Free HVAC calculators you can run in a browser.

These are the calculations a tech actually reaches for between calls: how big does the equipment need to be, how big does this duct run need to be, and is this system charged right. Each one runs entirely in your browser — nothing is sent to a server, nothing is saved, and there is no email gate in front of the answer.

They are deliberately narrow. Each tool answers one question well and then points you at the app that does the full job offline, because a browser tab is the wrong place to be standing in a mechanical room with no signal. Every result states the assumptions it made, and every tool says plainly where its scope ends.

• Live Free online Manual J calculator Whole-house heating and cooling load from eight inputs, using real 99% / 1% design temperatures resolved from your ZIP code rather than a five-region climate dropdown. Runs the same engine as the Manual J app, and tells you how precisely your ZIP matched. Related app: Manual J load calculator
• Live Free online CSA F280 calculator The Canadian rulebook, not the American one. Whole-house heat loss and heat gain in kW from eight metric inputs, with the design temperature resolved from your postal code to the building code's Appendix C location. Runs the same engine as the CSA F280 app; coefficients are provisional and every result says so. Related app: Heat Pump Sizing Canada
• Live Free online HVAC ductulator CFM ↔ velocity ↔ friction-band sizing for round and rectangular, with a clear path to the full offline Duct Tools app for static pressure and Manual D schedules. Related app: Ductulator & static pressure
• Live A2L P/T chart & superheat calculator Browser P/T lookup and superheat/subcooling for R-454B, R-32, and R-410A (bubble/dew where glide applies), with CTA into A2L Safe Charge for max charge and job history. Related app: R-454B superheat & A2L max charge

• HVAC Duct Tools Full ductulator, capacity, static pressure, and Manual D designer — free, offline.
• A2L Safe Charge R-454B superheat, subcooling, and A2L max charge with PT tables on device.
• Manual J load calculator MJ8 loads and Manual S sizing with ZIP climate data.
• CSA F280 heat loss calculator Canadian F280-12 loads with NRCan postal climate data.

These tools end at a number. The next question is usually where to buy, and for a homeowner or a tech working without a distributor account that is a genuinely annoying question to answer.

HVACDirect sells complete central AC, heat pump, and furnace systems online in standard sizes. That is an affiliate link — we may earn a commission on purchases, at no extra cost to you, and it never changes what any calculator on this site reports.

Whatever you buy and wherever you buy it, size it to a calculated load per Manual S. Referral code BZ07T66Q4DVJ at checkout credits the referral to us either way. It does not change your price. How our affiliate links work.