What Size AC Do I Need? Tonnage Calculator and Guide

A reader in Phoenix emailed me last summer. His contractor had quoted a 5-ton unit for a 1,900 square foot house. He pulled out an old utility bill, ran it through a rough load calculation, and realized the existing 3-ton system had kept the house at 75 even on 115-degree days. The contractor was upsizing by almost 70 percent. Had he installed it, the house would have felt damp all summer and the compressor would have burned out years early from short cycling.

AC sizing is one of the most expensive decisions in the entire installation, and it is the one most often gotten wrong. Below is how to size correctly, why bigger is not better, and what to insist on before authorizing a new install.

What "Tonnage" Actually Means

One "ton" of AC is 12,000 BTU per hour of cooling capacity. The term comes from how much heat it takes to melt one ton of ice over 24 hours. Residential central ACs come in half-ton increments from 1.5 to 5 tons:

• 1.5 tons = 18,000 BTU
• 2 tons = 24,000 BTU
• 2.5 tons = 30,000 BTU
• 3 tons = 36,000 BTU
• 3.5 tons = 42,000 BTU
• 4 tons = 48,000 BTU
• 5 tons = 60,000 BTU

Step 1: The Rule-of-Thumb Starting Point

The classic rule is 20 to 25 BTU per square foot. So a 2,000 sq ft home in a moderate climate needs roughly 40,000 to 50,000 BTU, which lands at 3.5 to 4 tons.

Square Footage	BTU Range	Tons (moderate climate)
600 to 1,000	14,000 to 24,000	1.5 to 2 tons
1,000 to 1,500	22,000 to 30,000	2 to 2.5 tons
1,500 to 2,000	30,000 to 42,000	2.5 to 3.5 tons
2,000 to 2,500	40,000 to 50,000	3.5 to 4 tons
2,500 to 3,000	48,000 to 60,000	4 to 5 tons
3,000 to 3,500	55,000 to 65,000	4.5 to 5 tons

Step 2: Adjust for Climate

A 2,000 sq ft home in Minneapolis and a 2,000 sq ft home in Houston have very different cooling loads. Use the DOE/IECC climate zone map to find your zone.

Climate Zone	BTU per Sq Ft	Example Cities
Cool (1, 2)	18 to 20	Seattle, Minneapolis
Moderate (3, 4)	20 to 25	Denver, Kansas City
Warm (5)	25 to 30	Atlanta, Charlotte
Hot/Humid (6)	28 to 33	Houston, New Orleans, Miami
Hot/Dry (7)	25 to 30	Phoenix, Las Vegas

Worked example: 2,000 sq ft home in Atlanta. 2,000 x 27 BTU/sq ft = 54,000 BTU = 4.5 tons. The same house in Denver: 2,000 x 22 = 44,000 BTU = 3.5 tons. Same square footage, very different equipment.

Step 3: Adjust for House Quality

Rules of thumb assume an "average" house: 8-foot ceilings, code-minimum insulation, normal window-to-wall ratios, average air leakage. Adjust if yours isn't average:

House Characteristic	Adjustment
10+ foot ceilings	+10 to 15%
Lots of west or south-facing glass	+10%
Heavy shade (mature trees)	-10%
Old or single-pane windows	+10 to 15%
Recently insulated/air sealed	-10 to 20%
5+ occupants regularly home	+5 to 10%
Kitchen open to living area	+5%

Step 4: Insist on a Manual J Load Calculation

Rules of thumb get you within one ton. A proper ACCA Manual J load calculation gets you within 5,000 BTU. It accounts for: square footage of each room, ceiling height, R-value of walls, attic, and floors, window U-factor and SHGC by orientation, infiltration rate (ideally from a blower-door test), duct location and leakage, occupancy, and your specific 1 percent and 99 percent design temperatures from ASHRAE data.

Insider tip: When a contractor visits to quote a new system, ask: "Will you perform a Manual J, and can I have the printout?" If they say "I'll just measure the existing system" or "I have it in my head," walk away. A real Manual J takes 30 to 60 minutes on a laptop with software like Wrightsoft or CoolCalc. Replacement-in-kind from a 30-year-old undersized or oversized system perpetuates the original mistake. I have personally never regretted asking for the printout, and I have seen quotes drop by an entire ton (and several thousand dollars) when contractors realized I was checking their work.

Why Oversizing Is Worse Than Undersizing

Most contractors err high "just to be safe." That instinct is the wrong one. An oversized AC causes:

• Short cycling: The system reaches the thermostat setpoint in 5 to 8 minutes, shuts off, and starts again 10 minutes later. Compressors are designed for long cycles. Each start is the hardest moment in a compressor's life, and constant cycling cuts lifespan from 18 years toward 10.
• Poor humidity removal: Dehumidification happens after about 10 minutes of runtime, once the indoor coil has cooled enough to condense water. Short cycles leave moisture in the air. Your house feels clammy at 72 degrees instead of comfortable at 76.
• Uneven temperatures: The AC cools the room with the thermostat fast and shuts off before air reaches farther rooms.
• Higher upfront and operating cost: A 5-ton install can run $2,000 more than a 3-ton install, and the larger unit pulls more current every time it starts.

An undersized AC runs constantly on hot days and may not keep up during a heat wave, but otherwise removes humidity well and lasts a normal lifespan. Given a choice, lean slightly small rather than large.

Step 5: Don't Forget Ductwork

A perfectly sized AC paired with undersized ductwork still performs badly. As of 2026, the ENERGY STAR ducts page notes that typical homes lose 20 to 30 percent of conditioned air through duct leaks. If ducts are undersized or leaky, ask the contractor about duct repairs or replacements as part of the install. Sometimes fixing ducts is cheaper than upsizing equipment.

Real-World Sizing Examples

Home	Climate	Recommended Size
1,200 sq ft ranch, decent insulation	Charlotte (warm)	2 to 2.5 tons
1,800 sq ft 2-story, average insulation	Denver (moderate)	3 tons
2,400 sq ft, 10ft ceilings, west-facing	Houston (hot/humid)	4.5 to 5 tons
3,000 sq ft, recently insulated and sealed	Atlanta (warm)	3.5 to 4 tons
1,500 sq ft older home, single-pane windows	Phoenix (hot/dry)	3 tons

What Size Is Your Current System?

Find the model number on the outdoor unit's data plate. Look for two digits indicating tonnage:

• 18 or 019 = 1.5 tons
• 24 or 024 = 2 tons
• 30 or 030 = 2.5 tons
• 36 or 036 = 3 tons
• 42 or 042 = 3.5 tons
• 48 or 048 = 4 tons
• 60 or 060 = 5 tons

If your current system has held the house comfortable on the hottest days, that size is a strong reference point. Run the model and serial through our free HVAC age tool to confirm.

What About Variable-Speed and Two-Stage Systems?

Modern variable-speed systems (like Carrier Infinity, Lennox Signature, Trane XV) are more forgiving of slight oversizing because they can run at 25 to 100 percent capacity instead of full-on or full-off. They still need correct sizing, but a half-ton over isn't the disaster it would be on a single-stage system. If you want the comfort benefits of long runtimes without paying for variable-speed, two-stage is a middle ground at about $500 to $1,000 less than full variable.

Putting It All Together

Start with square footage x BTU/sq ft for your climate. Adjust for house quality. Compare to the existing system's track record. Then insist on a Manual J printout before signing anything. Estimate replacement cost with our HVAC replacement calculator, and read our repair vs replace guide if the existing system is on its last legs. Brand-specific lifespan and pricing are on our Carrier, Trane, Lennox, Goodman, and Rheem brand pages.