BTU Calculator: What Size AC and Furnace a House Needs
Floor area, ceiling height, climate zone and which way the glass faces, and it returns the cooling load in BTU, the size in tons, and what the furnace has to put out. It is the square foot method, the same one behind every sizing chart online, and the page is honest about what that method can and cannot tell you.
When the figures look right, the HVAC equipment list downloads as a real PDF with your business details on it, built inside your own browser. Nothing you type is uploaded anywhere.
Worked example
These are the figures the calculator opens with and the answer it gives. Change anything above and every number below moves with it.
- Conditioned floor area1600 sq ft
- Ceiling height8 ft
- Climate zoneZone 4, mixed, mid Atlantic and the lower Midwest
- Sun exposureAverage
- People living there3 each
- Kitchen inside the conditioned spaceYes
Cooling size3 tons
This is a rule of thumb and it is not a load calculation. The real answer is a Manual J, which counts your windows, your insulation, your infiltration and which way the house faces, and a contractor should run one before anybody orders equipment. Use this number to sanity check a quote, not to place an order.
Oversizing is the classic mistake and it is worse than being slightly small. A unit with too much capacity satisfies the thermostat in a few minutes and shuts off, so it never runs long enough to pull water out of the air, and the house ends up cold and clammy with the compressor short cycling itself to an early grave.
| Cooling load | 36,600 BTU/hr |
|---|---|
| That is | 3.05 tons12,000 BTU makes a ton |
| Equipment size to shop for | 3 tonnearest half ton step |
| Cooling per square foot | 23 BTU |
| Heating output needed | 72,000 BTU/hr |
| Furnace input, 80% AFUE | 90,000 BTU/hr |
| Furnace input, 95% AFUE | 75,789 BTU/hr |
The material list this produces
| Condenser, tons | 3 |
|---|---|
| Matched air handler or coil, tons | 3 |
| Line set, ft | 25 |
| Equipment pad, each | 1 |
| Disconnect and whip, each | 1 |
| Programmable thermostat, each | 1 |
What this calculator assumes
- This is the square foot method, the same rule of thumb behind every free sizing chart: 20 BTU per square foot of cooling as the base, scaled for ceiling height, climate zone and sun, plus 600 BTU for each person over two and 4,000 for a kitchen.
- Heating is figured at 30 to 60 BTU per square foot depending on zone, which is output at the register. Furnace input is higher, because an 80 percent furnace throws a fifth of the gas up the flue.
- Equipment is sold in half ton steps up to 5 tons, so the number to shop with is the rounded one, not the raw one.
What a ton is, and why the number is not a guess about weight
A ton of cooling is 12,000 BTU per hour. It comes from the ice trade: the heat it takes to melt one ton of ice over twenty four hours. Residential split systems are sold in half ton steps from one and a half up to five, so the only sizes you can actually buy are 18,000, 24,000, 30,000 and so on.
That is why this calculator rounds. A load of 33,000 BTU is not a three ton house and not a two and a half ton house, it is a three ton house, because 30,000 is the box that exists.
Heating is quoted differently. The BTU on a furnace label is usually input, the gas going in, and what heats the house is output. An 80 percent furnace sends a fifth of it up the flue, so a 90,000 BTU furnace delivers about 72,000. The calculator shows both so you can read a quote properly.
Bigger is not safer, it is worse
An oversized air conditioner cools the air fast, hits the thermostat setting and shuts off. Cooling is only half its job. The other half is pulling moisture out, and that only happens once the coil has been cold for a while, which needs a long run.
So an oversized system leaves a house cold and damp, which feels worse than warm and dry, and it short cycles, which wears the compressor out years early. This is the single most common mistake in residential HVAC and it usually comes from somebody sizing off square feet alone and then rounding up for safety.
Undersizing has a floor to it. The house just stays a degree or two warm on the hottest afternoon of the year. That is a smaller problem than a clammy house and a dead compressor.
Where the square foot method stops
This page uses the published rule of thumb: 20 BTU a square foot as the base, scaled for ceiling height, climate zone and sun exposure, plus 600 BTU for each person over two and 4,000 for a kitchen. It is the method behind every free chart, and it is genuinely useful for checking whether a quote is in the right ZIP code.
It is not a load calculation. A Manual J counts the actual windows and which way they face, the insulation in the walls and attic, the tightness of the envelope, the duct losses and the local design temperature. Two identical 1,600 square foot houses, one from 1955 with single glazing and one built last year, can be a ton and a half apart.
The right use of this number is to walk into the conversation informed. If a contractor quotes five tons for a 1,600 square foot house in Ohio, this page tells you to ask why before you sign.
Questions people ask
What size air conditioner do I need for a 1,500 square foot house?
At 8 foot ceilings, three people and a kitchen, this calculator puts a 1,500 square foot house at 34,600 BTU, a 3 ton unit, in zone 4 with average sun. The same house comes out at 2.5 tons in zone 7 with heavy shade and 3.5 tons in zone 1 with sunny west or south glass. Put your own numbers in above, then get a Manual J before ordering.
How many BTU per square foot for cooling?
Twenty is the base figure this calculator starts from, before any adjustment. Climate, ceiling height, sun, occupants and a kitchen all move it, and this calculator shows the per square foot figure it ended on so you can sanity check it.
Is it better to oversize or undersize an AC?
Oversizing is worse than being slightly small. An oversized unit short cycles and never runs long enough to dehumidify, so the house ends up cold and clammy and the compressor wears out early.
What is the difference between furnace input and output BTU?
Input is the gas going in, output is the heat reaching the house. An 80 percent AFUE furnace loses a fifth up the flue, a 95 percent furnace loses a twentieth.
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Common sizes, worked out
The same calculator with the numbers already filled in, one page per size, each ending in its own material list.
More sizes, answered
Each row is this calculator run with those numbers when the page was built. Type the same numbers in above to see every line of the working.
| Example | What it assumes | Answer |
|---|---|---|
| 1,000 sq ft | 1,000 square feet, 8 foot ceilings, climate zone 4, average sun, two people, kitchen inside. | Cooling size: 2 tons |
| 1,200 sq ft | 1,200 square feet, 8 foot ceilings, climate zone 4, average sun, three people, kitchen inside. | Cooling size: 2.5 tons |
| 1,500 sq ft | 1,500 square feet, 8 foot ceilings, climate zone 4, average sun, three people, kitchen inside. | Cooling size: 3 tons |
| 2,000 sq ft | 2,000 square feet, 8 foot ceilings, climate zone 4, average sun, four people, kitchen inside. | Cooling size: 4 tons |
| 2,500 sq ft | 2,500 square feet, 8 foot ceilings, climate zone 4, average sun, four people, kitchen inside. | Cooling size: 4.5 tons |
| 3,000 sq ft | 3,000 square feet, 8 foot ceilings, climate zone 4, average sun, five people, kitchen inside. | Cooling size: 5.5 tonsAbove 5 tons you are past ordinary residential split system territory. |
| 12x12 room | 144 square feet, 8 foot ceilings, climate zone 4, average sun, one person, no kitchen. | Cooling load: 2,880 BTU/hr |