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Heat Loss Calculator
Calculate how much heat a room loses in BTU & watts. Room by room heat loss calculations with results saved.
Efficient quoting, scheduling & certificates. All in one place.
Gas Engineer Software helps you manage your heating & plumbing business and save hours each week on admin and paperwork.
How to Use Our Room Heat Loss Calculator
This calculator works room by room. Add each room individually, and your results are saved automatically as you go — so you can build up a full property calculation without losing your work. Results display in both watts and BTU/hr.
Here’s what you’ll need for each room:
- Room dimensions — length, width and ceiling height in metres
- Construction type — wall, floor and ceiling materials (e.g. solid brick, cavity wall, timber floor)
- Glazing details — number, size and specification of windows and external doors
The U-values and design temperatures are built into the calculator using standard assumed values for the UK and the type of room. Specific values listed below.
Work through each room, save your results, and repeat until you’ve covered the whole property. The running total gives you the whole-house peak heat demand.
Assumed values in this calculator:
Design temperatures
| Outdoor | −3°C |
| Living room | 21°C |
| Bedroom | 19°C |
| Bathroom | 24°C |
| Hall / utility | 18°C |
| Kitchen | 20°C |
Wall U-values
| Solid brick | 2.1 W/m²K |
| Uninsulated cavity | 1.0 W/m²K |
| Insulated cavity | 0.45 W/m²K |
Window U-values
| Single glazed | 5.0 W/m²K |
| Double glazed | 2.2 W/m²K |
| Triple glazed | 1.0 W/m²K |
Floor U-values
| Suspended timber | 0.7 W/m²K |
| Solid ground floor | 0.7 W/m²K |
Roof / ceiling U-values
| Uninsulated flat roof | 1.8 W/m²K |
| Insulated flat roof | 0.25 W/m²K |
| Loft — 100mm insulation | 0.25 W/m²K |
| Loft — 50mm insulation | 0.35 W/m²K |
| Uninsulated loft | 2.0 W/m²K |
Ventilation (ACH)
| Living room | 1.5 ACH |
| Bedroom | 1.0 ACH |
| Bathroom | 3.0 ACH |
| Hall / utility | 2.0 ACH |
| Kitchen | 2.0 ACH |
Other
| Thermal bridging y-value | 0.15 W/m²K |
| Intermittent heating uplift | 12 W/m² floor area |
What is a Heat Loss Calculator?
A heat loss calculator works out how much heat a room loses through its walls, floor, ceiling, windows and doors. That figure — measured in watts or BTU/hr — tells you the minimum output required from a radiator or heat emitter to keep the room at your target temperature.
For gas engineers and heating designers, getting this right isn’t optional. Undersized radiators leave customers with cold rooms and callbacks. Oversized ones inflate installation costs and cause short-cycling. An accurate heat loss calculation is the foundation of a properly sized heating system — and increasingly, it’s what Building Regulations and industry standards expect you to demonstrate.
How Is Heat Loss Calculated?
The fundamental formula is:
Heat Loss Formula
Q = U × A × ΔT
Q = heat loss (watts)
U = U-value of the building element (W/m²K)
A = area of the element (m²)
ΔT = temperature difference between inside and outside (°C)
This is built into the calculator above. If you were to complete the calculation manually, you would run this calculation separately for each element of the room — each wall, the floor, ceiling, windows and doors — then add the results together. The total gives you the fabric heat loss for that room.
On top of that, you add a ventilation heat loss to account for air leakage and deliberate ventilation. For most domestic properties, a standard air change rate of 0.5–1.0 ACH is used.
Worked example:
A living room: 4m × 5m floor plan, 2.4m ceiling height, with a 12m² solid brick external wall (U-value: 2.0 W/m²K). Internal target temperature 21°C, external design temperature –3°C. ΔT = 24°C.
Fabric heat loss (that wall): Q = 2.0 × 12 × 24 = 576 watts
You’d repeat this for every other element — the remaining walls, floor, ceiling, windows and doors — then total them up to get the full fabric heat loss.
Ventilation heat loss: Using the standard formula: Qv = 0.33 × N × V × ΔT
Where N = air change rate (0.5 ACH for a living room) and V = room volume (4 × 5 × 2.4 = 48m³):
Qv = 0.33 × 0.5 × 48 × 24 = 190 watts
Total room heat loss = fabric heat loss + ventilation heat loss
So for this room, once all fabric elements are calculated and added to the ventilation figure, you arrive at the total output your radiator needs to match.
Frequently Asked Questions
Why Do Room-by-Room Heat Loss Calculations?
A single whole-house heat loss figure tells you what size boiler or heat pump you need. But it doesn’t tell you what radiator to put in each room — and that’s where many heating systems go wrong.
Calculating heat loss room by room lets you:
- Size each radiator correctly
- Balance the system properly
- Meet modern standards
- Avoid callbacks
What is the heat loss calculation formula?
The standard formula is Q = U × A × ΔT, where Q is heat loss in watts, U is the U-value of the building element, A is the surface area in m², and ΔT is the temperature difference between inside and outside. You calculate this for every element of a room — walls, floor, ceiling, windows and doors — then add the totals together, including a ventilation allowance.
Is it hard to move from paper to software?
Not at all! Everything done on software is the exact same thing as you will be used to on paper. The only key difference is that you can use your phone or computer to handle all your paperwork and admin.
In fact, one of the key benefits of Gas Engineer Software is how easy it is to use and set up.
How accurate is this heat loss calculator?
The accuracy depends on the accuracy of the inputs. Generally, it is advisable to use a conservative estimate — it’s better to slightly oversize than under.
This calculator uses assumed U-values for the construction type and external design temperature, which will likely introduce a margin of error.
What U-values should I use for UK properties?
Some common starting points for UK domestic properties:
- Uninsulated solid brick wall: 2.0–2.2 W/m²K
- Unfilled cavity wall: 1.5 W/m²K
- Cavity wall with insulation: 0.35 W/m²K
- Modern insulated wall (Part L compliant): 0.18–0.28 W/m²K
- Solid concrete floor (uninsulated): 0.7 W/m²K
- Suspended timber floor (uninsulated): 0.7 W/m²K
- Single glazing: 4.8–5.6 W/m²K
- Double glazing (standard): 2.8–3.2 W/m²K
- Double glazing (low-e): 1.6–2.0 W/m²K
Can I use this for underfloor heating as well as radiators?
Yes. The heat loss calculation itself is the same regardless of the emitter type — you’re working out how much heat the room needs, not how it’s delivered. The output figures in watts and BTU/hr are what you need to specify either radiators or UFH circuits. Bear in mind that UFH typically operates at lower flow temperatures, which may affect your boiler or heat pump selection.
Does this work for heat pumps as well as gas boilers?
Absolutely. In fact, an accurate heat loss calculation is even more critical for heat pump installations. Heat pumps are sized more precisely than gas boilers, and they operate most efficiently when matched closely to the building’s actual heat demand. The room-by-room figures also help you identify any rooms where fabric upgrades (insulation, glazing) might be worth recommending to the customer before installation.
Make your job easier with
Gas Engineer Software
Efficient quoting, scheduling & certificates. All in one place.
GES helps you manage your heating & plumbing business and save hours each week on admin and paperwork.