What Size AC Do I Need? Calculator + Guide
AC Size Chart by Home Size
| Home Size | Moderate Climate | Hot Climate | BTU/h |
|---|---|---|---|
| 400–600 sq ft | 1 ton | 1.5 ton | 12,000–18,000 |
| 600–900 sq ft | 1.5 ton | 2 ton | 18,000–24,000 |
| 900–1,200 sq ft | 2 ton | 2.5 ton | 24,000–30,000 |
| 1,200–1,500 sq ft | 2.5 ton | 3 ton | 30,000–36,000 |
| 1,500–1,800 sq ft | 3 ton | 3.5–4 ton | 36,000–48,000 |
| 1,800–2,200 sq ft | 3.5 ton | 4–4.5 ton | 42,000–54,000 |
| 2,200–2,800 sq ft | 4 ton | 5 ton | 48,000–60,000 |
| 2,800–3,500 sq ft | 5 ton | 5 ton+ | 60,000+ |
Moderate climate = ASHRAE Zone 3–4 (e.g., Atlanta, New York, Denver). Hot climate = Zone 1–2 (e.g., Miami, Houston, Phoenix).
How to Calculate AC Size
The ASHRAE Manual J method is the industry standard for residential cooling load calculation:
Step 1: Calculate floor area (sq ft)
Step 2: Determine base cooling load from climate zone table
Step 3: Apply correction factors: insulation, windows, orientation, ceiling height
Step 4: Add internal loads (people, equipment, lighting)
Step 5: Convert to tons (divide BTU/h by 12,000)
Full worked example: see our cooling load calculation guide.
Types of AC Systems
| System Type | Size Range | Best For | SEER Range | Installed Cost |
|---|---|---|---|---|
| Window AC | 0.5–2 ton | Single room | 10–15 | $150–700 |
| Mini-Split | 0.75–4 ton | 1–4 rooms, additions | 16–33 | $2,000–6,000 |
| Central AC | 1.5–5 ton | Whole house | 14–21 | $3,500–10,000 |
| Heat Pump | 1.5–5 ton | Heating + cooling | 15–22 | $4,000–12,000 |
Signs Your AC Is the Wrong Size
AC Too Big (Oversized)
- Cools quickly but room feels clammy/humid
- AC turns on and off frequently (short-cycling)
- Higher electricity bills than expected
- Uneven temperatures between rooms
- Compressor fails early (10 years instead of 15–20)
AC Too Small (Undersized)
- Can't reach set temperature on hot days
- Runs continuously without shutting off
- Some rooms never get cool
- Electricity bills spike in summer
Common Mistakes
- Using a simple "tons per sq ft" rule without adjustments: This ignores insulation, windows, climate, and orientation — all of which significantly affect the load
- Replacing with the same size as the old unit: The old unit may have been oversized. If you've added insulation or windows, you may need less
- Letting the contractor choose the size: Get an independent Manual J calculation. Some contractors oversize to avoid comfort complaints
- Ignoring humidity: In humid climates, the AC must run long enough to dehumidify. Oversized units cool too fast and leave moisture behind
Frequently Asked Questions
What size AC do I need for 1,500 sq ft?
For 1,500 sq ft in a moderate climate: 2.5–3 ton AC (30,000–36,000 BTU). In hot climates: 3.5–4 tons. The exact size depends on insulation, windows, ceiling height, and local climate zone.
How do I calculate what size AC I need?
The ASHRAE Manual J method considers: floor area, ceiling height, insulation R-values, window area and type, orientation, climate zone, occupancy, and internal heat gains. Our AC size calculator automates this.
Is it OK to oversize an AC unit?
No. Oversized ACs short-cycle, waste energy, create uneven temperatures, and have shorter lifespans. ACCA recommends sizing within 15% of calculated cooling load.
Should I get a 2 ton or 2.5 ton AC?
It depends on your calculated cooling load. 2 tons covers ~800–1,000 sq ft in moderate climate. 2.5 tons covers ~1,000–1,300 sq ft. If your load falls between sizes, round up to the nearest half-ton — but don't go more than 1 ton above.