HVAC Load Sizing Engine
Stop relying on generic rules of thumb. Calculate your exact heating and cooling requirements based on climate zone, building envelope, and room usage.
Primary Variables
Envelope Modifiers
Standard thermal envelope.
Why accurate HVAC sizing matters
An oversized air conditioner will cool your house too quickly and shut off before it has a chance to remove humidity, leaving your home feeling "cold and clammy." It also wears out parts faster due to short-cycling. An undersized unit will run constantly and never reach your desired temperature on extreme days.
The Danger of the "20 BTU Rule"
Many generic calculators simply multiply your square footage by 20 to find your required BTUs. While this might work for an average house in a moderate climate, it falls apart at the extremes.
For example, a 1,500 sq ft home in hot, humid Miami (Climate Zone 1) with large, single-pane west-facing windows requires significantly more cooling capacity than a 1,500 sq ft home in Boston (Climate Zone 5) built with modern spray-foam insulation.
How to use Reverse Sizing
If you are installing a mini-split system or reusing existing equipment, you can use our Reverse Sizing mode. Enter the capacity of the unit (e.g., 18,000 BTUs or 1.5 Tons), input your climate and room variables, and the engine will calculate the maximum square footage that specific unit can effectively condition.
Professional Disclaimer
This engine uses a simplified Manual J estimation for planning and budgeting purposes. A certified HVAC contractor should always perform a comprehensive room-by-room Manual J load calculation prior to equipment purchase and installation.