BTU / HVAC Sizing Calculator
A rough sizing estimate for heating and cooling. Enter room dimensions, insulation quality, sun exposure, and climate zone. Result is a range, not a single number — proper HVAC sizing requires a Manual J calculation by a licensed HVAC professional.
Room
Environment
Materials Needed
Rough BTU rules of thumb
A common starting point for room air conditioner sizing is 20 BTU/hr per square foot of floor area. That gets adjusted up or down for ceiling height above 8 ft, insulation quality, sun exposure, and climate zone. This calculator combines those adjustments and returns a range (typically ±15% of the midpoint) because the exact number depends on things it can't see: window count and quality, air infiltration, occupancy, appliance heat gain, and duct condition.
Why oversizing is a real problem
HVAC oversizing is more common than undersizing and it's worse than most people realize. An oversized AC cools the air quickly, hits the thermostat cut-off, and shuts off before dehumidifying properly — you end up with a room that feels cold and clammy. Oversized systems also short-cycle, which wears out the compressor faster and wastes energy on the startup surge. Right-sized systems run longer, quieter cycles that dehumidify better and cost less to operate over their lifetime.
Manual J is the actual answer
For any whole-home HVAC installation, the industry-standard sizing method is Manual J from ACCA (Air Conditioning Contractors of America). It accounts for every window, insulation type, orientation, and infiltration rate. A rough estimate like this one is fine for buying a window unit or portable AC. Do not use it to size a central system — hire a contractor who does Manual J.