Room radiator sizing

Estimate the steady-state heat loss of one room. Whose figures these are, and how it is calculated.

What this 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.

The figures

In Simple mode the tool applies a YLE design assumption for each surface. Those are our own figures — not industry-standard, not typical, not recognised values. Every one is shown in the working.

The property

Asked once — every room in this property shares these.

The property — asked once

PostcodeNeeded to continue.

Property type

Recorded only. It never affects the result.

Where in the country

Needed to continue. Sets the outside design temperature. This one does change the result.

How warm you keep the house (°C)No default — this one is yours. Every room starts from it, and any room can be set differently.

The room

Measure it, say where it sits, and say how warm you're designing it to be.

The property

postcode · type · region · house temperature — set once

What to call itWhatever you call it on the job — "bedroom 2", "back kitchen". It names the room and nothing else: it does not set the temperature or any other figure.

Length (m)Width (m)Height (m)

Measured inside the room — wall face to wall face, floor to ceiling.

A bay, dormer or sloping ceiling is not modelled, so this room's result will be low. Do not enter the feature as a separate room — that would count its ventilation twice. Calculate the additional external wall, roof and glazing loss separately and add it to this result.

How warm this room is kept

Not set yet

the same as the rest of the house

Only this room. Everything else keeps the house figure.

Existing radiators

Recorded only. Nothing is calculated from these, and the result does not change.

How many in this room

Where is it in the building?

This decides whether the floor and the ceiling lose anything. Their areas are the room's own — never asked.

Draughtiness

Starts at Typical. Change it if the room is visibly draughty or has a flue, chimney or permanent vent.

Which walls face outside?

Tap them. Everything else is treated as a warm room next door, which loses nothing.

Enter the room's length and width and its plan appears here.

Windows and doors

Only the ones in an outside wall. They come out of the wall area.

Windows

How many

Each window is measured on its own — nothing is pre-filled and nothing carries over from another window or another room. Double is treated as uncoated — the figure errs high on modern units.

Outside doors

How many

Measured on its own — nothing pre-filled.

Room 1

No result yet

Something above is still needed.

Through the fabric—

Ventilation—

Based on YLE design assumptions for the constructions you chose, not on measured values for this building.

How this was calculated

Every assumption used, and every surface

Outside design temperature (°C)Unheated space inside (°C) (A-27)Detached unheated (°C) (A-28)Air changes per hour (A-20)Floor U-value (W/m²K) (from the construction)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.

Surface	Construction	U	Source	Net m²	Beyond °C	ΔT	Loss W
—	—	—	—	—	—	—	—

Ventilation: — × — air changes × — m³ × — K = — W

BTU/h conversion: displayed watts × 3.412142, rounded to the nearest BTU/h.

Every figure above is a YLE design assumption for the construction you chose — not a measured value for this building, and not an industry-standard or recognised figure.

Every room in this property

The property

—

Postcode

—

Property type

—

Where in the country

—

How warm you keep the house

Room 1
not measured
—

Don’t add these figures together to size a boiler or heat pump.

Something look wrong in this calculator? Tell us — it is the most useful message you can send.