Input Parameters
About This Calculator
Uses the unit index method with corrections based on empirical data:
- Base cooling index varies by building type
- Orientation correction (west-facing has highest load, north-facing lowest)
- Window-to-wall ratio correction (larger windows = more solar gain)
- Insulation level correction (better insulation = lower load)
- Design temperature difference correction
Results include a 10% design safety margin. Recommended capacity can be used directly for AC unit selection.
How this cooling load calculator works
This tool uses a simplified unit-index method. It starts from a typical cooling load per square meter, then adjusts for building type, orientation, insulation, window-to-wall ratio, and indoor-outdoor temperature difference.
Inputs explained
- Room area sets the base sensible cooling load estimate.
- Room height is used to report cooling load per volume.
- Building type selects a typical base cooling index.
- Orientation and window ratio approximate solar heat gain impact.
- Insulation level and design temperatures adjust the base estimate for envelope performance and climate severity.
FAQ
Does the result include latent load?
The result is a simplified sizing estimate and should not replace a full sensible and latent cooling load calculation.
Why does west-facing orientation increase the result?
West-facing rooms often receive stronger afternoon solar gains, so the calculator applies a higher orientation factor.
How should I use the recommended AC capacity?
Use it as an early screening value, then verify equipment selection with manufacturer data and a detailed load calculation.
Common Sizing Mistakes — What Engineers Get Wrong
Forgetting latent load in humid climates
This calculator outputs sensible load only. In Miami or Houston (design wet-bulb ≥77°F / 25°C), latent load can add 20–40% to total cooling capacity. A 36,000 BTU/h sensible result in a humid climate may need a 48,000 BTU/h unit after dehumidification is accounted for. Use ACCA Manual J or ASHRAE load procedures for final equipment selection.
Using gross floor area instead of conditioned area
Enter only the area that the system will condition. Including unconditioned garage, mechanical rooms, or unfinished basement inflates the result. A 2,500 ft² house with a 500 ft² unconditioned garage should be entered as 2,000 ft².
Oversizing "for safety" — why it backfires
An oversized AC unit short-cycles: it cools the space quickly but shuts off before running long enough to dehumidify. ASHRAE research shows a unit sized 15–25% above actual load causes more comfort complaints than a correctly sized unit. Add margin through a full Manual J, not by bumping up the estimate.
Design temperature vs. average temperature
Cooling load calculations use the ASHRAE 1% design dry-bulb (the temperature exceeded only 1% of hours per year), not the average summer temperature. For Phoenix, AZ this is 109°F (43°C); for Atlanta it is 92°F (33°C). Using average July temperatures (often 10–15°F lower) will produce undersized equipment.