AC Size Calculator: How to Choose the Right Air Conditioner
How AC Sizing Works
Air conditioner size is measured in BTU/h (British Thermal Units per hour) for cooling capacity, or in tons where 1 ton = 12,000 BTU/h. An undersized unit runs constantly without reaching the set temperature. An oversized unit cycles on and off too frequently, wasting energy and failing to dehumidify properly.
The gold standard for residential AC sizing is ASHRAE Manual J (ACCA 8th Edition), which calculates the cooling load based on:
- Local climate zone and outdoor design temperature
- Building envelope (insulation, windows, air leakage)
- Internal heat gains (occupants, appliances, lighting)
- Duct losses and ventilation requirements
AC Size Chart by Room Size
| Room Size (sq ft) | BTU Required (Moderate Climate) | BTU Required (Hot Climate) | Approximate Tons |
|---|---|---|---|
| 100–150 | 5,000 | 5,500–6,000 | 0.4–0.5 |
| 150–250 | 6,000 | 6,500–7,500 | 0.5–0.6 |
| 250–350 | 8,000 | 9,000–10,000 | 0.67–0.83 |
| 350–450 | 10,000 | 11,000–12,000 | 0.83–1.0 |
| 450–550 | 12,000 | 13,000–14,500 | 1.0–1.2 |
| 550–700 | 14,000 | 15,500–17,000 | 1.2–1.4 |
| 700–1,000 | 18,000 | 20,000–23,000 | 1.5–1.9 |
| 1,000–1,200 | 21,000 | 24,000–27,000 | 1.75–2.25 |
| 1,200–1,400 | 24,000 | 27,000–30,000 | 2.0–2.5 |
| 1,400–1,600 | 28,000 | 31,000–35,000 | 2.3–2.9 |
| 1,600–2,000 | 32,000 | 36,000–42,000 | 2.7–3.5 |
| 2,000–2,500 | 40,000 | 45,000–52,000 | 3.3–4.3 |
Note: These values assume 8-ft ceilings, standard insulation (R-13 walls, R-30 ceiling), and double-pane windows. Use the calculator above for precise sizing.
AC Size by Climate Zone (ASHRAE)
| ASHRAE Zone | Example Cities | BTU/sq ft | For 1,500 sq ft |
|---|---|---|---|
| Zone 1 (Very Hot) | Miami, Honolulu | 25–30 | 37,500–45,000 (3–3.75 ton) |
| Zone 2 (Hot) | Houston, Phoenix | 22–28 | 33,000–42,000 (2.75–3.5 ton) |
| Zone 3 (Warm) | Atlanta, Dallas, LA | 18–22 | 27,000–33,000 (2.25–2.75 ton) |
| Zone 4 (Mixed) | New York, Denver | 16–20 | 24,000–30,000 (2–2.5 ton) |
| Zone 5 (Cool) | Chicago, Boston | 14–18 | 21,000–27,000 (1.75–2.25 ton) |
| Zone 6 (Cold) | Minneapolis, Buffalo | 12–16 | 18,000–24,000 (1.5–2 ton) |
| Zone 7 (Very Cold) | Duluth, Anchorage | 10–14 | 15,000–21,000 (1.25–1.75 ton) |
How to Calculate AC Size: Step-by-Step
The ASHRAE cooling load formula for residential use:
Q = A × q × Corient × Cinsul × Cheight
- A = floor area in sq ft
- q = base cooling index from climate zone table (BTU/h per sq ft)
- Corient = 1.0 (north/east facing) to 1.2 (south/west with large glass area)
- Cinsul = 0.7 (super-insulated) to 1.4 (uninsulated)
- Cheight = actual ceiling height ÷ 8 ft
Worked Example
Scenario: 1,800 sq ft single-story home in Houston, TX (Zone 2), 9-ft ceilings, south-facing with 30% window-to-wall ratio, standard insulation.
Step 1: Base load = 1,800 × 25 = 45,000 BTU/h
Step 2: South-facing large windows → Corient = 1.15 → 51,750 BTU/h
Step 3: Standard insulation → Cinsul = 1.0 → 51,750 BTU/h
Step 4: 9-ft ceiling → Cheight = 9/8 = 1.125 → 58,219 BTU/h
Result: Need a 5-ton (60,000 BTU/h) system. Next standard size up is 5 tons.
