Rough estimate of air conditioning capacity using a common rule of thumb (20 BTU per sq ft). This is a starting point only—proper sizing requires a professional Manual J load calculation.
2026-04-09T00:00:00Z
In air conditioning terminology, "tonnage" refers to cooling capacity—not the physical weight of the unit. One ton of cooling capacity equals 12,000 British Thermal Units (BTUs) per hour. This term originated from the amount of heat needed to melt one ton of ice over 24 hours.
Proper AC sizing is critical for efficiency. An undersized unit cannot cool effectively, while an oversized unit "short cycles"—turning on and off frequently—leading to poor dehumidification, higher energy consumption, and reduced comfort. The rule of thumb is 20 BTUs per square foot for a standard 8-foot ceiling.
Follow these steps to determine the correct AC tonnage for your space:
A 15×15 ft bedroom with 8-ft ceiling, 2 occupants, moderate sun exposure:
Oversized units short cycle—turning on and off repeatedly without running long enough to remove humidity. This creates discomfort and wastes energy.
AC units must run long enough to both cool and dehumidify. A correctly sized unit maintains optimal humidity levels (30-50%) for comfort.
No. Buy the right size. A slightly oversized unit is worse than one that's slightly undersize. Trust proper calculations.
Poor window insulation increases heat gain. Professional calculations account for window types, orientation, and shading when sizing AC units.
Use average ceiling height or calculate by volume rather than square footage. Higher ceilings = more air to cool = more tonnage needed.
Yes, add the BTU requirements for each room and divide by 12,000. However, consult an HVAC professional for accurate ducting and loss estimates.
This heuristic provides a quick estimate per room. For whole-house systems, sum room loads and consult ducting and distribution requirements; professional Manual J calculations remain the recommended approach.
Yes. Improving insulation, windows, and sealing air leaks reduces heat gain and can lower the calculated BTU and tonnage needs. Use the metric/imperial toggle to test scenarios before purchasing equipment.
Related Tools
Calculate drain materials.
Calculate BTU requirements.
Calculate boiler capacity needed.
Calculate airflow requirements.
Calculate furnace capacity.
Calculate building heat loss.