Written October 2026 with our current installed prices and the sizes our estimator starts from.
Most houses need a 2 to 5 ton air conditioner, roughly one ton for every 500 to 600 square feet. A new central AC runs $5,942 to $14,450 installed from us, by size and efficiency. From about $99 a month at 0% APR for 60 months, the whole price financed, on approved credit.
So as a starting point: about 3 tons for a 1,500 square foot house, 4 for 2,000, 5 for 2,500 to 3,000. That’s the chart answer. The real one comes from a Manual J load calculation, the industry standard from the Air Conditioning Contractors of America (ACCA), and in a humid summer the difference matters more than people expect, because an AC that’s too big leaves the house cool and clammy.
If you only do one thing: before anybody quotes you a new AC, find the size of the one you have (how is below) and ask them how they decided whether that number is right. If the answer is “same as the old one,” nobody sized it. They copied it.
What size AC unit do I need? The short answer, by house size
These are the house sizes our free HVAC estimator asks about, the size it starts each one at, and what a new central air conditioner costs installed in Northern Virginia at each. The spread inside each row is mostly efficiency and how the compressor runs: a basic single-stage unit at the bottom, a two-stage or variable-speed one at the top.
| House size | AC size | At 0% for 60 months | AC installed |
|---|---|---|---|
| Up to 1,250 sq ft | 2 tons | From $99 a month | $5,942 to $11,950 |
| 1,251 to 1,400 sq ft | 2.5 tons | From $107 a month | $6,443 to $12,450 |
| 1,401 to 1,700 sq ft | 3 tons | From $116 a month | $6,944 to $12,950 |
| 1,701 to 1,900 sq ft | 3.5 tons | From $124 a month | $7,445 to $13,450 |
| 1,901 to 2,400 sq ft | 4 tons | From $132 a month | $7,946 to $13,950 |
| 2,401 to 3,000 sq ft | 5 tons | From $141 a month | $8,447 to $14,450 |
Monthly figures: 60mo @ 0% APR on each starting price, the whole price financed, on approved credit. Longer plans lower the payment and cost more in total. The lender sets the final terms.
Over 3,000 square feet, one row stops meaning much. The biggest standard home AC is 5 tons: federal rules define a home central air conditioner as one rated under 65,000 BTUs an hour, and 5 tons is 60,000. A bigger house may need two systems. The estimator prices one and flags it, and the measurement settles it.
Notice what the table is: where our estimator starts, so it can give you a price. Trane’s own chart, which it calls “a rough idea,” lands in the same place, 4 tons for 2,000 square feet. The Department of Energy is blunt about charts like that. The common shortcut of a ton per 400 to 500 square feet, its fact sheet says, “fails to take into account local climate and the efficiency of a house,” and in a very efficient house one ton can cool 800 to 1,000 square feet. The rest of this page is how to get from the chart to the right number for your house.
How to read the size of the AC you have
Go out to the outdoor unit and find its rating plate, a sticker or metal plate on the side or back of the cabinet. Find the model number. On most brands, a two-digit number inside it is the cooling capacity in thousands of BTUs an hour: 18, 24, 30, 36, 42, 48 or 60. Divide by 12 and you have tons, because a ton of cooling is 12,000 BTUs an hour. A 36 is 3 tons. A 48 is 4. A 60 is 5. That’s Trane’s own instruction, and Goodman numbers its units the same way.
It’s close, not universal. Lennox uses a few odd numbers (a 047 is 4 tons, a 059 is 5), so if yours won’t divide cleanly, look the model up before you trust the math.
And the outdoor unit is only half the system. A central air conditioner is rated as a pair: the outdoor unit plus the indoor coil, which in a house with a gas furnace sits right on top of the furnace. A new outdoor unit on an old coil isn’t the system on the spec sheet.
Why “same as the old one” is usually wrong
The old AC was sized for the house as it was, by whoever put it in, with whatever method they used. The house has probably changed since. New windows. More attic insulation. A finished basement. An addition. Every one of those changes how much heat gets in on a July afternoon.
And the old size may have been wrong on day one. For a long time the habit in this trade was to round up, because nobody calls to complain that the AC is too strong. Trane’s own product manager tells buyers to beware of a dealer who’d “just want to replace your system with whatever you currently have.” We agree with him.
Virginia’s residential code requires heating and cooling equipment in new homes to be sized with ACCA Manual S from a Manual J load calculation. For a replacement in an existing house, the code also lets a homeowner swap in equipment of similar size. Legal is not the same as right.