Common AC Sizing Mistakes
- Oversizing by 50%+: "Bigger is better" is wrong. An oversized AC short-cycles, wastes energy, and doesn't dehumidify. The ACCA recommends sizing within 15% of calculated load.
- Ignoring duct losses: Leaky ducts in unconditioned spaces (attic, crawlspace) can add 20–30% to actual load. Have ducts tested before sizing.
- Not accounting for window upgrades: After replacing single-pane with double-pane windows, recalculate — you may be able to downsize.
- Using rules of thumb for commercial: Commercial buildings need Manual N calculations. Internal loads from equipment and occupancy dominate.
- Forgetting humidity: In humid climates (Zone 1–2), a properly sized system runs longer cycles that remove moisture. Oversized systems leave indoor humidity at 60%+.
Central AC vs Mini-Split vs Window AC
| Feature | Central AC | Mini-Split | Window AC |
|---|---|---|---|
| Size Range | 1.5–5 tons | 0.5–3 tons per head | 5,000–25,000 BTU |
| SEER Rating | 14–21 | 16–42 | 10–15 |
| Ductwork Required | Yes | No | No |
| Zoning | Single zone (standard) | Multi-zone capable | Single room |
| Installation Cost | $3,000–$7,500 | $3,000–$8,000 per zone | $150–$700 |
| Best For | Whole-house, existing ducts | Additions, no ducts, zoning | Single room, budget |
SEER Rating and Energy Efficiency
SEER (Seasonal Energy Efficiency Ratio) measures cooling efficiency. Higher SEER = lower electricity bills. As of 2023, the federal minimum is SEER 14 in northern US and SEER 15 in southern US.
| SEER Rating | Annual Cost (3-ton, Zone 4) | Savings vs SEER 14 |
|---|---|---|
| 14 | ~$1,100/year | Baseline |
| 16 | ~$960/year | $140/year (13%) |
| 18 | ~$855/year | $245/year (22%) |
| 20 | ~$770/year | $330/year (30%) |
| 25 | ~$615/year | $485/year (44%) |
Standards Reference
- ASHRAE Fundamentals 2021 — Chapter 18 (Nonresidential Cooling and Heating Loads)
- ACCA Manual J — Residential Load Calculation (8th Edition)
- ACCA Manual S — Equipment Selection
- ASHRAE Standard 90.1 — Energy Standard for Buildings
Frequently Asked Questions
What size AC do I need for a 1,500 sq ft house?
In a moderate climate (Zone 3–4), a 1,500 sq ft house needs approximately 2.5 tons (30,000 BTU/h). In hot climates (Zone 1–2), increase to 3–3.5 tons (36,000–42,000 BTU/h). In cool climates (Zone 5–6), 2 tons (24,000 BTU/h) may suffice. Always verify with a Manual J calculation.
How many tons of AC per square foot?
The general rule is 1 ton per 400–600 sq ft in moderate climates, or 1 ton per 300–400 sq ft in hot climates. This translates to about 20 BTU per sq ft as a baseline. However, insulation quality, ceiling height, and window area significantly affect this ratio.
Is it better to oversize or undersize an AC?
Neither is ideal, but slightly undersizing is generally less harmful than oversizing. An oversized AC short-cycles (turns on/off frequently), wastes energy, increases wear, and fails to dehumidify properly. ACCA recommends sizing within 15% of the calculated Manual J load.
How do I know if my AC is the right size?
Signs of proper sizing: the system runs in long steady cycles (15–20 min), maintains set temperature on the hottest days, indoor humidity stays below 55%, and energy bills are reasonable. Signs of oversizing: frequent on/off cycling, cold but clammy air, high humidity. Signs of undersizing: runs constantly, can't reach set temperature on design days.
Does a higher SEER rating mean bigger AC?
No. SEER measures efficiency, not capacity. A 3-ton SEER 21 unit and a 3-ton SEER 14 unit both provide 36,000 BTU/h of cooling. The SEER 21 unit does it using less electricity. Higher SEER means lower operating costs, not more cooling power.