Calculator inputs
The page uses one main column. Input fields switch to 3, 2, and 1 columns by screen size.
Example data table
| Input item | Example value | Unit | Notes |
|---|---|---|---|
| Floor area | 180 | m² | Used for volume and internal loads. |
| Ceiling height | 3.1 | m | Volume becomes 558 m³. |
| Wall U-value | 0.42 | W/m²·K | Insulated wall assembly. |
| Window SHGC | 0.52 | - | Controls solar admission through glazing. |
| Heating ACH | 0.70 | ACH | Winter infiltration estimate. |
| Cooling ACH | 0.90 | ACH | Summer infiltration estimate. |
| Lighting density | 8 | W/m² | Internal sensible gain. |
| Safety factor | 10 | % | Applied at the end. |
Formula used
Q = U × A × ΔT
Each surface load equals the assembly U-value multiplied by exposed area and the design temperature difference. This is used for walls, roof, floor, windows, and doors.
Q = ρ × Cp × (ACH × Volume / 3600) × ΔT
Air density, specific heat, volume, and air change rate define the sensible heating or cooling effect from outside air leakage.
Q = Window Area × Solar Irradiance × SHGC × Exposure Factor
This captures solar heat entering through glazing during cooling design conditions.
Q = (Lighting W/m² × Floor Area) + (Equipment W/m² × Floor Area) + (People × W/person)
Internal loads are added to cooling gain and treated as offsets against heating demand.
Heating Design = max(Gross Heating Loss − Offsets, 0) × (1 + Safety Factor)
Cooling Design = Gross Cooling Gain × (1 + Safety Factor)
How to use this calculator
- Enter your project name and floor geometry.
- Provide winter and summer indoor and outdoor design temperatures.
- Add exposed areas and U-values for each main envelope element.
- Set infiltration rates for heating and cooling using ACH values.
- Fill in glazing solar inputs, occupancy, lighting, and equipment loads.
- Optionally enter a winter solar offset and a design safety factor.
- Press the calculate button to show results below the header and above the form.
- Review the summary cards, breakdown table, and Plotly chart.
- Use the CSV or PDF buttons to save the output.
Frequently asked questions
1) What does this calculator estimate?
It estimates sensible heating loss and cooling gain for a building zone or small project area. It combines conductive envelope transfer, infiltration, internal gains, glazing solar gain, and a final safety factor.
2) Can I use this for early-stage sizing?
Yes. It works well for concept design, budget studies, and quick comparisons between insulation levels or glazing choices. Final equipment selection should still be checked against a full mechanical design method.
3) Why are there separate heating and cooling temperatures?
Winter and summer design conditions are usually different. Separate inputs let you calculate heat loss in one scenario and heat gain in another, using more realistic peak design assumptions.
4) Why do internal gains reduce heating demand?
People, lighting, and equipment release sensible heat into the occupied space. That heat helps offset winter losses, so the net heating load can be lower than the gross envelope loss.
5) What is a reasonable ACH value?
That depends on envelope tightness, entrance traffic, and construction quality. Tighter buildings may sit near 0.3 to 0.5 ACH, while leakier or frequently opened spaces can be much higher.
6) What is SHGC?
SHGC means solar heat gain coefficient. It represents the fraction of incident solar energy transmitted through glazing into the space. Lower values generally reduce cooling loads.
7) Should I include a safety factor?
A modest safety factor can cover modeling uncertainty, small load omissions, and operating variations. Oversizing too much can still create inefficiency, so use it carefully rather than automatically choosing a large number.
8) Is this enough for code compliance documentation?
Usually no. Compliance submissions often require approved procedures, local climate data, detailed assemblies, ventilation schedules, and equipment assumptions. This page is best treated as an engineering support tool.