Heating power required · Full metric & imperial units · Instant conversion.
| Component | Area (m² / ft²) | U-value (W/m²·K) | Heat loss (W/K) | Share |
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How much heat a room loses depends on three things: how much surface area is exposed to the cold, how well each surface resists heat flow, and how large the temperature gap is between inside and outside. This calculator models all three. It works in metric and imperial units simultaneously, enter feet and inches, get metres and centimetres in the summary and it converts temperatures between Celsius, Fahrenheit, and Kelvin on the fly. The output is the heating capacity a radiator, heat pump, or space heater needs to hold the room at its target temperature on the coldest expected day. It’s built for homeowners sizing a heating system, installers sanity-checking a design, and students working through building physics. All calculations run in your browser with no data transmitted.
Enter the room’s length, width, and height. Each field has its own unit selector, metres, centimetres, millimetres, feet, inches, or yards.
Pick the level: basement, ground floor, middle floor, or top floor/attic. Each has different ceiling and floor exposure.
Choose the insulation quality, from “no insulation” to “excellent / modern airtight.”
Set the number of external walls: one for a room with a single outside face, four for a fully exposed corner room.
Enter the total window and door area on those external walls, choosing the unit that matches your measurements.
Select the air changes per hour (ACH) that best describes the room’s airtightness.
Enter the ambient (outdoor design) and internal (target) temperatures. The unit dropdown converts the values automatically.
Choose your preferred output unit: W, kW, BTU/h, kcal/h, or hp and read the power required at the top.
The calculation reduces to one identity: power equals heat loss coefficient times temperature difference.
Formula: Heat Loss Coefficient (W/K) = Σ(U × A) + 0.33 × ACH × Volume
Formula: Power Required (W) = Heat Loss Coefficient × ΔT
Formula: ΔT = Internal Temperature − Ambient Temperature
Where U is the U-value of each building element (in W/m²·K), A is its area in square metres, and 0.33 × ACH × Volume is the ventilation loss coefficient (with 0.33 being the volumetric heat capacity of air in Wh/m³·K).
Each construction element is treated separately. Walls lose heat through their net area total external wall area minus the glazed area. Windows and doors use their own U-value, which is much higher than a wall’s. Ceiling and floor losses are multiplied by level adjustment factors that account for what’s above and below: a middle-floor room has heated neighbours on both sides, so it loses less through ceiling and floor than a top-floor room does.
Ventilation is computed from air changes per hour times room volume. The 0.33 coefficient means one full air change per hour in a 50 m³ room adds roughly 16.5 W/K to the loss coefficient a substantial contribution in leaky buildings.
Five insulation presets drive the U-values: none, poor, standard, good, and excellent. These are the same bands used in simplified residential load calculations, and they range from a 2.50 W/m²·K wall (uninsulated masonry) down to 0.15 W/m²·K (modern airtight construction with high-performance glazing).
Consider a 6 m × 4 m ground-floor living room with a 2.7 m ceiling. Two of its four walls are external. The room has 5 m² of glazing, standard insulation, and 0.5 ACH airtightness. You’re designing for −10°C outside and 21°C inside.
Step 1 – Geometry:
Floor area = 24 m²
Ceiling area = 24 m²
Total wall perimeter area = 2 × (6 + 4) × 2.7 = 54 m²
Exposed wall area (2 of 4 walls) = 27 m²
Net wall area = 27 − 5 = 22 m²
Volume = 6 × 4 × 2.7 = 64.8 m³
Step 2 – U-values (standard insulation):
Wall U = 0.45, ceiling U = 0.35, floor U = 0.35, window U = 2.50
Step 3 – Loss coefficients (W/K):
Walls: 0.45 × 22 = 9.9
Windows: 2.50 × 5 = 12.5
Ceiling (ground floor, ×1.00): 0.35 × 24 × 1.00 = 8.4
Floor (ground floor, ×0.50): 0.35 × 24 × 0.50 = 4.2
Ventilation: 0.33 × 0.5 × 64.8 = 10.7
Total heat loss coefficient: 45.7 W/K
Step 4 – Power required:
ΔT = 21 − (−10) = 31°C
Power = 45.7 × 31 = 1,417 W
That’s about 4,835 BTU/h, a mid-size radiator or a small heat pump zone. Windows dominate the breakdown at 27% of total loss, followed by ventilation at 23%. The takeaway: in a room with standard insulation, glazing and air infiltration typically account for half the heating load. Upgrading to double glazing or sealing draughts changes the result dramatically, a modern low-E window at 1.6 W/m²·K instead of 2.5 would knock roughly 300 W off the required output, and dropping to 0.3 ACH would remove another 200 W.
Length and height in metres, centimetres, millimetres, feet, inches, or yards. Area in square metres, square centimetres, square feet, or square inches. Temperature in Celsius, Fahrenheit, or Kelvin. Output power in watts, kilowatts, BTU per hour, kilocalories per hour, or horsepower.
Heat loss is the rate at which heat escapes the room for a given temperature difference, expressed as W/K. Heat load is that loss multiplied by the design temperature difference, giving the heating power required in watts. The first is a property of the building fabric; the second depends on the climate you’re designing for.
A U-value measures how much heat passes through one square metre of a building element per degree of temperature difference, in W/m²·K. Lower values mean better insulation. A modern insulated wall might be 0.25 W/m²·K; an uninsulated masonry wall is closer to 2.5.
ACH stands for air changes per hour, the number of times the entire volume of air in a room is replaced by outdoor air per hour. A well-sealed new build runs around 0.3 ACH; a leaky older home can be 1.5 or higher. Higher ACH means more ventilation heat loss.
Because what’s above and below the room determines whether those surfaces are heat loss paths. A middle-floor room with heated neighbours on both sides loses very little through ceiling and floor. A top-floor room loses a lot through the roof. A ground-floor room loses to the ground, but the ground temperature is relatively stable so the effective loss is lower than an exposed surface.
They’re band-level approximations. The five presets – none, poor, standard, good, excellent correspond to commonly cited U-value ranges for residential construction, but real buildings vary within each band. A professional heat loss survey measures or looks up the actual U-values of your walls, roof, and glazing.
The math works for any enclosed space, but the U-value presets are calibrated for residential construction. Commercial spaces often have different wall systems, glazing ratios, and ventilation requirements. The calculator can give a rough estimate, but a commercial heat loss calculation should account for occupancy, lighting loads, and process heat.
Because BTU/h and watts measure the same thing on different scales. One watt equals approximately 3.41 BTU/h, so a 1,500 W result shows as about 5,118 BTU/h. Neither number is wrong, they’re the same physical quantity in different units.
Both express the heat loss coefficient, just in different unit systems. W/K is metric; BTU/(h·°F) is imperial. The conversion factor is roughly 1.8, so a 50 W/K coefficient is about 90 BTU/(h·°F). The calculator shows both side by side.
Use the coldest temperature your location is expected to see for a sustained period, typically the 1% design temperature used in local building codes. Common values are −20°C for cold climates, −10°C for temperate regions, and 0°C for mild climates. Designing for a colder extreme than necessary leads to oversized equipment.
Because the calculation is approximate. Real buildings have thermal bridges, infiltration paths, and construction details that aren’t captured by simple U-value tables. A 15% margin ensures the heating system can hold the room at its target temperature on the coldest day without running at full output continuously.
No. The calculation is a pure fabric and ventilation loss model. Solar gain through windows and internal gains from occupants, lighting, and appliances would reduce the net heating requirement during daylight and occupied hours, but they’re not included. For sizing a heating system, the worst-case design condition assumes no solar gain and minimal occupancy.
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