Updated 11 April 2026
What Size Central AC Do You Need? BTU and Tonnage Guide
Getting the right size is the single most important decision in a central AC installation. An oversized system wastes money and creates comfort problems. An undersized system runs constantly and never fully cools your home.
BTU and Tonnage Basics
Air conditioning capacity is measured in BTU/h (British Thermal Units per hour) or tons. One ton of cooling equals 12,000 BTU/h. Residential central AC systems range from 1.5 tons (18,000 BTU/h) to 5 tons (60,000 BTU/h).
1.5 ton
18,000 BTU/h
2.5 ton
30,000 BTU/h
3.5 ton
42,000 BTU/h
5 ton
60,000 BTU/h
Home Size to Tonnage Table
| Home Size (sq ft) | Tonnage | BTU/h | Equipment Cost |
|---|---|---|---|
| 600 to 800 | 1.5 ton | 18,000 | $1,200 to $2,200 |
| 800 to 1,200 | 2 ton | 24,000 | $1,500 to $2,800 |
| 1,200 to 1,500 | 2.5 ton | 30,000 | $1,800 to $3,200 |
| 1,500 to 2,000 | 3 ton | 36,000 | $2,200 to $3,800 |
| 2,000 to 2,500 | 3.5 ton | 42,000 | $2,500 to $4,200 |
| 2,500 to 3,000 | 4 ton | 48,000 | $2,800 to $4,800 |
| 3,000 to 3,500 | 4.5 ton | 54,000 | $3,200 to $5,200 |
| 3,500+ | 5 ton | 60,000 | $3,500 to $5,800 |
These are general guidelines for moderate climates. Hot/humid zones may need 10% to 20% more capacity. A high-SEER system does not change the sizing requirement.
Climate Zone Adjustment
The base sizing table assumes a moderate climate. If you live in a hot or cold region, adjust accordingly. A contractor performing a Manual J calculation will account for this automatically.
| Climate Zone | Regions | BTU per sq ft | Adjustment |
|---|---|---|---|
| Hot/Humid | FL, TX, AZ, LA, MS, AL, GA | 22 to 25 | +10% to +20% |
| Warm/Moderate | NC, SC, TN, VA, CA, NV | 20 to 22 | Baseline |
| Cool/Mixed | OH, PA, IL, NY, MA, MI | 18 to 20 | -5% to -10% |
| Cold | MN, WI, ME, VT, NH, MT | 16 to 18 | -10% to -20% |
Why Oversizing Is a Problem
Many contractors push larger systems because they cost more and are easier to sell with the promise of faster cooling. In reality, an oversized AC creates four serious problems:
Humidity Problems
An oversized system cools the air so fast that it shuts off before removing enough moisture. The result: a cold, clammy house that feels uncomfortable even at 72 degrees.
Short Cycling
The system turns on, reaches temperature in minutes, shuts off, then repeats. This constant on/off cycling wears out the compressor and contactor faster than normal operation.
Higher Energy Bills
AC systems are least efficient during startup. Short cycling means more startups per day, which uses more electricity than a properly sized unit running longer cycles.
Uneven Temperatures
The system shuts off before conditioned air reaches all rooms evenly. Rooms near the air handler feel cold while distant rooms stay warm.
Manual J Load Calculation
A Manual J calculation is the industry-standard method for determining exactly how much cooling your home needs. It accounts for factors that simple square-footage rules cannot:
Red flag: contractor who sizes by square footage alone
Any contractor who says "your home is 2,000 sq ft, so you need 3.5 tons" without asking about insulation, windows, or orientation is guessing. A proper Manual J takes 30 to 60 minutes and costs $100 to $300 when done independently. Many quality contractors include it free with an installation quote.
How SEER Affects Cost by Tonnage
Higher efficiency costs more at every size, but the dollar premium grows with tonnage. Here is the total installed cost (equipment + labor) by tonnage and SEER tier:
| Tonnage | 14 to 16 SEER2 | 17 to 20 SEER2 | 21+ SEER2 |
|---|---|---|---|
| 2 ton | $3,000 to $4,200 | $3,800 to $5,500 | $5,000 to $7,500 |
| 2.5 ton | $3,300 to $4,800 | $4,200 to $6,200 | $5,800 to $8,500 |
| 3 ton | $3,500 to $5,500 | $4,500 to $7,000 | $6,500 to $9,500 |
| 3.5 ton | $4,000 to $6,200 | $5,200 to $7,800 | $7,200 to $10,500 |
| 4 ton | $4,500 to $7,000 | $5,800 to $8,800 | $8,000 to $12,000 |
| 5 ton | $5,200 to $8,000 | $6,500 to $10,000 | $9,000 to $14,000 |
Costs include equipment, labor, and standard installation materials. Does not include ductwork. See our SEER guide for efficiency details.