What Size AC Do I Need? BTU Sizing Guide
Whether you're replacing an old unit or sizing new construction, picking the right AC size matters more than any other decision. Too small and it can't keep up; too big and it short-cycles, dehumidifies poorly, and wastes money. This guide gives you a simple field method — the BTU/sqft rule — plus the factors that change the answer.
The Quick Answer: BTU per Square Foot
The fastest way to estimate AC size is to multiply conditioned floor area by a climate-based BTU/sqft factor:
12,000 BTU/h = 1 ton of cooling
| Climate / Exposure | BTU/sqft | Example Region |
|---|---|---|
| Cool climate, well-shaded | 20 | Pacific Northwest, basement |
| Mild climate | 25 | Mid-Atlantic, ground floor |
| Hot climate, west-facing | 30 | Southeast US |
| Top floor / hot / poor insulation | 35 | Southwest, attic |
Quick Reference: Tons by Square Footage
For a typical mild-to-hot climate (25-30 BTU/sqft):
| Area (sqft) | Recommended Tons | BTU/h |
|---|---|---|
| 400-600 | 1.5 ton | 18,000 |
| 600-900 | 2 ton | 24,000 |
| 900-1,200 | 2.5 ton | 30,000 |
| 1,200-1,600 | 3 ton | 36,000 |
| 1,600-2,000 | 3.5 ton | 42,000 |
| 2,000-2,500 | 4 ton | 48,000 |
| 2,500-3,000+ | 5 ton | 60,000 |
Estimates for average insulation. Always verify with a proper load calculation.
Example: 1,500 sqft home in Florida
1,500 × 30 = 45,000 BTU/h
Add: 8 windows (+8,000) + 4 people (+1,600) + 3 appliances (+1,500)
Subtotal: 56,100 BTU/h
Insulation credit (×0.85): 47,685 BTU/h
Tons: 47,685 ÷ 12,000 = 3.97 → 4 ton unit
Factors That Change AC Size
- Climate & design temp — hotter regions need more tons per sqft
- Insulation quality — R-30 roof/R-13 walls cut load by ~15%
- Windows — each standard window adds ~1,000 BTU/h (west-facing worse)
- Ceiling height — vaulted ceilings increase conditioned volume
- Occupants & appliances — people add 400 BTU/h; major appliances 500 BTU/h each
- Duct losses — leaky ducts in attics can add 20-30% to capacity needs
- Sun exposure — west-facing walls and large unshaded windows increase load
Why Oversizing Is Bad
⚠️ Bigger is NOT better. Oversized units cool the space too quickly, then shut off (short cycling). Consequences: poor dehumidification (clammy air), temperature swings, higher bills, and premature compressor wear. Always size to the actual load.
Frequently Asked Questions
How many square feet will a 2-ton AC cool?
Typically 800-1,200 sqft, depending on climate and insulation. In hot climates, maybe 700 sqft; in mild climates, up to 1,200.
How do I calculate what size AC I need?
Multiply sqft by BTU/sqft factor (20-35 by climate), add corrections for windows/people/appliances, divide by 12,000 to get tons. For exact sizing, use full Manual J software.
Is it bad to oversize an air conditioner?
Yes — short cycling, poor dehumidification, temperature swings, higher bills, and compressor wear. Size to actual load.
📱 Size it right the first time
FieldCalc HVAC runs the full simplified Manual J calc — enter area, climate, windows, people. Get BTU/h, tons, and recommended unit size. Works offline.
Download Free →