Underfloor heating vs radiators: what does your result mean?
Start with your numbers, not a generic list of pros and cons. The three values below show whether each emitter can actually cover the same room heat demand.
Change any input above and this interpretation updates automatically.
Next: compare cost, running temperature, heat-pump compatibility and response time before deciding which system is better for your room.
Performance check: do both emitters cover the room heat demand?
This calculator is a performance and sizing check. It asks whether the underfloor-heating area and the radiator can each deliver the room heat demand at the entered operating conditions. Broader questions about comfort, retrofit work, installation cost and which emitter is the better choice belong in the linked guide.
Underfloor heating spreads heat over a large surface, so it can often deliver the required room output with lower water temperatures. Radiators concentrate the output in a much smaller area, which means their available power falls as flow and return temperatures are reduced. A radiator that looks comfortably oversized at catalogue conditions can become marginal when used in a low-temperature system.
The result above gives you the practical starting point: if one option is below 100% coverage, it does not meet the entered heat demand under the assumptions you selected. If both are above 100%, the decision moves on to comfort, controls, installation constraints, response time and the heat source.
Underfloor heating: strengths and limits
Water-based underfloor heating is a large low-temperature emitter. Its main advantage is the active floor area: a modest heat output per square metre can add up to substantial room output when most of the floor is available.
The limitation is that not every square metre is necessarily active. Fixed kitchen units, baths, built-in furniture or other areas may reduce the heated surface. Floor covering and floor construction also affect achievable W/m². The value entered in the calculator should therefore come from the planned system rather than from a generic assumption.
Underfloor heating also has greater thermal inertia. This can create stable comfort, but it normally reacts more slowly to rapid thermostat changes than a radiator.
Radiators: strengths and limits
Radiators are compact, familiar and fast to respond. They are often easier to install in an existing building because they do not require a complete floor build-up.
The key limitation in a low-temperature system is output correction. A radiator catalogue rating such as 2,000 W at ΔT50 is not the same as 2,000 W at 55/45/20°C or at heat-pump temperatures. The calculator applies the radiator exponent to estimate how much output remains at the actual mean water-to-room temperature difference.
Large or low-temperature radiators can still work very well. The important point is to size them from corrected output, not from the headline ΔT50 figure alone.
Compare cost only after checking emitter output
A cost comparison is useful only after both emitters have been shown to meet the same room heat demand. Otherwise you may be comparing a correctly sized option with an emitter that cannot actually deliver the required heat.
Installation cost depends heavily on whether the system is going into a new build, a full renovation or an occupied existing property. Water underfloor heating usually needs more floor work and more installation steps, while replacing or adding radiators can be simpler in an existing hydronic system. That does not automatically tell you which system will be cheaper to run.
Running cost starts with the building heat demand. If the room loses 1,200 W under the design condition, the emitter still needs to supply about 1,200 W whether that heat comes through the floor or through a radiator. The potential difference is that lower system temperatures can improve the efficiency of some heat sources, especially heat pumps and condensing systems. Controls, zoning, insulation, operating hours and energy tariffs can easily change the final bill.
For an annual estimate, use the Heating Cost Calculator after establishing the room or building heat demand.
Do not infer energy use from the heat emitter alone
Annual energy use cannot be calculated reliably from emitter type alone. The useful way to analyse cost is to follow the energy path from the room backwards.
- Room heat loss: insulation, airtightness, windows, outdoor temperature and ventilation determine how much heat the room needs.
- Emitter temperature: the floor or radiator must deliver that heat at a practical flow and return temperature.
- Heat-source efficiency: a heat pump, boiler or district-heating interface may behave differently at different system temperatures.
- Controls and operation: schedules, zoning, setbacks and user behaviour affect delivered energy.
- Energy price: the same useful heat can cost very different amounts depending on the tariff and heat source.
This calculator handles step two: it checks whether each emitter can supply the required heat at the entered conditions. That prevents a misleading cost comparison based on a radiator that is actually too small or an underfloor system that requires an unrealistic W/m² output.
Heat-pump check: will the emitters work at low flow temperature?
For a heat pump, the calculation should answer whether the available emitter area can cover the room heat loss at the intended low flow temperature. Underfloor heating uses a large surface, while radiators need to be checked at their corrected low-temperature output. Use the calculated coverage and required ΔT50 rating instead of assuming catalogue output remains available.
How much heat can underfloor heating provide?
The calculator uses a transparent first-pass relationship:
For example, a 20 m² room with 85% active floor has 17 m² of heated floor. At an entered 70 W/m², the calculated output is 1,190 W. The useful comparison is then against the room heat demand, not against room area alone.
The required active-floor output shown in the result works in the opposite direction. It tells you how many W/m² the available heated floor would need to deliver to meet the entered room demand. If that requirement is higher than the output supported by the planned pipe spacing, water temperature, screed and floor covering, the design needs to change.
Radiator output at lower flow temperatures
Radiators are commonly published with a nominal output at a reference temperature difference. When the system operates cooler, the output has to be corrected. This calculator uses:
Q50 is the catalogue output at ΔT50, ΔT is the difference between mean radiator water temperature and room temperature, and n is the radiator exponent. Use the manufacturer’s exponent when it is available.
The “Required ΔT50 rating” result is especially useful when planning a low-temperature conversion. It estimates how large the catalogue-rated radiator would need to be to cover the same room demand at the flow and return temperatures you entered.
What if underfloor heating or the radiator is below 100%?
A result below 100% is not just a red flag; it tells you where to look next.
- Underfloor heating below 100%: compare the required active-floor W/m² with the realistic manufacturer output. Check whether active floor area can be increased and whether room heat loss can be reduced.
- Radiator below 100%: compare the calculated corrected output with the room requirement. The required ΔT50 rating shows the approximate nominal size needed at the same temperatures.
- Both below 100%: do not choose between two undersized options. First verify the room heat loss and then change emitter size, active area, system temperature or the building fabric.
- Both above 100%: output is no longer the deciding factor. Compare comfort, response, installation work, controls and heat-source efficiency.
How to use this underfloor heating vs radiator calculator
For the most useful comparison, start with a realistic room heat demand. If you already have a room-by-room heat-loss calculation, select “Known total power in W”. Otherwise you can use a W/m² estimate for an early-stage check, but the quality of the result depends on the quality of that assumption.
For the floor, enter only the area that can actually emit heat and use an output in W/m² appropriate to the planned system. For the radiator, use its ΔT50 nominal output and the intended flow and return temperatures. The calculator then puts both options on the same basis: watts delivered versus watts required.
Print the report when you want to compare variants. The printout contains all active input data and all calculated results, so you can keep one version for a low-temperature scenario and another for a higher-temperature scenario without losing the assumptions behind each result.