Underfloor Heating Calculator

Estimate heated floor area, W/sq ft or W/m2 output, room heat loss, floor finish derate, supply temperature, warmup energy, and total zone load for electric mats or hydronic underfloor heating.

Heated area Zone load Floor finish derate Warmup energy

🏠Room Presets

Choose a real underfloor heating scenario, then adjust dimensions, clearances, floor finish, heat-loss profile, and system type for the actual room.

📏Underfloor Heating Inputs

Room shape

UFH system type

Room length (ft)

Room width (ft)

Room diameter (ft)

Circle area method

Room base (ft)

Room height (ft)

Room area (sq ft)

Custom area use

Usable heated area (%) Subtract islands, tubs, permanent cabinets, and clearances where heat cable or pipe will not be installed.

Design output density

Floor finish derate

Room heat-loss profile

Custom heat loss (BTU/hr sq ft)

Floor assembly

Heated mass thickness (in)

Custom thickness (in)

Warmup temperature rise (F) Warmup energy uses the selected assembly, heated area, thickness, and sensible heat of the floor mass.

Sizing buffer

Enter positive room dimensions and a usable heated area between 30% and 100%.

Heated area formula Room area x usable heated area percent. Custom mode lets you enter a measured room area directly.

Zone load formula Max of adjusted UFH output and room heat loss, then multiplied by the selected sizing buffer.

Supply temperature logic Hydronic temperature is estimated from output density, assembly response, floor derate, and heat-loss severity.

Heated Area

sq ft and m2

Zone Load

W and BTU/hr

Output Density

W/sq ft and W/m2

Temp Or Warmup

supply temp or kWh

Full Calculation Breakdown

Use manufacturer limits for cable spacing, thermostat sensor placement, floor maximum temperature, and hydronic water temperature before final specification.

⚙UFH System Spec Grid

12-15 W/sq ft mats Electric mats are common below tile in bathrooms, kitchens, and small comfort-heating zones.

8-10 W/sq ft cable Loose cable adapts to irregular floor plans and lets the clear heated area be shaped more precisely.

6-12 W/sq ft hydronic Hydronic tubing output depends on spacing, supply temperature, plates, slab depth, and floor finish.

27-29C finish limit Many wood, laminate, vinyl, and comfort floors use a surface limit near 80-85 F.

📊Electric Mat And Hydronic Comparison

Electric Mat Or Cable

Typical output8-15 W/sq ft
Best scaleSingle rooms
ResponseFast to medium
Load sizingWatts and amps
Design limitFloor finish temp

Hydronic Tube System

Typical output6-12 W/sq ft
Best scaleLarge zones
ResponseMedium to slow
Load sizingBTU/hr and gpm
Design limitSupply temp

📘Reference Tables

UFH system	Typical output	Metric output	Common use	Calculator assumption
Electric heating mat	12-15 W/sq ft	129-161 W/m2	Tile bathrooms and kitchens	12 W/sq ft default
Loose electric cable	8-12 W/sq ft	86-129 W/m2	Irregular rooms and clearances	10 W/sq ft default
Electric foil heater	6-10 W/sq ft	65-108 W/m2	Laminate and floating floors	8 W/sq ft default
Hydronic slab tubing	8-12 W/sq ft	86-129 W/m2	Concrete slabs and screeds	10 W/sq ft default
Hydronic plates	6-10 W/sq ft	65-108 W/m2	Wood decks and retrofit plates	8 W/sq ft default

Floor finish	Derate used	Typical surface limit	Effect on output	Design note
Tile or stone	3%	85-90 F	Very small reduction	Best heat transfer for UFH
Polished concrete	2%	85-90 F	Very small reduction	High thermal mass, slow response
LVT or vinyl	8%	80-85 F	Moderate output reduction	Respect finish maker limit
Laminate	12%	80-85 F	Moderate output reduction	Foil or low-temperature system common
Engineered wood	18%	80-82 F	Large output reduction	Keep temperatures conservative
Low-tog carpet	25%	80-82 F	Large output reduction	Total tog strongly affects output

Room heat-loss profile	BTU/hr sq ft	W/sq ft	W/m2	Typical condition
Low loss	15	4.4	47	Well insulated interior room
Average	22	6.4	69	Typical insulated room
High loss	30	8.8	95	Older envelope or more exterior wall
Very high loss	40	11.7	126	Sunroom, slab edge, or cold climate room

Common project size	Clear heated area	Example output	Estimated zone load	Likely system
Small bathroom	55-75 sq ft	12 W/sq ft	660-900 W	Electric mat
Kitchen tile area	120-180 sq ft	10-12 W/sq ft	1.2-2.2 kW	Mat or cable
Open living zone	250-450 sq ft	8-10 W/sq ft	2.0-4.5 kW	Hydronic plates
Basement slab	350-650 sq ft	10-13 W/sq ft	3.5-8.5 kW	Hydronic slab
Whole-house UFH	1,200-2,500 sq ft	6-10 W/sq ft	7-25 kW	Hydronic zones

💡Practical UFH Sizing Tips

Use clear heated area, not wall-to-wall area.

