Hi Product Hunt. I'm Mike, I build HVAC field calculators as a one-person shop. I built this because every free Manual J calculator on the web asks you to pick a climate region from a dropdown of four or five. Phoenix and Minneapolis are not the same load at the same square footage, and a 20°F miss on design temperature throws the whole number by about the same proportion. This one resolves your ZIP against published 99% winter / 1% summer design temperatures and tells you whether it matched a weather station, a reference city, or just the 3-digit prefix, so you know how much to trust it. Honest scope: it's a whole-house block load from eight inputs. It won't do room-by-room, duct design, or Manual S equipment selection. Those live in the app, which runs the same engine (an automated test fails the build if the web and app numbers ever disagree). No account, no email gate, nothing stored. Question for anyone who's had an HVAC quote: did the contractor show you a load calculation, or just a tonnage?

HVAC Load Calculator

Free Manual J calculator with real ZIP-code design temps

Free Manual J calculator with real ZIP-code design temps