Air Conditioning Sizing Guide
Why correct sizing matters
Accurate cooling size starts with heat gain. A room gains heat through walls, windows, roofs, people, lights, electronics, and outdoor air leakage. An air conditioner must remove that heat each hour. The result is normally shown in BTU per hour. It can also be shown in tons, where one ton equals 12,000 BTU per hour.
Small and large units behave differently
A small unit may run constantly. It can still fail to reach the target temperature. It may also leave humidity high. A very large unit can cycle too fast. Short cycles cool the air, yet they may not remove enough moisture. This calculator balances both problems by combining room geometry with load adjustments.
Inputs that change the load
Area is the starting point. Many quick rules use about 20 BTU per square foot for normal rooms. That rule is useful, but it is not complete. Ceiling height changes the air volume. Poor insulation raises wall and roof heat gain. Strong afternoon sun adds a major window load. Extra occupants add body heat. Appliances add their electrical power as heat. Outdoor air leakage adds a load based on air changes and temperature difference.
How this tool estimates capacity
The calculator uses these physics based ideas in one form. It converts metric inputs when needed. It estimates base cooling from floor area and ceiling height. It then adjusts for insulation, climate, sunlight, occupants, window area, appliances, kitchen use, humidity, and safety margin. It also estimates common equipment size. This helps you compare portable units, window units, mini splits, and central systems.
Energy and practical judgment
The energy estimate uses SEER as a seasonal efficiency value. Higher SEER means lower electric demand for the same cooling output. The daily cost estimate depends on operating hours and local electricity price. Use it as a planning estimate, not a guarantee.
Good sizing also needs judgment. Rooms with large glass, unsealed ceilings, server equipment, or heavy cooking may need professional load analysis. Multi-room systems need duct losses and airflow balance. Still, this tool gives a strong first estimate. It explains each load part, so the result is easier to review. Use realistic inputs. Then choose the next available unit size above the calculated load. Review manufacturer tables too. They may show airflow limits, installation rules, and minimum circuit requirements for clear safe operation.