Underfloor heating should be sized from the area where cable, mat, foil, or tubing can actually run. Permanent cabinets, tubs, islands, hearths, and no-heat perimeter clearances reduce the heated area and raise the effective load per square foot.

Compare output against room heat loss.

A warm floor comfort system can be smaller than a primary heat source. When the calculated heat loss exceeds the floor output after derating, the zone needs more active area, tighter hydronic design, a different floor finish, or supplemental heat.

Underfloor heating systems requires precise sizing because the sizing of the underfloor heating system must equal the heat output of the floor relative to the heat loss of the room. If the heat output of the floor dont equal the heat loss of the room, then the underfloor heating system will struggle to achieve and maintain the desired temperature within the floor and the room on cold days. Thus, to size the underfloor heating system, you must calculate the heat loss of the room and you must set that equal to the output of the underfloor heating system.

To determine the usable floor area of a room, you must measure the area of the floor. However, you must subtract areas of the floor that will not be able to recieve the heat from the underfloor heating system, such as areas that include built-in cabinet, countertops, islands, or tubs. If you dont subtract these area from the total measurement of the floor, you will calculate the area of the usable floor area incorrectly.

How to Size Underfloor Heating

An incorrect area of the usable floor area will result in calculating the heat output of the underfloor heating system incorrectly. The amount of heat that the floor will have to output is based off the heat loss of the room. For instance, if the room is an interior room with good insulation, it may lose only 15 BTU of heat per square foot per hour.

In contrast, a sunroom that has three exterior wall may lose more heat than the interior room. Thus, the sunroom will have to have a higher heat output from the underfloor heating system. The type of floor finish that you plan to use will also affect the performance of your underfloor heating system.

Floors made with tile and stone conduct heat readily from the underfloor heating system. Thus, an underfloor heating system that use these types of floor finishes can meet the heat needs of the room even if the underfloor heating system operates at modest temperature. Floors that are finished with wood, laminate, or carpet, however, present a resistance to the heat that the underfloor heating system is distributing.

Thus, the underfloor heating system will have to work harder to heat these types of floor finishes to the desired temperature. If the underfloor heating system has to work harder to distribute heat to these types of floor finishes, the temperature of the floor may be higher than if it were tiled or made of stone. Thus, you will have to ensure that the underfloor heating system does not heat the floor to a temperature that exceeds the limit of that type of floor finish.

The better the conducting properties of the floor finish, the lower the water temperature and wattages of the underfloor heating system that will have to be used. The type of underfloor heating system that you will use will also have to be considered when sizing the system. For instance, electric mats and electric cable have the benefit of being able to heat quickly to new temperatures, so they are often used in underfloor heating systems in rooms of small sizes.

Underfloor heating systems that use hydronic tubing that is embedded within a thick slab of flooring will take longer for the floor to reach the desired temperature due to the thermal mass of the hydronic tubing. Thus, hydronic tubing will have to take more time to warm up to the same temperatures as the electric mats and electric cables. Hydronic tubing will have the benefit of being able to store more thermal energy due to the thermal mass of the tubing; it will heat up to the desired temperature, and the floor will retain that heat for longer periods of time.

Thus, a room that experiences high heat loss will have to use underfloor heating systems with hydronic tubing at closer distance to the floor finish; a room that has low heat loss the underfloor heating system with hydronic tubing can heat to a lower temperature. One more factor to consider when sizing an underfloor heating system is the energy that will be required for warmup. Warmup energy is the amount of energy that will be required to heat the mass of the floor by a specific number of degree.

For example, thin floor assemblies will contain less mass than thick floor assemblies. Thus, thin floor assemblies will require less warmup energy to heat to the desired temperature. Thin floor assemblies are beneficially used in rooms that is used only occasionally.

Reference tables are also provided for underfloor heating systems to assist in checking the inputs of the underfloor heating system. These reference tables display the typical heat output, limits of floor finishes, heat loss profiles of rooms of different sizes, and the number of heats that various sizes of zones within the underfloor heating system will provide. These reference tables will help you to determine if you will use one large zone or several smaller zone in your underfloor heating system.

Finally, you should be able to measure the usable floor area, determine the heat loss of the room, and decide the type of floor finish that you will use. By following these steps in sizing your underfloor heating system, you will ensure that the underfloor heating system will remain within the limits of the temperature requirements of the floor finish and the underfloor heating system wont short-cycle.