AC BTU Calculator
Calculate the exact BTU for your air conditioner. Enter room dimensions, climate zone, insulation, and occupancy factors for a precise result.
Air Conditioner BTU Calculator
AC BTU Calculation Formula
The standard formula for air conditioner BTU is based on ACCA Manual J methodology:
Basic cooling BTU formula:
BTU = Sq Ft × 20 × Climate Factor × Insulation Factor
Then add adjustment factors for ceiling height, sun, occupants, and kitchen.
Step-by-step example
- 1. Room: 20 ft × 15 ft = 300 sq ft
- 2. Base BTU: 300 × 20 = 6,000 BTU
- 3. Hot climate (TX): × 1.20 = 7,200 BTU
- 4. Average insulation: × 1.00 = 7,200 BTU
- 5. 10 ft ceiling (÷8 × 1.25): = 9,000 BTU
- 6. Sunny south-facing: + 10% = ~10,000 BTU
All Adjustment Factors
Climate Multipliers
- Hot (FL/TX/AZ): × 1.20
- Mixed (Midwest): × 1.05
- Moderate (Pacific): × 0.95
- Cool (Northeast): × 0.85
- Very cold: × 0.80
Insulation Multipliers
- Poor (pre-1970): × 1.15
- Average (1970–2000): × 1.00
- Good (2000–2015): × 0.90
- Excellent (2015+): × 0.82
Sun Exposure Adjustments
- Heavy shade: – 15%
- Mostly shaded: – 10%
- Normal: + 0%
- Sunny: + 10%
- Very sunny: + 20%
Other Adjustments (Add)
- Each extra occupant: + 600 BTU
- Kitchen nearby: + 4,000 BTU
- Top floor / under roof: + 15%
- Basement room: – 10%
- Ceiling above 8 ft: × (h ÷ 8)
- Each large window: + 1,000 BTU
Source: US DOE Air Conditioning Guidelines & ACCA Manual J.
AC BTU by Room Size — Quick Reference (2026)
Baseline cooling BTU at 20 BTU/sq ft (moderate climate, average insulation). Adjust for your conditions above.
| Room (sq ft) | Room Example | Cooling BTU | AC Tonnage | Window AC Size | Central / Mini Split |
|---|---|---|---|---|---|
| 100–150 sq ft | Small bedroom | 5,000 BTU | 0.4 ton | 5,000 BTU window AC | N/A — use window AC |
| 150–250 sq ft | Master bedroom | 6,000 BTU | 0.5 ton | 6,000 BTU window AC | N/A — use window AC |
| 250–350 sq ft | Large bedroom / studio | 8,000 BTU | 0.67 ton | 8,000 BTU window AC | 9,000 BTU mini split |
| 350–450 sq ft | Studio apartment | 10,000 BTU | 0.83 ton | 10,000 BTU window AC | 9,000 BTU mini split |
| 450–550 sq ft | 1-bedroom apartment | 12,000 BTU | 1 ton | 12,000 BTU window AC | 12,000 BTU mini split |
| 550–700 sq ft | Large 1-BR / 2-BR apartment | 14,000 BTU | 1.17 tons | 14,000 BTU window AC | 12,000–15,000 BTU mini split |
| 700–1,000 sq ft | 2-bedroom home | 18,000 BTU | 1.5 tons | 18,000 BTU window AC | 18,000 BTU (1.5 ton) mini split |
| 1,000–1,200 sq ft | 2–3 bedroom home | 21,000 BTU | 1.75 tons | 21,000–24,000 BTU | 2-ton central AC |
| 1,200–1,500 sq ft | 3-bedroom home | 24,000 BTU | 2 tons | Multiple window units | 2-ton central AC |
| 1,500–2,000 sq ft | 3–4 bedroom home | 30,000 BTU | 2.5 tons | Multiple window units | 2.5-ton central AC |
| 2,000–2,500 sq ft | Large 4-bedroom home | 36,000 BTU | 3 tons | Multiple units | 3-ton central AC |
| 2,500–3,500 sq ft | Large home / open plan | 48,000 BTU | 4 tons | Multiple units | 4-ton central AC |
* Baseline: 20 BTU/sq ft at moderate climate, average insulation. Adjust by 20–40% for hot climates or poor insulation. Full BTU calculation guide →
AC BTU Calculator — FAQ
Answers to the most common air conditioner BTU questions.
