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Heat loss calculator
Room-by-room heat loss for sizing a boiler or radiators, broken down element by element in watts — walls, windows, roof, floor and ventilation — rather than one number.
Heat loss
Room by room across a property, with radiators sized for the system you're actually fitting.
1976–1990, or an older house that's had cavity fill
Override for an exposed or high-altitude site.
75/65 — traditional boiler. Radiators are catalogued at ΔT50; this corrects the schedule for what the system actually runs at.
Outside walls only.
Override temperature and air changes for this room
Raise it for a draughty room, an open flue or a large opening.
W/m²K. Editing any value switches the construction to custom.
Whole property heat loss
0.8kW
Floor area
14m²
Average
56.1W/m²
Where the heat goes
Radiator schedule
rated at ΔT50 · system 75/65
Whole property heat loss
0.8kW
Floor area
14m²
Average
56.1W/m²
Where the heat goes
Radiator schedule
rated at ΔT50 · system 75/65
Guidance only. Follow Gas Safe procedures and the manufacturer's instructions. Gas work in the UK must be carried out by a Gas Safe registered engineer.
How it's worked out
Element by element, room by room. Each surface loses heat according to its area, its U-value and the temperature difference across it; ventilation loses heat according to the room's volume and its air change rate. The room total is the sum of those losses, and the property total is the sum of the rooms.
fabric loss (W) = area × U-value × temperature difference
ventilation loss (W) = volume × air changes × 0.33 × temperature difference
Fabric presets cover common construction types, so you aren't looking up U-values standing in somebody's hallway. Pick the wall build-up that matches what you can see and the calculator applies a representative value; override it if you know better.
Walls
Windows
Roof
Floor
Ventilation
Worked example
One room · 4.2 × 3.6 × 2.4 mA rear lounge in a 1930s semi. Two external walls, cavity, unfilled. Large single-glazed bay. Room above is heated, so no roof element. Suspended timber floor.
- Windows1,180 W
- Walls530 W
- Ventilation240 W
- Floor180 W
- Room total2,130 W
Illustrative figures for the method — your inputs will give your own. The shape is what matters: 55% of the loss in this room is going out of one bay window, and that's a sentence you can say out loud to the customer.
Why the breakdown, not the total
A single figure tells you the answer but not what to do about it
“This room loses 2,130 watts” sizes a radiator. It doesn't do anything else. Broken down element by element, the same calculation becomes a conversation you can have standing in the room: sixty per cent of the heat in here is going out of those windows.
That's the reason a customer agrees to spend money — not because you told them the number, but because they can see where it's going. It also stops you being blamed for a room that never gets warm when the real problem is a single-glazed bay nobody wanted to discuss.
Do the whole property room by room and you have a document, not an opinion. That is a different quote to be reading.
What the result means
- Per room, in wattsWhat that room needs to hold its design temperature. This is the figure to size the emitter against.
- Per elementWhere the loss actually is. Windows dominating is common in older housing and it's the most useful thing you'll show the customer.
- Property totalThe sum of the rooms — the number to size the appliance against, not the sum of the radiators.
A surprising answer is usually an input. A room total far higher than its neighbours normally means a fabric preset that doesn't match what's actually there, or an area entered in the wrong units. Ventilation carrying most of the loss suggests the air change rate is set for a draughtier room than the one you're in.
The calculator reports figures. Sizing decisions, and whether the installation is appropriate, are your judgement against the manufacturer's instructions.
The mistake people make
Sizing the boiler to the sum of the radiators
Radiators get rounded up. They're picked from what's available, in the size that fits the wall, with a bit in hand. Add them all together and you have a number considerably larger than the property's actual heat loss — and if that's what sizes the boiler, the boiler is oversized.
Which costs the customer. An oversized appliance cycles, spends less time condensing, and runs at a worse efficiency than the one on its label. It's a bill they pay every winter for a decision made in an afternoon.
Size the emitters to the room losses. Size the appliance to the property loss. They're two different numbers and only one of them comes off a merchant's shelf.
Questions
How do you calculate heat loss for a room?
Work out each element separately. For every surface, area × U-value × the temperature difference between inside and outside. For ventilation, room volume × air changes per hour × 0.33 × the same temperature difference. Add them up for the room total, and add the rooms for the property.
Do I need U-values for every wall?
No. Fabric presets cover common construction types, so you pick the build-up that matches what you can see and it applies a representative value. You can override it where you know the actual specification.
Why not just use watts per square metre?
Because two rooms of the same floor area lose completely different amounts of heat depending on how many external walls they have, what the glazing is, and what's above and below. A rule of thumb gets you a number; it doesn't tell you which element is costing the customer money.
Should I size the boiler to the radiators or the heat loss?
The calculated heat loss. Radiators are rounded up to what fits and what's stocked, so their total is larger than the property actually loses. Sizing to that total is how appliances end up oversized, cycling, and running at worse efficiency than the label.
Can I show the breakdown to a customer?
That's largely what it's for. Element-by-element figures turn a sizing exercise into an explanation — “most of the heat in this room goes out of that window” is the argument for spending money, and it comes from the same calculation you were doing anyway.
Is this free?
Yes. No sign-up, no email, no cap on rooms, and nothing held back behind a signup.
Guidance only. Follow Gas Safe procedures and the manufacturer's instructions. Gas work in the UK must be carried out by a Gas Safe registered engineer.
This calculator is part of OnPoint, which does certificates, scheduling and invoicing for UK trades. £19 for the first seat, £9 for each after.