📖HVAC Sizing Guides

Every one of these questions turns up at the same moment: you have a quote in one hand, a tape measure in the other, and no way to tell whether the number you have been given is sensible. The arithmetic is not difficult. What is missing is somebody writing it down in the order you actually need it, with the corrections that matter and without a product page at the end.

These nine guides do that. They cover where the twenty-BTU rule comes from and when it stops being true, why an oversized air conditioner makes a room worse rather than better, how to measure a room so the number means something, what an ACCA Manual J calculation adds that a rule of thumb cannot, how climate zones double the heating answer while barely touching the cooling one, the difference between BTU, tons and kilowatts, how to choose between a window unit, a portable and a mini split, what happens to a heat pump’s capacity when it gets genuinely cold outside, and why humidity decides comfort as much as temperature does.

Every figure on these pages comes either from a calculation you can repeat here or from a cited source — ENERGY STAR, the Department of Energy, ACCA, AHAM or HVI. Where a rule is a convention rather than a standard, it says so.

If you want the number rather than the reasoning, start with the air conditioner size calculator and come back afterwards. The guide most people need first is why an oversized air conditioner is worse — it is the one that changes what people buy.