HVAC Guides
Plain-English explanations of the concepts behind HVAC sizing - the standards, the formulas, and the how-tos - written to pair with our free calculators.
What Is a Manual J Load Calculation?
The ACCA standard behind every correctly sized HVAC system: what it measures, how it's performed step by step, what professionals charge ($99-$400), when building code requires it, and how Manual J relates to Manual S, T, and D.
Read the guideHow to Find AC Tonnage by Model Number
Read the size of your existing air conditioner straight off its data plate: where the model number hides, how the capacity code works (divide by 12), brand-by-brand quirks, and a decoder tool that does it for you.
Read the guideSACC vs ASHRAE BTU, Explained
Why the same portable AC lists two BTU ratings - and why the DOE's SACC number is roughly half the ASHRAE figure. What each test measures, a conversion table for common sizes, and which rating to trust when sizing.
Read the guideOur Calculation Methodology
The exact formulas behind every calculator on this site - AC tonnage, SEER savings, mobile home sizing, and more - with the ASHRAE-inspired assumptions spelled out step by step.
Read the guideLooking for the tools instead? Browse the cooling calculators, heating calculators, or HVAC utilities.