Electric Radiant Floor Mat Calculator

How it works

Floor-warming mats go only where the floor is open. The heated area is the room's length × width minus the floor under cabinets, vanities, the toilet, the tub and other built-ins; SunTouch's TapeMat manual says never to install mats under cabinets or built-ins with no floor clearance. Mats come in fixed sizes and cannot be cut, so the mat area is the heated area rounded down to the mat size step.

The heating load is the mat area × the watt density, and the current is watts ÷ volts. At 12 W/sq ft that is 1 A per 10 sq ft at 120 V and 1 A per 20 sq ft at 240 V, which is what SunTouch's TapeMat specification sheet lists for every mat it sells.

SunTouch's TapeMat manual loads a circuit to less than its breaker rating: up to 12 A of mats on a 15 A breaker and 12–15 A on a 20 A breaker, with 15 A the most one SunStat control can switch. The calculator allows the continuous-load share of the breaker you enter (80% by default, which matches SunTouch's 12 A on 15 A) or the thermostat's own maximum, whichever is lower, and reports whether the mats fit on one circuit and how many circuits the load would need at least.

Formula

room area       = length × width
heated area     = room area − area not heated
mat area        = heated area rounded down to the mat size step
watts           = mat area × watt density
amps            = watts ÷ volts
allowed amps    = min(breaker × continuous-load limit %, thermostat rating)
fits            = amps ≤ allowed amps
largest mat     = allowed amps × volts ÷ watt density
circuits needed = amps ÷ allowed amps, rounded up (a lower bound; whole mats cannot be split)

Example

A 10 × 8 ft bathroom is 80 sq ft. Taking out 20 sq ft for the vanity, toilet and tub leaves 60 sq ft to heat, which is a 60 sq ft mat.

At 12 W/sq ft the mat draws 720 W, or 6 A at 120 V. A 15 A circuit at 80% allows 12 A, so it fits on one circuit; the largest mat area that circuit carries at 12 W/sq ft and 120 V is 120 sq ft.

Assumptions and limitations

• The room is a rectangle and the area not heated is entered as one total.
• Watt density is the mat's rated figure; the nameplate amp rating governs where it differs.
• Circuits needed (amps ÷ allowed amps, rounded up) is a lower bound. Whole mats cannot be split between circuits, so the mats' sizes can require more: three 8 A mats on circuits allowing 12 A each need 3 circuits, not 2.
• The 80% continuous-load limit and the 15 A thermostat rating are typical, editable values. SunTouch's manual uses them for its own products; other makers and controls have their own limits.
• Only the mats are counted. Other loads on the same circuit, such as lights or an exhaust fan, are not included.
• This is an estimate, not an electrical design or code compliance check. It is not a substitute for the mat and thermostat manufacturer's instructions, the electrical code in force where you live or a licensed electrician.

Frequently asked questions