Heat Loss Calculator

Work out room or whole-house heat loss the proper way — element by element with Q = U·A·ΔT, plus ventilation — in watts and BTU/h, with the formula shown rather than hidden. This is the design heat loss used to size a heat pump, radiators or a boiler. For a quick room-size sizing instead, see the BTU calculator.

Enter each surface by its U-value and area, set the indoor and design-outdoor temperatures, and add a ventilation rate. The total updates live in W, kW and BTU/h, broken down into fabric and ventilation. Both unit systems are supported, and nothing leaves your device.

Building elements (each surface to outside)
ElementU-value (W/m²·K)Area (m²)Loss

Pick an element to prefill a typical U-value, then edit it to your real figure. Each element’s loss is U × A × ΔT.

Ventilation / infiltration
Volume = floor area × ceiling height. ~0.5–1 tight, 1–1.5 average, 1.5–2+ draughty.

A physics estimate using the steady-state formula Q = U·A·ΔT and a simplified ventilation term. It uses the U-values and rates you enter and does not model thermal bridging, party-wall and ground heat-flow detail, intermittent heating or solar gains. For a certified or MCS-compliant design, use a full room-by-room calculation to the relevant standard.

Where the numbers come from Sanity-check sizing against the rule-of-thumb BTU calculator, and for a solar or off-grid heating setup see the off-grid solar calculator.

The heat loss formula

Heat loss is the rate at which a heated space sheds heat to colder surroundings, and it has two parts: heat conducted out through the fabric, and heat carried out by air leaving and being replaced. The equation for heat loss adds them together.

Q_fabric = Σ (U × A × ΔT) · Q_vent = 0.33 × ACH × volume(m³) × ΔT · Q_total = Q_fabric + Q_vent

The fabric part is the heat loss formula applied to each surface: its U-value times its area times ΔT, the temperature difference between inside and outside. The rate of heat loss formula for the whole element is just the sum over every wall, the roof, the floor, the windows and the doors. The ventilation part uses 0.33 Wh per m³ per kelvin — the heat capacity of air — times the air changes per hour, the room volume in cubic metres, and ΔT. The answer is in watts; multiply by 3.412 for BTU/h, or divide watts by 1000 for kilowatts.

Setting ΔT

ΔT is the indoor design temperature minus the outdoor design temperature. Indoors, a living space is usually taken at 21 °C (70 °F), a bathroom warmer and a bedroom cooler. The outdoor figure is the cold design temperature your heating has to cope with, not the average — around −2 to −3 °C across much of the UK, lower in colder regions and many parts of North America. A surface against an unheated room or a neighbour uses that space’s temperature instead of the outdoor one, which lowers its ΔT.

Heat loss and heat gain

The same U·A·ΔT relationship describes heat flow in either direction, so the heat gain and heat loss formula is identical in form — only the sign of ΔT changes. In summer, when it is hotter outside than in, the fabric gains heat at U·A·ΔT; the difference is that cooling loads also include solar gain through glazing and internal gains from people and appliances, which a pure fabric calculation does not capture. This tool is built for the heating case, where fabric and ventilation losses dominate.

Sizing a heat pump, radiators or a boiler

The design heat loss in watts is exactly what a heat source has to replace on the coldest design day, which is why a room-by-room heat loss calculation underpins heat-pump and MCS sizing in the UK: size the heat pump or boiler to the whole-house total, and each radiator or underfloor circuit to its own room’s loss. Over-sizing a heat pump hurts efficiency and short-cycles it, so an honest calculation — not a padded one — gives the best result. The room-by-room mode totals individual rooms for this.

Worked example

A room at 21 °C with a −2 °C design outdoor temperature has ΔT = 23 K. Say it has 40 m² of wall at U 0.30 and 6 m² of window at U 1.4. Fabric loss is 0.30 × 40 × 23 = 276 W plus 1.4 × 6 × 23 = 193 W, so 469 W of fabric loss. With a 50 m³ volume at 1 air change per hour, ventilation adds 0.33 × 1 × 50 × 23 = 380 W. The total is about 849 W, which is roughly 2,900 BTU/h or 0.85 kW. Load this with the example link in the tool.

Typical U-values for reference

Edit the prefilled U-values to your real figures; these typical values just give a starting point.

Element (typical)U-value (W/m²·K)U (BTU/h·ft²·°F)

There is also a small standing loss from hot pipework outside heated rooms (uninsulated pipe runs in a loft or under a floor), usually minor next to fabric and ventilation losses but worth insulating regardless.

Frequently asked questions

What is the formula for heat loss?

Heat loss has two parts. Fabric loss through each surface is Q = U × A × ΔT — its U-value times its area times the inside–outside temperature difference. Ventilation loss is roughly Q = 0.33 × air changes per hour × volume in m³ × ΔT. Add fabric and ventilation for the total. The result is in watts; multiply by 3.412 for BTU/h.

How do I calculate the heat loss of a house?

List every surface separating heated space from outside — walls, roof, floor, windows, doors — with its area and U-value, and add U × A × ΔT for each. Then add ventilation loss, 0.33 × air changes × volume × ΔT. ΔT is the indoor design temperature minus the outdoor design temperature. The tool does this per element and totals it in watts and BTU/h; room-by-room mode sums rooms.

What temperatures should I use for ΔT?

Use a comfortable indoor design temperature — commonly 21 °C (70 °F) for living spaces — and the outdoor design temperature for your area, the cold figure your heating must handle rather than the average. In much of the UK that is around −2 to −3 °C; colder regions use lower. ΔT is the difference. A surface against an unheated room or neighbour uses that room’s temperature instead.

Can I use this to size a heat pump or radiators?

Yes — the design heat loss in watts is what a heat pump, boiler or set of radiators must replace at the design outdoor temperature, which is why a room-by-room heat loss calculation is the basis of heat-pump and MCS sizing in the UK. Size the heat source to the whole-house total and each emitter to its room’s loss. This is a sound estimate; a certified design also handles thermal bridging and detailed ventilation.

How is heat loss different from a BTU rule-of-thumb sizing?

A rule-of-thumb BTU calculator multiplies floor area by a fixed figure (around 20 BTU per square foot for cooling) and adjusts for ceiling height, sun and occupants — quick, but it ignores how well the building is insulated. This heat loss calculator works from the real fabric: the U-value and area of each surface and the ventilation rate, so a well-insulated and a leaky room of the same size differ greatly. For the quick version, see the BTU calculator.

What air change rate should I use?

Air changes per hour (ACH) depend on how airtight and ventilated the room is. Typical design figures: ~0.5–1 for a draught-proofed modern room, 1–1.5 for an average room, and 1.5–2+ for a draughty older room, kitchen or bathroom. When unsure, start near 1 and adjust. The ventilation term is 0.33 × ACH × volume × ΔT, so doubling the air changes doubles that part.