Sizes cooling and heating together, and adjusts for the four things that actually move the number: climate, insulation, ceiling height and sun.
Use the area being conditioned, not the whole house, unless you are sizing for the whole house.
Each person beyond two adds about 600 BTU of cooling load; a kitchen adds roughly 4,000.
Cooling capacity needed
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In tons—
Nearest standard AC size—
Volume conditioned—
Heating output needed—
Furnace input rating—
Cooling load per sq ft—
Heating in kW (if electric)—
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The calculation
The starting point is 20 BTU per square foot of cooling for an average, reasonably insulated space with 8 ft ceilings in a moderate climate. A 500 ft² room therefore needs about 10,000 BTU/hr — just under one ton, since a ton of cooling is 12,000 BTU/hr.
Four multipliers then move that figure: ceiling height (divided by 8), climate zone, insulation quality and sun exposure. Fixed additions follow for occupants beyond two, at roughly 600 BTU each, and for a kitchen at about 4,000 BTU.
Heating is sized differently
Climate zone
Heating BTU per ft²
Zone 1 — hot
30–35
Zone 3 — warm
40–45
Zone 5 — cool
50
Zone 7 — very cold
60
Note that the heating figure is output. A furnace is sold by its input rating, so an 80% AFUE unit needs an input of output ÷ 0.8 — a 20,000 BTU heat requirement means a 25,000 BTU furnace.
Why bigger is worse
An oversized air conditioner reaches the thermostat setpoint quickly and shuts down. Because dehumidification happens gradually as air passes over a cold coil, a unit that runs in short bursts never removes much moisture. The result is a room that is cold and damp rather than cool and dry, along with more wear from constant cycling. Undersizing is uncomfortable; oversizing is uncomfortable and expensive.
Rule-of-thumb sizing like this is genuinely useful for scoping a window unit, a mini-split, or a budget for a replacement system. It is not a substitute for a Manual J load calculation, which accounts for the actual construction of your walls, the orientation and specification of every window, duct losses and infiltration rates. For a whole-house system, that calculation is worth paying for.
Where estimates go wrong
Glazing is the biggest omission in any square-footage method. A room with one small north-facing window and the same room with a wall of west-facing glass have wildly different cooling loads, and the sun exposure multiplier here is only a coarse approximation of that difference. Vaulted ceilings, uninsulated attics above and rooms over garages are similarly under-represented.
Heat pumps blur the two numbers
If you are sizing a heat pump rather than a separate AC and furnace, one unit must satisfy both loads. In warm climates the cooling load dominates and sizing to it is fine. In cold climates the heating load is larger, and sizing to that would badly oversize the cooling. This is what variable-capacity inverter heat pumps solve — they modulate rather than cycle, so they can cover a wide range without the short-cycling penalty.
Check the ductwork too
Fitting a larger unit to ducts sized for a smaller one produces noise, poor airflow and disappointing performance regardless of the capacity on the nameplate. If the calculated size is much above what is installed now, the distribution system needs looking at as well as the equipment.
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Frequently asked questions
How many BTU do I need per square foot?
About 20 BTU per square foot for cooling in an average, moderately insulated space with 8 ft ceilings. Heating ranges from roughly 30 BTU per square foot in hot climates to 60 in very cold ones.
What size air conditioner do I need for a 500 square foot room?
Around 10,000 BTU, which is just under one ton, for an average room in a moderate climate. Add capacity for high ceilings, heavy sun exposure, a kitchen or more than two regular occupants.
How many square feet does 1 ton of AC cool?
Roughly 600 square feet in an average home, since one ton is 12,000 BTU and typical load is 20 BTU per square foot. That falls to about 500 ft² in hot climates and rises to 700 ft² or more in a well-insulated new build.
Is it better to oversize or undersize an air conditioner?
Neither, but oversizing causes the worse problems. An oversized unit reaches the setpoint and shuts off before it has removed humidity, leaving the space cold and clammy, and the constant cycling wears the compressor. Size to the calculated load rather than rounding up.
What is the difference between furnace input and output BTU?
Input is the fuel energy consumed, output is the heat actually delivered. An 80% AFUE furnace rated 100,000 BTU input delivers 80,000 BTU of heat. Size to the output you need, then divide by the efficiency to find the input rating to shop for.