What sizing actually is: a Manual J load calculation
The real method is a load calculation, and the standard is ACCA’s Manual J, which ACCA calls “the national ANSI-recognized standard” for sizing loads in houses, townhouses and condos. The idea is simple even if the math is long: add up the heat that gets into the house on a hot afternoon through the windows (glass facing west most of all), the walls and ceilings, air leaks, ducts running through a hot attic, and the people and appliances inside. Add the moisture. Then pick equipment, with Manual S, that covers both without blowing past them.
“A hot afternoon” has a specific meaning. It’s the design temperature, the one it’s hotter than only about 1 percent of the hours in a year: 91 at Dulles, 92 at Reagan National and Leesburg, against 75 inside. Manual J is also explicit that the contractor shouldn’t pad those conditions with a “safety factor.” Padding is how a house ends up with an oversized AC, which in a Virginia summer is worse than it sounds.
Online AC size calculators skip nearly all of that and multiply your square footage by a number, which is why two calculators give you two answers for the same house. Square footage is one input. Windows, insulation, air leaks, ceiling height, shade and which way the house faces are the rest.
Why bigger isn’t better in a Virginia summer
An air conditioner does two jobs at once. It cools the air, and it pulls water out of it: damp air crosses the cold indoor coil, the moisture condenses, and it runs down the drain. Manual J counts those as two separate loads, the heat and the moisture, and here the moisture is a big share of the work.
How big? The same ACCA table that sets the design temperature also counts the extra water in that air, in grains per pound. Against a 50 percent indoor target, Dulles comes out at 37. Atlanta is 36. Phoenix, at 108 degrees, is below zero: the air outside is drier than the air in your house. On a design day, our air carries as much water as Atlanta’s.
That’s why bigger isn’t better here. An oversized AC drops the temperature fast, and the thermostat shuts it off before it has run long enough to do the second job. Trane calls that short cycling and says it can leave you feeling “cold and clammy.” The thermostat says 72 and the house still feels sticky. An AC that’s too big:
- Cools the air around the thermostat and quits before much of the moisture comes out.
- Runs short, frequent cycles, which Trane says means premature wear and higher energy bills.
- Needs more air than the ducts may be able to carry: roughly 350 to 400 cubic feet a minute for every ton. More on that below.
- Costs more to buy. Every row up our table adds $501 to the starting price, for capacity you only use on paper.
The Department of Energy’s sizing guide, working from ACCA’s Manual S, puts a ceiling on it: cooling equipment shouldn’t exceed the calculated load by more than 15 percent.
Too small shows up differently. It runs longer to get there, and on the hottest afternoons the house creeps up past the setting. One thing that isn’t a sign of a small AC: running nearly nonstop on a 91 degree afternoon. A system sized to the design day is supposed to, and those afternoons add up to about 88 hours of the year. A little drift then is the trade for a house that’s dry the rest of the summer.
The better fix for humidity is how the AC runs, not how big it is. Trane says a two-stage AC pulls moisture out better than a single-stage, and variable speed does it best; its variable-speed units run anywhere from 25 to 100 percent. Most of a Virginia summer they sit on low, long and steady, which is exactly what drying a house takes.
The furnace and the ducts get a vote
This is the part most AC sizing articles leave out. In a house with a gas furnace, the AC has no fan of its own indoors. It borrows the furnace’s blower and your ducts. The coil needs roughly 350 to 400 cubic feet of air a minute for every ton of cooling, and the Department of Energy’s guide is plain that the blower has to be able to supply it. A 4-ton AC wants something like 1,400 to 1,600 a minute. Go up a ton and you’re asking the same blower and the same ducts for another 350 to 400.
So a bigger outdoor unit isn’t just a bigger box in the yard. If the ducts were built for 3 tons, the honest fix for a hot upstairs may be the ducts, not the tonnage.
When the furnace is due too, size and replace them together. The free HVAC estimator prices the pair: $8,898 to $23,000 installed, by size. From about $148 a month at 0% APR for 60 months, the whole price financed, on approved credit. The heating side gets sized its own way: here is what size furnace you need, by house size.
What the measurement checks
Our free in-home measurement takes about 45 minutes, and it sizes the system to your house, not to the label on the old unit. The questions are the ones a calculator can’t ask. How big are the rooms, really, and which ones are finished? What are the windows, and which way do they face? What’s in the attic, and do the ducts run through it? Can they carry the air a new system will move, or is a duct the real bottleneck? Does the coil on the furnace match the new outdoor unit, and is the drain under it sound?
