What size AC do I need for my house?

A common rule of thumb is one ton of cooling (12,000 BTU) for every 400 to 700 square feet, or roughly 20 BTU per square foot. A 1,600 square foot home in a moderate climate often needs about 2.5 to 3 tons, and a 2,400 square foot home about 3.5 to 4 tons. Hot-humid climates size higher. This is an estimate; a contractor's Manual J load calculation sizes the actual system.

How many BTU do I need per square foot?

Around 18 to 30 BTU per square foot for cooling in most homes, landing near 20 for a typical mixed climate. Hot-humid regions run 25 to 35, and cooler or well-insulated homes run lower. Ceiling height, sun exposure, insulation, and window area all shift the number, which is why a load calculation beats the rule of thumb.

Is it bad to oversize an AC or furnace?

Yes. An oversized system short-cycles, turning on and off rapidly. It cools the air fast but does not run long enough to pull out humidity, leaving the home clammy, and the frequent starts wear the equipment and waste energy. Right-sizing matters more than bigger, which is exactly why contractors run a Manual J calculation rather than guessing high.

What is a Manual J load calculation?

Manual J is the industry-standard method for calculating a home's heating and cooling load. It accounts for square footage, insulation, windows, orientation, air leakage, and local climate to size the system precisely. A reputable HVAC contractor performs one before recommending equipment; a rule-of-thumb estimate like this calculator is only a starting point.

What Size HVAC System Do You Need?

Size, not brand, is the decision that makes or breaks a new system. This turns your square footage and climate into a rough tonnage and BTU target, so you can sanity-check what a contractor recommends. It is a starting estimate; a Manual J load calculation sizes the real system.

Conditioned home size: 1,800 sq ft

Your climate

Home factors

3 tons

About 36,000 BTU of cooling

Rule-of-thumb range

2.5 to 3.5 tons

Per square foot

~20 BTU

Confirm with

Manual J

Estimate only. Uses the standard cooling rule of thumb (roughly 18 to 35 BTU per square foot by climate) adjusted by your home factors. It does not replace a Manual J load calculation, which accounts for insulation, windows, orientation, and air leakage.

How HVAC sizing works

Cooling capacity is measured in tons, where one ton equals 12,000 BTU per hour. The quick math is square footage times a BTU-per-square-foot figure for your climate, divided by 12,000. The climate figure does the heavy lifting: the same 2,000 square foot home needs noticeably more capacity in Phoenix than in Portland.

Climate	Typical cooling load	Rough coverage per ton
Hot / humid (South, Southwest)	25 to 35 BTU per sq ft	about 400 to 500 sq ft
Mixed (most of the country)	18 to 30 BTU per sq ft	about 500 to 600 sq ft
Cool / mild (Northwest, upper Midwest)	16 to 22 BTU per sq ft	about 600 to 700 sq ft

Why bigger is not better

The instinct to "size up to be safe" backfires with HVAC. An oversized air conditioner cools the air quickly but shuts off before it can pull humidity out, leaving a cold, clammy house, and the constant short-cycling wears the compressor and wastes energy. An undersized unit runs constantly and never quite catches up on the hottest days. The right size is the goal, which is why the guesswork stops at a Manual J calculation.

What the rule of thumb misses

Insulation and air sealing. A tight, well-insulated home can need a full ton less than a drafty one of the same size.
Windows and orientation. Big west-facing glass adds serious afternoon cooling load.
Ceiling height. Vaulted ceilings mean more air volume to condition than the floor area suggests.
Ductwork. Leaky or undersized ducts can undercut even a correctly sized unit. Our cost calculator lets you factor duct work into the price.

Frequently asked questions

What size AC do I need for 2,000 square feet?

Roughly 3 to 3.5 tons in a moderate climate, using about 20 BTU per square foot. Hot-humid areas can push it to 4 tons; cooler, well-insulated homes may need less. Enter your exact size and climate above for a tailored estimate, then confirm with a contractor's load calculation.

How do I convert tons to BTU?

Multiply tons by 12,000. A 3-ton system is 36,000 BTU per hour of cooling; a 2.5-ton system is 30,000 BTU. Residential central systems are commonly sold in half-ton steps from 1.5 to 5 tons.

Does this size the furnace too?

This calculator targets cooling (AC or heat pump) tonnage, the most common sizing question. Furnace sizing is measured in BTU input and depends heavily on climate and heat loss, so a cold-climate furnace is sized separately, again best done with a Manual J heating-load calculation.

Can I trust a contractor who sizes by square footage alone?

Be cautious. A quality contractor performs a Manual J load calculation rather than sizing off floor area or simply matching your old unit, which may itself have been wrong. If a bidder skips the load calc and quotes a size on the spot, ask why, and get a second opinion.

Know your size? Compare it against real quotes.

With a target tonnage in hand, you can spot a contractor who oversizes to upsell or undersizes to win the bid. Pair this with the cost calculator before you get quotes.

Estimate the cost

General information only, not engineering advice. Proper HVAC sizing requires a Manual J load calculation performed by a licensed contractor, accounting for your home's insulation, windows, orientation, and air leakage. Rule-of-thumb ranges from standard HVAC sizing references (roughly 18 to 35 BTU per square foot for cooling, one ton per 400 to 700 square feet by climate).