Two rooms of the same size can need quite different radiator outputs. This page explains each input and shows how much it moves the result.
All the examples below start from the same room: a living room 4 m × 3 m × 2.4 m with 7 m of external wall, 3 m² of double glazing, a heated room above and a ground floor below, kept at 21 °C. With the default inputs, its estimate is 850 W (2900 BTU/h). Each example changes one input and keeps everything else the same, so you can reproduce it in the calculator.
The two kinds of heat loss
The calculator adds two things together:
- Fabric loss: heat conducted through walls, windows, floor and ceiling. For each surface it is area × U-value × temperature difference.
- Ventilation loss: heat carried away by air leaving the room and being replaced by colder air. It is 0.33 × room volume × air changes per hour × temperature difference.
Indoor temperature and room type
The room type sets a default indoor temperature and a default air-change rate:
| Room type | Indoor temperature | Air changes per hour |
|---|---|---|
| Living room | 21 °C | 1.0 |
| Bedroom | 18 °C | 1.0 |
| Bathroom | 22 °C | 2.0 |
| Kitchen or hall | 18 °C | 1.5 |
| Other | 21 °C | 1.0 |
The indoor temperature matters because every term depends on the difference between inside and outside. Keep the example room at 18 °C instead of 21 °C and the estimate falls from 850 W to 700 W (2388 BTU/h).
Outdoor design temperature
The calculator assumes an outdoor design temperature of −3 °C. With 21 °C inside, that gives a temperature difference of 24 for the walls, windows and ventilation. A different outdoor temperature can be entered in Advanced mode; the calculator does not look up a local value from your postcode.
Air changes per hour
Air changes per hour (ACH) describe how many times the room's volume of air is replaced each hour. In the example room, ventilation is the largest single item: about 228 W of the 734 W total before the allowance.
A minimum of 0.5 ACH for most heated rooms is widely cited, with CIBSE Domestic Heating Design Guide (2026) Table 2-18, published via the MCS Heat Load Calculator, as the primary reference. The calculator's defaults are deliberately higher than that minimum: they are conservative estimates for a typical UK room of each type. Real homes vary with draughts, extract fans and trickle vents.
Raise the example room from 1.0 to 1.5 ACH and the estimate goes from 850 W to 950 W (3242 BTU/h). If you know your ventilation rate, enter it in Advanced mode.
Glazing
With the default values, a square metre of double glazing loses more than four times as much heat as a square metre of external wall (U-value 2.8 against 0.60). The calculator's window U-values are referenced to RdSAP 10 Table 24. These are the table's PVC or wooden-frame values for single and older 12 mm double and triple glazing; the older double row also covers an unknown installation date:
| Glazing | U-value, W/(m²·K) |
|---|---|
| Single | 4.8 |
| Double | 2.8 |
| Triple | 2.1 |
Swap the example room's 3 m² of double glazing for single glazing and the estimate rises from 850 W to 1000 W (3412 BTU/h). The glazing area matters as much as the type. In Simple mode the calculator assumes glazing covers 25% of the external wall area; in Advanced mode you can enter the real area, up to the size of the external wall.
Walls, floor and ceiling
The default U-values for the other surfaces are indicative averages for a simplified mid-performance UK dwelling:
| Surface | Default U-value, W/(m²·K) |
|---|---|
| External wall | 0.60 |
| Floor | 0.80 |
| Ceiling or roof | 0.40 |
They are not a full RdSAP age-band assessment. If an EPC or a builder's specification gives you the U-values for your walls, floor or roof, enter them in Advanced mode.
The number of external walls matters too. In Simple mode, choosing 0 to 4 external walls sets how much wall area is exposed, and with it how much glazing is assumed.
What is above and below the room
What lies on the other side of the floor and ceiling decides the temperature difference for those surfaces:
- a heated room next to it adds nothing to the heat loss;
- an unheated space is taken as 10 °C;
- a roof open to the outside uses the outdoor temperature;
- the ground under a floor uses the under-floor temperature, 10 °C by default.
The 10 °C ground temperature is a conservative planning value, not a measured site value. MCS uses the mean annual external air temperature as the assumed ground temperature for solid ground floors (MCS MGD 007 Table B1). Across the 18 UK regions it ranges from 8.5 °C in North East Scotland (Dyce) to 11.3 °C in the Thames Valley (Heathrow). A lower assumed ground temperature increases the floor's temperature difference and therefore the estimate.
Room size
Bigger rooms lose more heat: more surface area for fabric loss and more volume for ventilation loss. Ceiling height raises both the volume and the area of every external wall, so measure it rather than relying on the 2.4 m starting value.
The planning allowance
After adding fabric and ventilation losses, the calculator applies a planning allowance, 10% by default and adjustable from 0% to 25% in Advanced mode. With the allowance set to 0%, the example room comes out at 750 W (2559 BTU/h) rather than 850 W.
What the estimate does not include
The calculator does not count heat gains from people, appliances or sunshine, does not handle rooms that are not rectangular and does not use a property age-band selector. The Methodology page sets out every assumption.
Sources
Figures and formulas on this page come from the calculator's specification, which is based on: CIBSE Guide A for the heat-loss formulas; CIBSE Domestic Heating Design Guide (2026) Table 2-18, via the MCS Heat Load Calculator, for the 0.5 ACH minimum; MCS references, based on CIBSE guidance, for indoor and outdoor design temperatures; BRE RdSAP 10 Table 24 as the reference for window U-values; MCS MGD 007 Table B1 for the regional ground temperatures; and NIST SP 811 for the conversion 1 W = 3.412142 BTU/h. Wall, floor and ceiling U-values are the calculator's indicative averages; the air-change defaults and the 10 °C ground temperature are its conservative estimates.
Disclaimer
This calculator provides an indicative room heat-loss estimate for planning radiator purchases. It is not a substitute for a qualified heating engineer’s design, a Gas Safe registered installer’s advice, or an MCS-compliant heat-pump system design. Radiator outputs should be compared at ΔT50 (BS EN 442).