What size ac unit do I need?
Central air conditioning, sized by cooling capacity in BTU per hour. Enter your details and the answer appears with every step of the arithmetic shown, so you can check it against any quote you have been given.
This is an estimate from the standard industry rule of thumb, not an engineering calculation. For anything you are about to spend money on, have the figure confirmed by a load calculation.
How ac unit capacity is worked out
The starting point across the industry is roughly 20 BTU per square foot of conditioned living space. That figure is then adjusted for ceiling height, how much sun the space takes, how many people live there, and the climate you are in.
The calculation is shown step by step under the result rather than hidden behind the answer. That is deliberate: most sizing guides online are lead capture forms that give you a number and a phone call, and a number you cannot check is a number you cannot argue with when a contractor disagrees. Every adjustment is listed, so you can see what changed the answer and by how much.
Nothing you type is sent anywhere. The arithmetic runs in your browser, there is no form to fill in, and no one calls you afterwards. That is the whole reason this page exists — you should be able to what size ac unit do i need without trading your phone number for it.
What going too big or too small actually costs
Oversizing is the more common and the more expensive mistake. A unit with too much capacity cools the air to the thermostat setpoint quickly and then shuts off — which sounds efficient and is not. Removing humidity takes sustained runtime, so a short-cycling oversized unit leaves the house cold and clammy, wears the compressor out faster, and costs more to buy in the first place.
An undersized unit runs continuously on the hottest days and never quite reaches the setpoint. It is more comfortable than an oversized one on the humidity front, but the running cost is high and the equipment life is short.
Both directions cost money, which is why it is worth ten minutes to what size ac unit do i need before you take a quote at face value. The asymmetry is worth knowing though: for most equipment on this site, being slightly under is a comfort problem you notice on a handful of extreme days a year, while being significantly over is a permanent efficiency and equipment-life problem you pay for every month.
Where a rule of thumb stops being enough
Every number on this page comes from the standard industry rule of thumb, and a rule of thumb is not an engineering calculation. The real method is a Manual J load calculation, which models your wall and attic insulation, your window area and orientation, how leaky the building envelope is, and how much your ducts lose. It routinely lands a size away from any square-footage estimate — in either direction, which is the part people forget.
So treat this as a sanity check rather than a specification. Ten minutes to size it properly tells you whether a quote is in the right neighbourhood, which is genuinely useful — it is enough to catch a contractor who has quoted you two sizes up because that is what was on the truck. It is not enough to order equipment from.
A Manual J load calculation is the real answer. It models your insulation, window area and orientation, air leakage and duct losses, and it routinely lands one size away from any rule of thumb — in either direction. Any contractor who sizes your system by square footage alone and nothing else is guessing.