers heated bathroom floors under tile.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Output | 12 W per sq ft (130 W/m²) | Typical mats and cables (10 to 15 W per sq ft); Schluter DITRA-HEAT about 12.7 at standard spacing |
| Wall gap | 4 in (10 cm) | Typical maker instructions: 3 to 6 in from walls, 4 in from a toilet flange |
| Voltage | 120 V (230 V metric) | Typical US bathroom circuit; larger floors use 240 V |
| Circuit sizing | 125% of the load | NEC continuous-load rule for space heating |
| GFCI | required | NEC 424.44 |
| Thermostat limit | 15 A | Typical floor-heat thermostats; larger loads use a relay or power module |
Assumptions last reviewed October 8, 2026.
The calculator handles electric mats and cables only; hydronic floors with water pipes are sized by a heating designer. It does not design the circuit, choose the wire gauge or check the panel’s capacity, which is the electrician’s job. It does not account for heat lost into an uninsulated slab, which can halve the warmth felt; insulating board under the heater fixes that. And it treats the fixtures as one total area, so a complicated bathroom is best checked against the maker’s free layout service.
Two worked examples
A 9 × 7 ft bathroom
18 sq ft under the vanity, toilet and tub edge, 4 in margin, 12 W per sq ft, 120 V, 20 in mat.
- Inside the margin: 8.33 × 6.33 = 53 sq ft; less 18 sq ft fixtures = 35 sq ft heated
- Heater: 35 × 12 = 417 W; current 417 ÷ 120 = 3.5 A
- Circuit: 3.5 × 1.25 = 4.3 A, a 15 A circuit; mat about 21 ft long at 20 in wide
Just over half the room is heated, which is normal in a bathroom; the heat is where the feet are. A 35 sq ft kit is too big if your measurements are generous; choose the size below.
A 16 × 12 ft kitchen at 240 V
48 sq ft under the cabinets and island.
- Inside the margin: 15.33 × 11.33 = 174 sq ft; less 48 = 126 sq ft heated
- Heater: 126 × 12 = 1,509 W; current at 240 V = 6.3 A
- Circuit: 6.3 × 1.25 = 7.9 A, a 15 A circuit
At 120 V the same heater would draw 12.6 A and need a 20 A circuit, which is why kitchens and larger rooms are usually wired at 240 V. The running cost is the same either way; the watts decide it.
Where to find your inputs
Room. Wall to wall at floor level.
Fixtures. The footprint of the vanity, toilet, tub or shower base, cabinets and island, added together.
Wall gap. From the heating system’s installation manual.
Output and voltage. From the kit you are considering; the voltage from the circuit the electrician will provide.
Common mistakes
- Sizing by the room. Heat goes only where there are no fixtures. Use the open floor.
- A kit bigger than the space. The cable cannot be cut. Choose the size below.
- No GFCI. Floor heat in a bathroom must be GFCI-protected.
- Testing the cable once. Test resistance before, during and after tiling, as the maker says.
- No insulation under a slab. Much of the heat goes down. Insulate where you can.
- DIY wiring. The connection is electrician’s work in most places.
Questions people ask
- What size radiant heating mat for a bathroom?
- Measure the open floor where people stand. A 5 × 8 ft bathroom with about 12 sq ft under the vanity, toilet and tub edge, and a 4 inch gap at the walls, has about 20 sq ft to heat, so a kit of about 20 sq ft at 12 W per sq ft, 240 W, drawing 2 A at 120 V.
- How many watts per square foot for floor heating?
- Most electric mats and cables deliver 10 to 15 W per sq ft; 12 W is typical for a mat and about 12.7 W for a membrane system at standard cable spacing. More watts per square foot heat faster and cost more to run.
- What circuit does electric floor heat need?
- A dedicated circuit with GFCI protection, sized at 125% of the heating load because it is a continuous load. Up to about 1,440 W at 120 V fits a 15 A circuit; a larger floor goes to 240 V or a second circuit. Most thermostats switch up to 15 A.
- Can I cut a floor heating mat?
- You can cut the mesh backing to turn a corner, but never the heating cable itself; cutting it changes its resistance and can cause a fire. That is why the kit must be sized to fit the heated area without trimming the cable.
- Does floor heat go under the toilet or vanity?
- No. Heating cable must not run under fixtures, cabinets, or a bathtub, where the heat is trapped. Most makers also keep the cable at least 4 inches from a toilet wax ring and a few inches from walls.
- Do I need an electrician for radiant floor heat?
- The mat can be laid by a careful DIYer, but the connection to the circuit and the thermostat is electrical work that in most places needs a licensed electrician and an inspection. Check the rules where you live.