TradesWhat this is for
You got a quote for a new air conditioner or furnace and you want to know if the size makes sense. That is what this does. Put in your square footage, your climate, and a few details about the house, and it gives you a rough load and a tonnage to compare against the quote. If the two are close, the quote is in a sensible range. If the quote is a lot bigger, that is worth a question.
The one thing to be clear about: this is a sanity check, not a load calculation. The real method is called Manual J, and it accounts for things a square-footage estimate cannot, like which way your windows face and how leaky the house is. Use this to spot a quote that looks off, then ask for the Manual J.
Why bigger is not better
The most common mistake in residential HVAC is oversizing. A unit that is too big cools the house in short bursts and shuts off before it has run long enough to dry the air out. The result is a house that feels cold and damp, equipment that cycles on and off all day, and a system that wears out sooner. Right-sizing runs longer, gentler cycles that hold both temperature and humidity. That is why a smaller, correctly sized unit often beats a big one.
Common questions
Is this a Manual J?
No, and it is not trying to be. A Manual J looks at every wall, window, and duct in your specific house. This is a rough estimate from floor area and a few details, meant to tell you whether a quote is in a sensible range. Always get a real Manual J before buying equipment.
Why is oversized air conditioning a problem?
It sounds backwards, but bigger is usually worse. An oversized system cools the air fast and shuts off before it has run long enough to pull the humidity out. You get a house that is cold and clammy, more on and off cycling, higher up-front cost, and shorter equipment life.
My contractor quoted a much bigger unit than your estimate. What do I do?
Ask to see the Manual J that supports the size. A good contractor runs one and can show you the numbers. If the only reason for the bigger unit is "to be safe," that is a sign it was sized by rule of thumb rather than a real calculation.
Does this size the ductwork too?
No. Duct sizing is a separate step (Manual D). This tool only estimates the load so you can sanity-check the equipment size.
Sources: Rule-of-thumb area method (planning heuristic, not a code table) · ACCA Manual J is the authoritative load method (not implemented here). Engine version 1.0.0. Risk tier 2.