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How many BTUs does this space actually need?
Oversized air conditioners short-cycle and leave the air clammy; undersized ones run forever and never catch up. This calculator applies the standard sizing factors — climate baseline, insulation quality, ceiling volume, sun exposure, occupant heat — to produce a defensible BTU/hr estimate and tonnage before you talk to a contractor.
Homeowners use it before contractor season: knowing your tonnage converts "you need a 5-ton unit, trust me" into a conversation. It is also the reality check for window units and garage mini-splits — right-sizing beats oversizing every time.
The rule of thumb runs 20–35 BTU per sq ft depending on climate and insulation — a 1,500 sq ft average home in a mixed climate lands near 45,000 BTU/hr (about 3.5–4 tons). It is a screening number; a Manual J calculation is the real design.
No — oversizing is the more expensive mistake. An oversized unit cools the air before it dehumidifies, then shuts off, leaving cool, sticky rooms and short equipment life. Right-sized equipment runs longer cycles and controls humidity.