What size AC unit do I need?

Enter your floor area and climate and the answer appears with the arithmetic shown. No form, no email, and nobody calls you afterwards.

Why most sizing tools ask for your phone number

Search for equipment sizing and nearly every result is a lead capture form. You enter your details, and instead of a number you get a contractor calling within the hour. The sizing was never really the product.

This site gives you the number and shows how it got there. That matters more than it sounds, because the most useful thing an estimate does is let you push back: when a quote comes in two sizes above what the arithmetic suggests, you want to be able to ask why rather than take it on trust. Oversizing is the single most common thing that goes wrong in this industry, and it is not always accidental.

How AC 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 size it properly 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.

Other equipment

What size ac unit do I need What size mini split do I need What size dehumidifier do I need What size furnace do I need What size ceiling fan do I need What size generator do I need What size solar system do I need What size water heater do I need

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 what size ac unit do i need 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.

Frequently asked questions

What size AC unit do I need for my house?

Start from roughly 20 BTU per square foot, then adjust for ceiling height, sun exposure, occupancy and climate. For a 1,500 square foot home in a mixed climate that lands around 30,000 BTU, or 2.5 tons. The calculator applies the adjustments and shows its working.

How many square feet will a 3 ton AC cool?

Around 1,500 to 1,800 square feet in a mixed climate with 8 foot ceilings — less in a hot humid one, more in a cold one. Three tons is 36,000 BTU, and the conversion is 12,000 BTU per ton.

Is a bigger AC unit better?

No, and this is the most useful thing on this page. An oversized unit cools quickly, shuts off, and never runs long enough to pull humidity out — so the house ends up cold and clammy, the compressor wears faster, and you paid more for the privilege.

Is this accurate enough to buy equipment from?

No, and any site telling you otherwise is overselling a rule of thumb. Use it to work out the capacity approximately, sanity-check a quote, or know what range to expect before you call anyone. Before money changes hands, get a proper load calculation — it is the difference between an estimate based on floor area and one based on your actual building.

Do I have to give my email or phone number?

No. There is no form and nothing is transmitted — you can get the sizing right without giving up anything, because the arithmetic runs in your browser. Most sizing tools in this category are lead capture for contractors, which is why they ask. This one is funded by the advertising on the page instead.