Size is half of it. The other half is the install delivering the size you paid for: the refrigerant charge weighed in on a scale and checked on gauges at your house, not guessed.
The rest of a proper install, from the matched coil to the vacuum and the startup readings, is in AC installation, step by step.
If the old AC is still running, fall is a good time to get the number: no hot house, no rush, and a published price to hold any other quote against. More on what drives that price: what AC replacement costs in Northern Virginia.
AC size questions people ask us
How many tons of AC do I need?
Divide your square footage by 500 to 600 for a starting point, which puts most houses between 2 and 5 tons. A ton is 12,000 BTUs an hour. Then get it measured, because the same square footage can need a ton more or a ton less depending on the windows, insulation and air leaks.
What size AC do I need for a 1,500 square foot house?
About 3 tons. That’s where our estimator starts a 1,401 to 1,700 square foot house, and Trane’s chart says the same. A newer, tight 1,500 square feet can measure at 2.5. The price for each size is in the table above.
What size AC do I need for a 2,000 square foot house?
About 4 tons by the chart: it’s in our estimator’s 1,901 to 2,400 row, and Trane’s chart puts 2,000 at 4 too. It’s also the size most worth measuring. At 600 square feet a ton it’s closer to 3.5, and in a very efficient house the Department of Energy says a ton can cool 800 to 1,000 square feet.
What size AC do I need for a 2,500 square foot house?
Our estimator puts 2,401 to 3,000 square feet at 5 tons, the biggest standard home size, and Trane’s chart says 5 for 2,500. Sitting at the bottom of that row, a 2,500 square foot house is the one most likely to measure smaller than the row says, so don’t buy a 5 without a load calculation.
What size AC do I need for a 3,000 square foot house?
Five tons is the ceiling for one home system, and 3,000 square feet is right at it. Above that, or in a tall house where the upstairs runs hot, ask about two smaller systems, one per floor. Our estimator flags anything over 3,000 square feet for exactly that, and the measurement settles it.
Is an AC size calculator accurate?
As a ballpark, sure. Most multiply square feet by a fixed number, so they can’t see your windows, insulation, attic or which way the house faces. That’s why two calculators give you two answers. A Manual J load calculation looks at all of it.
What happens if my AC is too big?
It short cycles: it cools the air fast, shuts off before it has pulled much moisture out, and starts again soon after. You get a cool, clammy house, more wear and higher bills. In a humid Virginia summer, that’s the wrong trade.
What happens if my AC is too small?
It runs longer to catch up and falls behind on the hottest afternoons. Don’t mistake that for an AC running nearly nonstop on a 91 degree day, though. A system sized right is supposed to.
Should my new AC be the same size as my old one?
Maybe, but only if somebody checked. The house has probably changed since the old one went in, and the old size may never have been right. Virginia’s code allows a same-size swap. That doesn’t make it the right size.
Does a heat pump get sized the same way?
The cooling side, yes: same Manual J, same tonnage. A heat pump also heats, so the heating load gets checked too. For how each brand holds up at 10 to 20 degrees, see the best heat pump brands.
How do I find the size of my AC?
On the rating plate on the outdoor unit. Find the model number and the two-digit number inside it (18 to 60), then divide by 12: a 36 is 3 tons. Lennox uses a few odd ones, like 047 for 4 tons.
My AC isn’t old enough to replace. Can you just fix it?
Yes, and often that’s the right call. Our AC repair starts with a $99 diagnostic visit, and you get the problem and the price in writing before any work starts. Not sure which way to go? Here is how to decide between repair and replacement.
See your AC price, or book the free measurement
The free HVAC estimator asks a few questions about your home, shows the installed price for your size, and lets you book the free in-home measurement from the same page. Holding someone else’s quote? We’ll give you a free second opinion on it, and whatever we’re running this month is on our offers page.
Brands are their own question: the best HVAC brands and the best heat pump brands, ranked by the people who install them.
Call (703) 997-9757 or book the free measurement online.
More straight answers from our crew: how long a water heater lasts, how long a furnace lasts, how long an AC unit lasts and heat pump vs. air conditioner.
SwiftPro Heating, Cooling & Plumbing sizes and installs Trane, Carrier and Rheem air conditioners across Northern Virginia, based in Chantilly. SwiftPro is a Trane Comfort Specialist and a Rheem Pro Partner, and we’re Carrier experts. Family owned, BBB Accredited with an A rating, 24/7 emergency service, Virginia Class A contractor license 2705192276, and rated 5.0 stars across 1,131 reviews on Google, Angi, Yelp, the BBB and Facebook. Read the reviews.
