Recommended AC Size
3.5 Ton
BTU Capacity
42,000
Annual Cost
$315
Efficiency
Good
3.5 Ton
42,000
$315
Good
Inputs
Result
Base = 1,500/600 = 2.5 tons. ×1.0 climate ×1.1 age ×1.1 duct = 3.03. Windows (8, only 2 above 6, not enough for +0.1). Rounds to 3.0 ton standard size.
Inputs
Result
Base = 2,500/600 = 4.17. ×1.3 climate ×1.25 age ×1.25 duct = 8.47. +0.5 for 2nd story. +0.2 for windows above 6. Rounds to max 5.0 ton standard unit.
A general rule is 1 ton of AC per 600 square feet in a moderate climate. A 1,500 sq ft home needs about 2.5 tons. Hot climates need more (1 ton per 450 sq ft) while cool climates need less (1 ton per 700 sq ft).
| Home Size | Cool Climate | Moderate Climate | Hot Climate |
|---|---|---|---|
| 1,000 ft² | 1.5 tons | 1.5–2.0 tons | 2.0–2.5 tons |
| 1,500 ft² | 2.0 tons | 2.5 tons | 3.0–3.5 tons |
| 2,000 ft² | 2.5–3.0 tons | 3.0–3.5 tons | 4.0 tons |
| 2,500 ft² | 3.0–3.5 tons | 3.5–4.0 tons | 4.5–5.0 tons |
| 3,000 ft² | 3.5–4.0 tons | 4.0–5.0 tons | 5.0+ tons |
The federal minimum is 13-14 SEER depending on region. A 16 SEER unit offers good efficiency at reasonable cost. Units rated 20+ SEER provide the lowest operating costs but have higher upfront prices. In hot climates, higher SEER pays back faster.
| SEER Rating | Efficiency Class | Annual Cost (3-ton) | Best For |
|---|---|---|---|
| 13–14 | Minimum | $480–$520 | Budget installs, mild climates |
| 16 | Good | $390–$420 | Best value for most homes |
| 18–20 | High | $310–$350 | Hot climates, long-term savings |
| 21–25+ | Premium | $240–$290 | Max savings, eco-conscious |
Older homes (25+ years) typically need 25% more cooling capacity due to less insulation, single-pane windows, and air leaks. Homes under 10 years old with modern building codes need the baseline calculation with no added capacity.
| Home Age | Sizing Multiplier | Typical Insulation | Common Issues |
|---|---|---|---|
| Under 10 years | 1.0× | R-13+ walls, R-38+ attic | None—meets modern codes |
| 10–25 years | 1.1× | R-11 walls, R-19 attic | Some air leaks, aging seals |
| 25–50 years | 1.25× | R-4 to R-11 walls | Poor seals, single-pane windows |
| 50+ years | 1.3–1.4× | Minimal or none | No vapor barrier, balloon framing |
Leaky ducts can waste 20-30% of conditioned air, requiring a larger AC unit to compensate. Sealing ducts is often more cost-effective than upsizing the AC. New sealed ductwork operates at full efficiency with no added capacity needed.
The most common residential AC sizes are 2.5 to 3.5 tons (30,000-42,000 BTU). A 3-ton unit (36,000 BTU) fits most 1,500-2,000 sq ft homes in moderate climates. Larger homes or hot climates may need 4-5 ton units.
| AC Size | BTU/hr | Typical Home Size | Avg. Install Cost |
|---|---|---|---|
| 1.5 tons | 18,000 | 600–900 ft² | $3,000–$4,500 |
| 2.0 tons | 24,000 | 900–1,200 ft² | $3,500–$5,000 |
| 2.5 tons | 30,000 | 1,200–1,500 ft² | $3,800–$5,500 |
| 3.0 tons | 36,000 | 1,500–2,000 ft² | $4,000–$6,000 |
| 3.5 tons | 42,000 | 2,000–2,500 ft² | $4,500–$6,500 |
| 5.0 tons | 60,000 | 2,500–3,500 ft² | $5,500–$8,000 |
Read our guide
Read our guide
Read our guide
Calculate insulation R-values
Calculate electrical capacity
Calculate room square footage
Calculate materials for your building projects
Last Updated: Mar 9, 2026
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.