Understanding your result
The headline is the heat the floor delivers in BTU per hour, with the output per square foot and the share of the room’s heating load it covers in the sub-line. The two bars put the floor’s output against the room’s load; when the first bar is shorter than the second, the room needs another heat source on the design night, and the notes say so.
The stats give the output per square foot, the room load in your climate zone and the approximate supply temperature. The table adds the floor-to-room temperature difference, the output in watts and watts per square metre (the units electric mats are sold in), the floor covering with its R-value and temperature limit, and the coverage percentage. A supply temperature above about 130 °F is a sign that the covering is working against the system; a condensing boiler stops condensing and a heat pump’s efficiency falls fast above that.
The room load is the same climate-zone rule of thumb the radiator output calculator and the baseboard heater calculator use, so the three emitters can be compared for the same room. For a room with large windows, the heat loss through walls, windows and roofs calculator gives a better load than the per-square-foot figure, and the boiler size calculator adds up the floors and radiators a boiler must serve.
How we calculate this
The coefficient of 2.0 BTU per hour per square foot per °F is the ASHRAE combined radiant and convective heat transfer coefficient for a heated floor at typical room conditions; it is the same figure the radiant panel chapter of the ASHRAE Handbook uses for first estimates. Output in watts is BTU/hr ÷ 3.412, and watts per square metre is that divided by 0.0929. The supply temperature estimate treats the path from the water or element to the floor surface as a series of resistances, the covering’s R-value plus about R-0.5 for the embedment in a slab or the subfloor over the tubing, and asks how much hotter the source must be to drive the output through them.
The room load per square foot is on the BTU per square foot table; the covering R-values are typical published values for flooring materials.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Floor heat transfer coefficient | 2.0 BTU/hr per sq ft per °F | ASHRAE Handbook, HVAC Systems and Equipment, panel heating (combined radiant and convective) |
| Floor temperature limit | 85 °F (80 °F for wood) | ASHRAE comfort limit for occupied floors; wood flooring manufacturers’ limits |
| Covering R-values | Tile 0.1, vinyl 0.3, laminate 0.6, wood 0.8, carpet with pad 1.5 | Typical published thermal resistances of flooring materials |
| Embedment resistance | R-0.5 | Typical for tubing in a thin slab or under a subfloor with plates; thick slabs are higher |
| Room load | Climate-zone heating load per sq ft × insulation factor | See the BTU per square foot table |
| Floor temperature | 80 °F | A common design surface temperature for living spaces |
Assumptions last reviewed October 7, 2026.
The calculator is a steady-state estimate. It does not model tubing spacing and the striping it causes, slab thickness and response time, downward losses into an uninsulated slab (which can be 10 to 25% without underslab insulation), or the bathroom case where a higher floor temperature is accepted. It does not size tubing loops or electric mats; that is the system designer’s job, and our sister site Floorings Calculator covers radiant mat sizing by floor type. The guide to radiators and baseboards compares emitters at low water temperatures.
Two worked examples
A tile floor in a mixed-climate living room
200 sq ft of heated tile floor at 80 °F in a 70 °F room, zone 4, average insulation.
- Output per sq ft: 2.0 × (80 − 70) = 20 BTU/hr per sq ft
- Total: 20 × 200 = 4,000 BTU/hr (1,172 W, 63 W/m²)
- Room load: 200 × 38 = 7,600 BTU/hr; coverage 53%
- Supply temperature: 80 + 20 × (0.1 + 0.5) = 92 °F
The floor covers half the room’s design load. Raising the floor to the 85 °F limit gives 30 BTU/hr per sq ft and 6,000 BTU/hr, still 21% short on the coldest night; good insulation would bring the load to 6,460 and nearly close the gap. In practice the room needs a small radiator or the floor plus a few degrees of tolerance on the two or three coldest nights a year, and 92 °F water is ideal for a heat pump.
A warm wood floor in a well-insulated warm-climate bedroom
120 sq ft of engineered wood floor at 84 °F in a 68 °F room, zone 3, good insulation.
- Output per sq ft: 2.0 × (84 − 68) = 32 BTU/hr per sq ft
- Total: 32 × 120 = 3,840 BTU/hr (1,125 W, 101 W/m²)
- Room load: 120 × 30 × 0.85 = 3,060 BTU/hr; coverage 125%
- Supply temperature: 84 + 32 × (0.8 + 0.5) = 126 °F
The floor covers the room with a quarter to spare, but the note objects to the floor temperature: 84 °F is above the 80 °F limit for wood, and the boards will dry and gap. At 80 °F the output is 24 BTU/hr per sq ft and 2,880 BTU/hr, 94% of the load, which with good insulation in Atlanta is enough. The 126 °F supply through R-0.8 wood is also at the edge of what a heat pump supplies efficiently.
