HVAC Load Calculator

Last updated: October 6, 2026

Free BTU room chart — print it.

Room-by-room HVAC sizing for DIY mini-split installs.

Room-by-room BTU, mini-split size, duct CFM, and baseboard watts — Manual-J-inspired rule-of-thumb math that shows its work. Free, no signup.

How does this HVAC load calculator work?

Every tab here is built on the same skeleton, borrowed from ACCA Manual J and simplified to the inputs that move the number most. This calculator’s baseline is 25 BTU/hr per square foot. From there it applies four factors:

Then a 10% margin, rounded up to the nearest 500 BTU.

Sun factors are sunny ×1.15, average ×1.0, and shaded ×0.90. Insulation factors are poor ×1.25, average ×1.0, and good ×0.85. Those multipliers are this calculator’s, printed on this page.

Methodology & disclosure: the calculator applies the factors on this page in your browser. It is a planning estimate, not a measured result and not a substitute for a full ACCA Manual J. Amazon links on this site are affiliate links; they do not change the math.

How do you determine what size HVAC system you need?

The calculator above is one planning pass. The U.S. Department of Energy's Guide to Home Heating and Cooling describes a different, older estimate: one ton (12,000 Btu) of cooling for each 400 to 500 square feet of floor area. DOE says that rule does not account for local climate or how well the house is sealed and insulated. In a very efficient house, the same guide says one ton can condition 800 to 1,000 square feet. On a 1,000 sq ft house the old estimate is 2 to 2.5 tons (1,000 ÷ 500 = 2; 1,000 ÷ 400 = 2.5). The efficient-house range is 1 to 1.25 tons (1,000 ÷ 1,000 = 1; 1,000 ÷ 800 = 1.25). DOE says oversized cooling systems can be noisy, provide inadequate thermal comfort, and lead to mold and poor indoor air quality, and it says to insist that the contractor use ACCA Manual J.

This site's 25 BTU/hr per square foot path, walked through on the 1,000 sq ft page, is not the same rule as that ton-per-area estimate. Use the tabs for a room-by-room planning number. Do not treat either shortcut as the purchase. What BTU stands for is the unit on the quote. BTU to kW is the resistance-heat conversion, and it is not a furnace size by square footage.

How many BTU does each room size need?

There is no typical-BTU table on this page. Ceiling, windows, sun, and insulation change the result. Use the calculator above.

Why is oversizing the #1 DIY HVAC mistake?

Bigger feels safer and isn't. An AC that's too big cools the air fast, shuts off before it has dehumidified anything, and you get a cold clammy room plus compressor short-cycling that kills the equipment years early. Inverter mini-splits modulate. A head far above the load is capacity you can't use. Undersizing means the unit runs longer on the worst days.

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