Underfloor Heating Calculator

Underfloor Heating Guide

What's the difference between Electric and Water UFH?

Electric underfloor heating uses resistive heating mats or cables installed under the floor. It is ideal for smaller areas (bathrooms, en-suites, kitchens), straightforward DIY or professional installation, and retrofitting in existing properties. Water (hydronic) underfloor heating uses pipes circulating hot water — lower running cost per hour but requires significant infrastructure (manifold, pump, mixer valve) and is most cost-effective in new builds or major renovations. For areas under 20m², electric systems are usually the simpler and more cost-effective choice, since the higher running cost per hour matters less over a smaller space.

What do I need to know about Wattage Requirements?

Standard electric UFH mats run at 150–200W per m². For bathroom floor warming: 150W/m² is sufficient. For primary heating (bathroom or kitchen): 200W/m². Areas next to exterior walls or ground-floor rooms in cold climates may need higher wattage. Typical bathroom (3.5m² heated area): 3.5 × 150W = 525W total. Running for 4 hours per day at 24.5p/kWh: 0.525kW × 4h × 24.5p = 51.5p per day. Annual cost at this usage: approximately £188.

What do I need to know about Floor Covering Compatibility?

Best performance (lowest thermal resistance): ceramic and porcelain tiles, natural stone. These heat quickly and retain warmth well. Good performance: LVT/luxury vinyl tile (modern UFH-compatible products). Moderate: engineered wood and laminate — check the manufacturer's maximum surface temperature limit (usually 27°C). Poor: carpet significantly insulates against the heat, reducing efficiency and potentially exceeding floor temperature limits. Traditional underlay adds additional resistance to heat flow, which is why underfloor heating manufacturers usually specify a thin, UFH-compatible underlay rather than a standard thick one.

What's the key thing to understand about Installation Essentials?

Always install a thermostat with a floor sensor (not just air sensor) — floor sensors prevent overheating and protect the floor covering. Two-hour concrete screed (for wet UFH) or self-levelling compound (for electric mat systems) must cover the mat before tiling. Electric UFH mats should never be overlapped or cut (except at the turning loops). Plan the mat layout before purchasing — the number of turns and room shape affect which mat size you need. Have an electrician connect the thermostat — this is notifiable electrical work under Part P and should not be attempted without the correct qualifications.

What do I need to know about Insulation Beneath UFH?

Underfloor heating relies on warming the floor's surface so heat radiates upward into the room, which means any heat able to escape downward or sideways into the subfloor, rather than upward into the room, is wasted energy that still shows on the running cost this calculator estimates. For this reason, proper insulation beneath an underfloor heating system, whether electric matting or water-filled pipes, is just as important as the heating element itself, and is sometimes overlooked when retrofitting UFH into an existing floor rather than installing it as part of new construction. On a ground floor over a concrete slab, rigid insulation boards are typically laid beneath the heating system specifically to direct heat upward rather than losing it into the ground below, which otherwise acts as a significant, continuous heat sink. On an upper floor between heated rooms, insulation beneath the UFH matters less for heat loss, since the room below is also heated, but can still help direct heat upward more quickly and efficiently, improving how fast the room responds to the heating being switched on, which is a particular consideration for electric UFH systems where responsiveness affects running cost over short on-off cycles.

What should I know about Response Time — Electric vs Wet UFH?

Electric underfloor heating mats generally warm up and respond to being switched on considerably faster than a water-based, or wet, underfloor heating system, which matters for how it's used day to day and for the running cost figures this calculator produces. Electric systems typically have a relatively thin heating element close to the floor's surface, meaning they can bring a room noticeably closer to its target temperature within a shorter period, which suits electric UFH well to being used on a timer for short, specific periods, such as warming a bathroom floor just before and during use, rather than running continuously. Wet UFH systems, where warm water circulates through pipes typically embedded in a thicker screed layer, store and release heat much more slowly, since the pipes have to first warm a substantial mass of screed before that heat reaches the surface, meaning wet systems respond more sluggishly to being turned on or off but also hold their heat for longer once the mass of screed has warmed through. This is why wet UFH is generally better suited to being run for longer, steadier periods at a lower, consistent temperature across a whole floor area, while electric UFH suits shorter bursts of use in smaller areas such as a bathroom or ensuite floor.

What should I know about Zoning and Thermostats for UFH Systems?

Dividing underfloor heating into separate zones, each with its own thermostat, allows different rooms to be heated to different temperatures and on different schedules, which both improves comfort and reduces running costs compared with one thermostat controlling the whole system. A typical setup uses a manifold (for water-based systems) with a separate circuit per room or zone, each fitted with an actuator controlled by that zone's thermostat, while electric UFH naturally allows a thermostat per room since each mat or cable run is usually independent anyway. Zoning matters because rooms vary enormously in heat loss and use pattern — a well-insulated bedroom used only at night has very different heating needs from an open-plan kitchen-diner used all day — and a single thermostat for a large area will inevitably leave some parts too warm and others too cool. Smart thermostats with weather compensation or load compensation can further improve efficiency for water-based UFH by adjusting the flow temperature based on outside conditions rather than just cycling on and off. When planning a system, decide on zoning at the design stage, since retrofitting additional zones to a water-based system later usually means reworking the manifold and pipe runs, which is far more disruptive than specifying it correctly from the outset.

Reference: Typical Wattage by Room and Floor Covering

Electric underfloor heating output is specified in watts per square metre, and the right figure depends heavily on both the room's heat loss and what sits on top of the heating mat, because floor coverings vary enormously in how much heat they let through.

Manufacturers rate wood and laminate floors for a maximum surface temperature, usually around 27°C, both for comfort and to avoid drying out or warping the covering, so a kit intended for tile cannot simply be fitted under wood at full output — the thermostat must be set to a floor-temperature limit rather than a room-temperature target.

Worked Example: Sizing and Costing a Bathroom Install

Take a 4m2 bathroom being retiled, with electric underfloor heating as a comfort boost alongside a towel radiator. At 150 W/m2, the mat needs a rated output of 4 x 150 = 600W. Running cost depends on how long the thermostat actually calls for heat, not the rated wattage alone — a well-insulated bathroom on a timer for two hours a morning draws power for roughly 60-70% of that window once it reaches temperature and starts cycling. At a typical UK electricity price of around 25p/kWh: Energy used per day = 0.6 kW x 2 hours x 0.65 (duty cycle) = 0.78 kWh Daily cost = 0.78 x £0.25 = roughly 19-20p Cost over a year (used most days) = approximately £65-75 That is a modest running cost for the comfort gained, which is why electric UFH is popular in bathrooms even though it is more expensive per kWh to run than a wet (water-based) system — the area and daily runtime are both small. A wet UFH system heating an entire ground floor runs for far more hours a day over a much larger area, so its running cost comparison against radiators depends much more on the efficiency of the boiler or heat pump supplying it, covered in the heat pump vs gas boiler guide on this site.