How to use the BTU Calculator
- 1
Enter the area you want to condition and the ceiling height.
- 2
Choose your climate, how sunny the space is and how well it's insulated, then add people and whether it includes a kitchen.
- 3
Read the cooling load, the nearest AC size in tons and the heating load. Use it for budgeting, then confirm with a Manual J calculation.
How many BTUs per square foot?
A common starting point for cooling is about 20 BTU per square foot, a little less in cool climates and up to about 26 in hot ones. Heating needs run the other way, from about 30 BTU per square foot in hot climates to 60 in cold ones. One ton of cooling is 12,000 BTU per hour.
Ceiling height, sun, insulation, people and kitchens all move the number. Each extra person adds roughly 600 BTU, and a kitchen adds about 4,000.
Why oversizing an AC is a mistake
An oversized air conditioner cools the house quickly and shuts off before it has removed the humidity, so rooms feel cold and clammy. It also cycles on and off more, which wears it out sooner and costs more to run.
That's why HVAC contractors use an ACCA Manual J load calculation for the final size. This calculator gives you a realistic range for a quote or a first conversation.
Frequently asked questions
What size AC do I need for 1,500 square feet?
About 3 tons (36,000 BTU) in a mixed climate, and 3.5 to 4 tons in a hot, sunny one. Good insulation and shade can bring it down half a ton.
How many BTUs do I need per square foot?
About 16 to 26 BTU per square foot for cooling depending on climate and sun (20 is the usual rule of thumb), and 30 to 60 for heating.
How many BTUs is one ton of AC?
One ton is 12,000 BTU per hour of cooling.
Is a BTU calculator the same as a Manual J?
No. Manual J looks at walls, windows, orientation, ductwork and local design temperatures. This is a rule-of-thumb estimate for budgeting.
What furnace size do I need?
Divide the heating load by the furnace efficiency. A 60,000 BTU heating load needs about 63,000 BTU input at 95% AFUE, or 75,000 at 80% AFUE.