Where to find your inputs
Heated floor area. The area actually covered by tubing or mats. Installers leave out the space under cabinets, bathtubs and fixed furniture, which can be a third of a kitchen or bathroom.
Floor surface temperature. The design temperature, which the floor thermostat or the mixing valve will hold; 80 °F is typical for living rooms, up to 85 for tile, lower for wood per the flooring manufacturer.
Covering. What will be on top of the heated layer. For carpet, the pad matters as much as the carpet; a radiant-rated pad is about R-0.5 instead of R-1 or more.
Zone and insulation. The climate zone by county and the room’s insulation; a room with large windows is “poor” for this purpose even if the walls are good.
Common mistakes
- Expecting a floor to carry a cold-climate house alone. Floors top out around 30 BTU/hr per sq ft; zone 5 to 7 rooms with average insulation lose 45 to 60.
- Running a wood floor above 80 °F. It gaps and cups; respect the flooring maker’s limit.
- Putting thick carpet and pad over radiant heat. It needs water 30 to 40 °F hotter than the floor and wastes the low-temperature advantage.
- Counting the whole room as heated. Cabinets and fixtures cover a lot of floor; count only the area with tubing or mats.
- Skipping insulation under the slab. Without it a quarter of the heat can go into the ground.
- Sizing mats from the output. Electric mats are sized by the installer in watts per square foot; this calculator says how much heat the floor can deliver, not how to lay it out.
Questions people ask
- How many BTU per square foot does radiant floor heating deliver?
- About 2 BTU per hour per square foot for every degree Fahrenheit the floor surface is above the room air. A floor at 80 °F in a 70 °F room gives about 20 BTU per hour per square foot; at the 85 °F comfort limit in a 68 °F room, about 34. That ceiling of roughly 20 to 35 BTU per hour per square foot is the defining constraint of radiant floors, and a room that loses more than that per square foot needs supplemental heat or better insulation.
- What is the maximum temperature for a heated floor?
- ASHRAE puts the comfort limit for a floor people stand on at about 85 °F (29 °C); above it feet feel uncomfortably hot over time. Wood flooring is usually limited to 80 to 82 °F by its manufacturer, because higher temperatures dry and gap the boards; some engineered products allow 85. Bathrooms where people are barefoot briefly are sometimes run to 90 °F. The calculator warns when the floor temperature exceeds the covering's limit.
- Can radiant floor heating heat a whole house?
- In a well-insulated house, yes; in an average or poorly insulated one in a cold climate, usually not on its own. A room with average insulation in zone 4 loses about 38 BTU per hour per square foot on the design night and in zone 6 about 52, while a floor at the comfort limit delivers about 30. Good insulation brings the zone 4 figure down to about 32, which a tile floor can nearly meet. Radiant floors suit tight houses, mild climates, and rooms where a second heat source handles the coldest nights.
- Does the floor covering matter for radiant heat?
- A lot. Heat has to pass through the covering, and its resistance decides how much hotter the tubing or element must be than the floor surface. Tile and stone (R-0.1) are almost transparent; vinyl and laminate (R-0.3 to 0.6) are fine; engineered wood (about R-0.8) needs noticeably warmer water; carpet with a standard pad (R-1.5 or more) can need water 30 to 40 °F hotter than the surface, which pushes a heat pump or condensing boiler out of its efficient range. Low-R pads made for radiant floors help.
- What water temperature does a radiant floor need?
- Roughly the floor surface temperature plus the output per square foot multiplied by the thermal resistance between the water and the surface (the covering plus about R-0.5 for the slab or subfloor over the tubing). For 20 BTU per hour per square foot under tile, that is about 12 °F above the floor, so 92 °F water for an 80 °F floor; under carpet it is about 40 °F above, so 120 °F. Low water temperatures are why radiant floors pair well with heat pumps and condensing boilers.
- How is electric radiant floor heating sized?
- Electric mats and cables are sold by watts per square foot, typically 10 to 15 W per square foot (34 to 51 BTU per hour per square foot of input), and they heat the floor to whatever temperature the thermostat allows up to the covering's limit. The output into the room is still governed by the floor surface temperature; a 12 W per square foot mat simply reaches the limit faster. Sizing mats to a specific floor is the job of the installer or of our sister site Floorings Calculator.