Heat loss calculator
Work out the design heat loss of each room and the whole property, in watts and kW. Enter the rooms, the construction or your own U-values, and a postcode for the outdoor design temperature. Free, no signup — it runs the same heat-loss engine as the heat-pump design in Vento.
1. The property
U-values (optional) — enter your own instead of the construction defaults
W/m²K, applied to every room. Leave a box blank to keep the default shown in it.
2. Room by room
6 rooms · 2.38 kW totalInternal dimensions in metres. Glazing is the total area of windows and glazed doors in external walls.
- 01
Design temperature and air changes
615 W36 W/m² · 17.1 m² · ΔT 23 KWalls 135 · Windows 132 · Floor 98 · Ventilation 249 W - 02
Design temperature and air changes
609 W38 W/m² · 16 m² · ΔT 20 KWalls 117 · Windows 108 · Floor 80 · Ventilation 304 W - 03
Design temperature and air changes
234 W33 W/m² · 7.2 m² · ΔT 20 KWalls 42 · Windows 36 · Floor 36 · Roof 29 · Ventilation 91 W - 04
Design temperature and air changes
417 W30 W/m² · 14 m² · ΔT 20 KWalls 112 · Windows 72 · Roof 56 · Ventilation 177 W - 05
Design temperature and air changes
308 W32 W/m² · 9.6 m² · ΔT 20 KWalls 94 · Windows 54 · Roof 38 · Ventilation 122 W - 06
Design temperature and air changes
198 W41 W/m² · 4.8 m² · ΔT 24 KWalls 39 · Windows 26 · Roof 23 · Ventilation 109 W
Design heat loss
2.38 kW
2,381 W · 35 W/m² over 68.7 m²
Space heating only, rounded up to the next 0.5 kW. Hot water is on top, and check the unit's output at your design temperature — not its nameplate.
Radiator output needed
Catalogue (ΔT50) output each room's emitter needs at this flow temperature, 5 K drop.
| Room | Loss | ΔT50 rad |
|---|---|---|
| Living room | 615 | 1,842 |
| Kitchen / diner | 609 | 1,539 |
| Hall and stairs | 234 | 592 |
| Bedroom 1 | 417 | 1,054 |
| Bedroom 2 | 308 | 779 |
| Bathroom | 198 | 631 |
Do this for every room and turn it into a quote
In Vento the same heat-loss engine runs on the rooms you survey on the job, alongside the design and the proposal — no re-keying from a calculator.
Start a free trialA design estimate, not an MCS heat-loss survey. It can't be used to certify an installation or for a Boiler Upgrade Scheme application. How it works is set out below.
How the calculation works
Each room's design heat loss is the heat it loses through its fabric plus the heat carried away by air leaving it, at the coldest conditions the system is designed for:
Fabric = Σ (U × A) × ΔT — external walls (less glazing), glazing, floor, roof
Ventilation = 0.33 × n × V × ΔT
ΔT = room design temperature − outdoor design temperature
where U is the element's U-value (W/m²K), A its area, n the air changes per hour and V the room volume. It is the same steady-state method that BS EN 12831 and CIBSE Guide A are built on, with simplified inputs.
Room design temperatures
Each room type has its own design temperature and a base air change rate, scaled up for older, leakier construction and down for new build. Override either for any room.
| Room type | Design temp | Base air changes/h |
|---|---|---|
| Living room | 21 °C | 1 |
| Dining room | 21 °C | 1 |
| Study | 21 °C | 1 |
| Kitchen | 18 °C | 1.5 |
| Bedroom | 18 °C | 1 |
| Bathroom | 22 °C | 1.5 |
| Hall / landing | 18 °C | 1 |
| Other | 21 °C | 1 |
Construction and U-values
The construction you pick sets these U-values (W/m²K). If you know the real figures — from a survey, an EPC or a construction table — enter them instead, element by element.
| Construction | Wall | Glazing | Floor | Roof |
|---|---|---|---|---|
| Older, little insulation | 1.5 | 4.8 | 0.9 | 1.2 |
| Insulated, double glazed | 0.35 | 1.8 | 0.25 | 0.2 |
| New build / high performance | 0.18 | 1.2 | 0.15 | 0.13 |
Walls, floors and ceilings
- • External walls— each one counted is a quarter of the room's wall area, so it assumes walls of similar length. "Not sure" uses a typical share for the property type. Glazing is taken out of the wall area.
- • Floors and ceilings— with a heated room on the other side, no heat is counted as lost through them. Otherwise the floor or roof U-value applies to the room's full floor area (half the roof figure for a flat).
- • Outdoor design temperature — by postcode area, from about −1 °C on the south-west coast to −5 °C in northern Scotland, or −2 °C when no postcode is given. Regional approximations; enter your own figure if you have one for the site.
- • Radiator output — mean water temperature is the flow temperature less half a 5 K drop, and a radiator gives (ΔT ÷ 50)1.3 of its ΔT50 catalogue output.
What it doesn't include
Thermal bridging, the ground-floor perimeter-to-area method, heat flow to adjoining rooms or neighbours at different temperatures, an allowance for intermittent heating, and heat recovery from mechanical ventilation. That makes it a sound design estimate for scoping and checking a job — not the heat-loss calculation MCS requires for a certified heat pump installation or a Boiler Upgrade Scheme application.
Frequently asked questions
How do you calculate the heat loss of a room?
Add the fabric loss to the ventilation loss. Fabric loss is, for each external wall, window, floor and roof, its U-value × its area × the temperature difference between the room's design temperature and the outdoor design temperature. Ventilation loss is 0.33 × air changes per hour × room volume × the same temperature difference. The calculator above does exactly that for each room and adds the rooms up.
What is a typical heat loss per m² in the UK?
It depends almost entirely on the construction. On this calculator, the example two-storey semi comes out at about 35 W/m² built to roughly 2000s standards and about 103 W/m² with older, uninsulated walls, roof and glazing, at a −2 °C outdoor design temperature. Individual rooms vary around that: a bathroom designed to 22 °C or a room with three external walls loses more per m² than a bedroom with one.
What outdoor design temperature should I use?
The coldest outdoor temperature the system is designed to hold the room's temperature at. Enter a postcode and the calculator uses a regional figure for that postcode area, from about −1 °C on the south-west coast to −5 °C in northern Scotland. These are regional approximations for sizing; if you have the design temperature for the site's own location, enter it instead.
Is this an MCS heat loss calculation?
No. It follows the same basic method — fabric plus ventilation loss at a design temperature difference — but with simplified inputs, and it leaves out thermal bridging, the ground-floor perimeter method and heat flow between rooms at different temperatures. Use it to scope and check a job. A certified heat pump installation, and a Boiler Upgrade Scheme application, need the full heat-loss calculation MCS requires.
How do I size radiators for a heat pump?
Radiators are catalogued at ΔT50 — 50 K between the mean water temperature and the room. A heat pump runs far cooler, so the same radiator gives much less: at 45 °C flow into a 21 °C room it gives about 33% of its catalogue figure. The calculator works that out per room — pick a flow temperature and it shows the ΔT50 output each room's emitter needs to meet its heat loss.
Every room, every job, straight into the quote
Vento runs this heat-loss engine on the rooms you survey, then carries the result into the design and the proposal your customer signs. Try it free for 14 days.
Start your 14-day trial14-day free trial