The right answer for a 700 sqft NYC apartment is wildly different from the right answer for a 2,400 sqft house in northern Maine. Climate, square footage, insulation, ductwork (or lack of it), and what you’re actually trying to do all change the answer.

Rather than make you read another forty-page buyer’s guide, we built something that does the math for you.

The selector is inspired by the same sizing principles licensed HVAC contractors use (ACCA Manual J), adjusted for climate zone and insulation quality. It is a fast consumer approximation, not a full Manual J. It will not replace a contractor’s site survey, but it will tell you whether you’re being quoted something reasonable or something that will short-cycle your house into either a sauna or a meat locker.

Note on Manual J: When you actually buy, your contractor should do a proper Manual J load calculation for your specific home. Any contractor who skips this step and “just sizes based on square footage” or “matches what you had before” is doing it wrong. The selector tells you whether their answer is in the right ballpark.