Room radiator sizing — method

Whose figures these are:

In Simple mode, you choose what each surface is made of and the calculator applies a YLE design assumption for that construction. These are YLE's own figures, derived from physics and stated material properties — not industry-standard, typical or recognised values.

The working shows the assumptions used in the calculation. Five can be changed there: outdoor temperature, the temperatures assigned to unheated and detached spaces, air changes per hour, and the floor U-value. Wall, roof and glazing U-values cannot be entered in this version.

It estimates a room's heat loss. It does not tell you which radiator to buy, and it does not judge whether a radiator is adequate.

What the U-value is:

It is how much heat passes through one square metre of a surface for each degree of temperature difference across it, in W/m²K. The lower the figure, the less that surface loses. A solid wall loses more than an insulated one; a single-glazed window loses more than a sealed unit — the number is how much more.

Property-specific information:

A specification, drawing, survey, EPC or manufacturer’s documentation can help you identify the construction in front of you. This calculator does not reproduce U-values from those sources; it applies its own assumption to the construction you select.

If you need to use a property-specific wall, roof or glazing U-value, this version cannot accept it. Do not treat its Simple-mode assumption as a substitute for that evidence.

Telling a plain sealed unit from a coated one:

Look at the spacer bar at the edge of the unit — a gas marking, a maker's name or a date stamp, or a grey or black plastic spacer rather than bright aluminium, all indicate a later coated unit. Viewed at a shallow angle against something white, a coated pane carries a faint tint. The installation certificate or window order states the specification outright.

If none of these settles it, choose the plain unit — it gives the larger result. That does not make it accurate: if the unit is coated, it overstates the window's heat loss and therefore the room total.

Rooms with a flue, chimney or permanent vent:

High uncertainty — this one is yours to set. The rate depends on flue or vent size, appliance type, chimney height and how exposed the building is. This tool cannot estimate it, and a wrong figure here can move the result by more than any other single input.

Open flue serving an operating appliance — others have found this between 1.5 and 4.0 air changes per hour. It varies with flue size, appliance type, and the purpose-provided ventilation sized alongside it. Stack effect drives it, so the rate rises as it gets colder — the loss is largest exactly when the radiator is most needed.
Unblocked chimney, no appliance — others have found this between 1.0 and 3.5 air changes per hour. It varies with throat size and chimney height. A fitted throat restrictor or partly closed damper sits near the bottom of the range; a wide throat on a tall stack above the top of it.
Permanent ventilator, no flue — others have found this between 1.0 and 2.5 air changes per hour. It varies with free area, height, and which face of the building it is on. A vent is a much smaller opening than a chimney.

Where a room has more than one flue, chimney or permanent vent, or one of them plus visible draughts, none of these ranges applies.

A room-sealed or balanced-flue appliance is not this condition — it takes its air from outside and does not ventilate the room at all.

Internal walls — solid or studwork:

Tapping is an indication only, never a test. Dry-lined masonry — plasterboard fixed over a solid wall — can sound hollow in exactly the way a stud partition does, so a hollow sound distinguishes nothing on its own.

Use evidence that already exists: an exposed edge visible without making one, such as a doorway reveal, a service or meter opening, a cut-out behind a removed fitting, or an unfinished cupboard or loft-hatch return; the property's own records — drawings, a previous survey, a building-control or improvement record; or what was found last time something was fixed to that wall.

Dense or lightweight: where an unfinished or exposed edge is already visible, dense brick and dense block are dark and close-grained, while aerated block is pale, open-textured and obviously lighter.

If you cannot identify it, choose "I don't know". The calculator will not produce a result for that room until the wall construction is known.

A wall facing a heated space contributes nothing and needs no construction at all, so only walls facing somewhere cooler need entering.

The floor figure:

Approximate. This simplified floor calculation does not model the room’s exposed perimeter, the ground conditions beneath it, or the complete floor construction. The true figure may differ materially from the value used. This is the roughest number in the calculation. Change it if you have better information.

How this is calculated

element loss = area × U-value × (room temperature − temperature beyond)

ventilation = 0.33648 × air changes per hour × volume × (room temperature − outdoor temperature)

estimated steady-state heat loss = Σ element losses + ventilation

Every surface is worked out separately and shown separately, net of any openings cut out of it. An opening takes the conditions of the surface it sits in unless you give it its own, and the figure actually used is shown either way. A surface you mark as bordering a heated space contributes nothing, and is still listed contributing nothing rather than disappearing.

The air constant in the ventilation line is derived, not looked up. It is the volumetric heat capacity of dry air reduced to hourly units, at 20.00 °C and 101325 Pa:

specific gas constant = 8.314462618 ÷ 0.0289647 = 287.055 J/(kg·K)
density = 101325 ÷ (287.055 × 293.15) = 1.2041 kg/m³
constant = density × 1006 ÷ 3600 = 0.33648 W·h/(m³·K)

The reference condition is stated because it is a choice: air changes are reckoned on the room's own volume, so the density that turns that into a mass flow is the density of air in the room. It is a condition for evaluating a physical property, not a design temperature — your design temperatures are the ones you enter.

Figures that look unusual are flagged and still calculated. A flag says what was seen; it does not say you are wrong, and it never stops a result.

This calculator provides an estimate based on the dimensions, construction values and temperatures entered by the user. Actual heat loss can be affected by building condition, thermal bridges, ventilation, exposure, intermittent heating and system operating temperatures. Verify the result before purchasing or installing equipment. Where a formal design, regulatory calculation or manufacturer warranty requires a specified method, use a suitably qualified designer and the required documentation.

It is an estimation tool for an initial sizing assessment. It does not replace a room-by-room heat-loss calculation where one is required for system design, regulatory compliance or a manufacturer's warranty.