Heat loss / heating load calculator
Room and building heat loss from dimensions, design temperatures and U-values for walls, windows, doors, ceiling and floor — pick a typical construction from the dropdowns or enter your own — plus ventilation and infiltration. Required heating load in kW, broken down by element.
How to use this calculator
- Enter the room dimensions and indoor/outdoor design temperatures.
- Pick wall, glazing, door, ceiling and floor constructions, or enter your own U-values.
- Enter ventilation and infiltration air change rates — use the country reference to find a typical outdoor design value.
- Read the required heating load, broken down by element, plus the annual energy and cost if you add a heat source.
What this calculates
Steady-state heat loss splits into two parts: heat conducted straight through the building envelope (transmission), and heat carried away in the air that leaves the room (ventilation and infiltration):
Q_transmission = Σ (Ui × Ai × ΔT) for walls, windows, doors, ceiling, floor
Q_air = 0.34 × (nvent + ninf) × V × ΔT
Q_total = Q_transmission + Q_air
U = thermal transmittance of the element, W/(m²·K)
A = element area, m²
ΔT = indoor minus outdoor design temperature, K
n = air change rate, 1/h
V = room volume, m²
0.34 = the heat capacity of air per m³ per K, Wh/(m³·K)
Wall area is the NET area — the gross perimeter-×-height envelope minus the window and door area entered separately — so glazing and doors are never counted twice, once as "wall" and once as their own element.
U-value presets
Each element (wall, window, door, ceiling, floor) has a dropdown of typical construction tiers, from older uninsulated stock through modern (roughly WT2021-grade) insulation to a passive-house level — or choose "Custom" to enter your own U-value directly, e.g. from a manufacturer's datasheet or an as-built energy audit. Set ceiling or floor to "Internal — no loss" when that surface faces another heated space rather than the outdoors or the ground.
Ventilation and infiltration
Both are entered as an air change rate (room air volumes exchanged per hour) and combined with the same air-heat-capacity relationship, then shown as two separate line items so you can see how much of the air-side loss is deliberate ventilation versus uncontrolled leakage. A typical hygienic minimum for occupied rooms is around 0.5 h-1; infiltration through a reasonably airtight modern building is commonly in the 0.1-0.2 h-1 range, higher for older, leakier construction.
Annual energy and cost
The design-point heating load above assumes one fixed outdoor temperature — but the outdoor temperature obviously changes all year. The standard way to turn a single design-point calculation into an annual figure without re-running it at every temperature the year sees is the degree-day method: divide the total heating load by the design ΔT to get how much the room loses per degree of temperature difference, then multiply by the heating season's total heating degree days — a published per-location figure that already sums the whole year's varying outdoor temperature into one number (a cold day contributes a lot, a mild day almost nothing, a summer day nothing at all).
Heating degree days is not a number most people have memorised, so the "Reference location" dropdown fills in a rough, order-of-magnitude starting figure for a handful of countries (base ~18°C, one representative city) — pick the closest one and the field below fills in, then adjust it if you have a more exact figure for your own location from a national meteorological or energy agency. It stays a plain number either way; the dropdown is only a shortcut to a sensible starting point, not a lookup the calculation depends on.
Q/K = Q_total / ΔT_design [W/K]
Annual thermal energy = Q/K × HDD × 24 / 1000 [kWh/yr]
Annual purchased fuel = Annual thermal energy / efficiency / (kWh per fuel unit)
Annual cost = Annual purchased fuel × price per fuel unit
Heat source
Picking a heat source sets two things at once: a typical seasonal efficiency (a boiler, below 1 — combustion and distribution losses) or COP (a heat pump, above 1 — it moves more heat than the electricity it consumes), and the unit that fuel is normally priced in — m³ of gas, litres of oil, tonnes of coal or pellets, or kWh directly for electricity, a heat pump's electricity, or district heat. The conversion between a fuel's natural unit and kWh uses its typical calorific value, so the fuel price you already know from a bill or a supplier's quote — not a kWh-equivalent you'd have to calculate by hand — is what you enter. Switching heat source resets the price field to a rough placeholder for that fuel; replace it with your own tariff. Choose "Custom" to enter an efficiency/COP directly and price per kWh, the same as before.