What Size HVAC Do I Need? Tons, BTUs & Sizing (2026)

Here's the short answer: to size an HVAC system, take your home's conditioned square footage, multiply by 20–30 BTU per square foot (the low end for cold climates, the high end for hot ones), adjust for your home's envelope, then divide by 12,000 to get tons of cooling. For a typical 2,000 sq ft home in a mixed climate that's about 4 tons of cooling and a 100,000 BTU furnace. This is the same Manual-J-lite screen our calculator runs — here's the whole method.

The sizing formula

Two loads drive an HVAC system: how much heat it has to remove in summer (the cooling load, in BTU/h) and how much it has to add in winter (the heating load). Both start from floor area:

1. Cooling load ≈ conditioned sq ft × cooling BTU/sq ft (climate) × envelope factors
2. Cooling tons = cooling load ÷ 12,000, snapped to the nearest real size
3. Heating load ≈ conditioned sq ft × heating BTU/sq ft (climate) × envelope factors
4. Furnace size = the smallest unit whose output (input × AFUE) covers the heating load

"Conditioned" area is the finished space you actually heat and cool — leave out the garage, the unfinished basement, and the attic.

Climate sets the base factor

The single biggest input is where you live. A square foot of house in Phoenix sheds far more heat gain in July than the same square foot in Portland, so the cooling factor climbs with the climate — while the heating factor moves the opposite way, since cold winters, not hot summers, drive the furnace:

Climate zone	Example regions	Cooling BTU/sq ft	Heating BTU/sq ft
Hot (Sun Belt)	AZ, TX, inland FL	30	25
Humid subtropical	Gulf Coast, Southeast	28	30
Mixed	Mid-Atlantic, lower Midwest	25	40
Temperate / marine	Pacific NW, coastal CA	22	35
Cold-winter (freeze-thaw)	Upper Midwest, Northeast, Mountain	20	50

Notice the inversion: a cold-climate home needs the least cooling per square foot but the most heating, which is exactly why the same 2,000 sq ft house takes a 3.5-ton AC and a 120,000 BTU furnace up north, but a 5-ton AC and only an 80,000 BTU furnace in the South.

Adjust for the envelope

Two identical floor plans don't always take the same equipment — the building envelope shifts the load. These are the multipliers our calculator applies on top of the climate factor:

Factor	Setting	Effect on load
Insulation	Poor / average / good	+15% / — / −15%
Home age	Pre-1980 / 1980–2000 / post-2000	+10% / — / −7%
Ceilings	8 ft / 9 ft / 10 ft+	— / +13% / +25% (cap)
Windows	Few / average / many	−5% / — / +10%
Sun exposure	Shaded / mixed / full	−6% / — / +8% (cooling only)

They stack. A leaky, pre-1980, full-sun home with 10-ft ceilings can carry well over 50% more cooling load than a tight, shaded, modern one of identical size — easily a full ton or two of difference. Ceilings matter because you're conditioning volume, not floor area; sun exposure only touches the cooling side.

Worked example: 2,000 sq ft, mixed climate

Run our standard home through the formula — 2,000 sq ft, mixed climate, average insulation and windows, 8-ft ceilings, built in the 1990s (every envelope factor neutral):

• Cooling: 2,000 sq ft × 25 BTU/sq ft = 50,000 BTU → 50,000 ÷ 12,000 = 4.17 tons → snaps to 4 tons.
• Heating: 2,000 sq ft × 40 BTU/sq ft = 80,000 BTU. A standard 80% AFUE furnace needs 80,000 ÷ 0.80 = 100,000 BTU of input, so it takes a 100,000 BTU furnace (100k × 0.80 = 80k of usable output). A 96% high-efficiency furnace would need 83,000 BTU of input — still a 100k unit, since an 80k unit only outputs 76,800 BTU.

That's the reference case behind every dollar figure in our HVAC replacement cost guide.

Sizing by home size and climate

Put the climate factor and floor area together and you get the cooling tonnage for common house sizes. Contractors often shorthand the same math as 400 to 600 square feet per ton — the reciprocal of the BTU factor — and the cost guides from Angi and HomeGuide lean on that rule too. This is the nearest-SKU tonnage our calculator returns:

Home size	Hot	Humid	Mixed	Temperate	Cold
1,000 sq ft	2.5 t	2.5 t	2 t	2 t	1.5 t
1,500 sq ft	4 t	3.5 t	3 t	3 t	2.5 t
2,000 sq ft	5 t	5 t	4 t	3.5 t	3.5 t
2,500 sq ft	6 t †	6 t †	5 t	5 t	4 t
3,000 sq ft	8 t †	7 t †	6 t †	5 t	5 t
3,500 sq ft	8 t †	8 t †	7 t †	6 t †	6 t †

† Above the ~5-ton single-unit ceiling, so it's typically two systems — common in larger and hotter homes, and the point where whole-house sizing really needs a professional. Assumes an average envelope; adjust up or down with the multipliers above.

Why snapping to the nearest ton matters

Residential equipment only comes in half-ton steps, so the calculated load gets rounded to the nearest real size — not rounded up "to be safe." Oversizing backfires: an AC or heat pump that's too big cools the air to the thermostat setpoint and shuts off before it has run long enough to wring humidity out of the air, so the house ends up cold and clammy. Those short run cycles — short-cycling — also start and stop the compressor constantly, which wears it out early and wastes energy. Bigger is not better; correctly sized is.

The furnace follows the same discipline from the other direction: you size it by output, not the input number on the label. Usable heat = input BTU × AFUE, so an 80,000 BTU heating load takes a 100,000 BTU / 80% furnace (80k output), not an 80,000 BTU one that would only deliver 64,000.

When it's more than one system — and when Manual J is non-negotiable

The square-foot method is a screen. It's accurate enough to budget, compare quotes, and catch a contractor trying to sell you two tons too much — but it can't see your actual windows, air leakage, or duct losses. Before anyone installs equipment, a contractor should run an ACCA Manual J room-by-room load calculation; many inspectors now require it on a permit. Treat the number here as your check against that calc, and treat any of these as a signal to slow down and get the real load run:

• Homes roughly above 2,200–3,300 sq ft (depending on climate), where a single residential unit can't cover the load (see the two-system rows above).
• The coldest zones, where the heating load can exceed even the largest residential furnace.
• A ducted heat pump in a cold climate, which may need cold-climate equipment or a dual-fuel backup — the heat pump vs. furnace + AC comparison covers that call, and a ductless mini-split sizes per zone instead.

Get your number

You don't have to do the arithmetic by hand. Enter your square footage, climate, ceiling height, insulation, and windows, and our calculator runs the full load calculation — cooling tons, furnace size, and the priced-out system — in one step.

Open the HVAC cost calculator →