What is the BTU formula for air conditioners?
Basic formula: BTU = Room sq ft × 20 × Climate Factor × Insulation Factor
Key adjustments on top of the base:
- Ceiling above 8 ft: multiply base by (height ÷ 8)
- Sunny room: add 10% — shaded room: subtract 10%
- Each occupant beyond 2: add 600 BTU
- Kitchen: add 4,000 BTU
- Top floor / under roof: add 15%
See the full BTU calculation guide for worked examples.
How many BTU do I need for a 12×12 room?
A 12×12 room (144 sq ft) needs 5,000–6,000 BTU for cooling.
- Moderate climate, average insulation: 5,000–5,500 BTU
- Hot climate (FL, TX, AZ): 6,000–7,000 BTU
- Sunny room or poor insulation: +10–15%
- Basement / shaded: 4,500–5,000 BTU
A 5,000 BTU window AC is the standard recommendation for a 12×12 bedroom.
What size window air conditioner do I need for 300 sq ft?
A 300 sq ft room needs approximately 7,000–8,000 BTU for a window air conditioner.
- Moderate climate, average insulation: 7,000 BTU
- Hot climate or sunny room: 8,000–9,000 BTU
- Poor insulation or top floor: 9,000–10,000 BTU
Standard 8,000 BTU window AC units are the best fit for most 300 sq ft rooms.
Is it better to oversize or undersize an air conditioner?
Neither is ideal, but oversizing is the bigger problem.
An oversized AC "short-cycles" — it cools the air temperature quickly but shuts off before dehumidifying properly. This leaves rooms feeling clammy, uses more energy, and causes premature wear.
An undersized AC runs constantly but can't reach setpoint temperature on hot days. Maximum safe oversizing: no more than 10–15% above calculated BTU.
What SEER rating should I look for in a new AC?
As of 2023, the US minimum is SEER2 13 (South/Southwest) or SEER2 14 (North). ENERGY STAR requires SEER2 ≥ 15.
- Window AC: look for EER ≥ 12
- Central AC: SEER2 ≥ 16 recommended
- Mini split: SEER 18–22+ for best efficiency
Higher SEER is worth it in warm climates — the payback period is 5–8 years in areas running AC 6+ months per year.
How many BTU for a 1,500 sq ft house?
A 1,500 sq ft home typically needs 24,000–30,000 BTU (2–2.5 tons) of central air conditioning.
- Cool climate (Pacific NW, NE): 21,000–24,000 BTU
- Moderate climate (Midwest): 24,000–27,000 BTU
- Hot climate (TX, FL, AZ): 28,000–36,000 BTU
For mini split coverage, use multiple zones totalling the same BTU. Use the Mini Split BTU Calculator for multi-zone sizing.
Can a portable AC cool the same as a window AC of the same BTU?
No — portable ACs are less efficient than window units of the same listed BTU.
The DOE's SACC (Seasonally Adjusted Cooling Capacity) rating reflects real-world output. A 10,000 BTU portable AC typically has a SACC of 6,000–7,000 BTU. Size a portable unit 25–35% larger than your calculated BTU when using listed (not SACC) ratings.
For a 300 sq ft room needing 8,000 BTU: choose a 10,000–12,000 BTU portable AC.
How do I calculate BTU for an open floor plan?
For open floor plans, calculate the total square footage of the connected area as one room.
If a living room (400 sq ft) flows into a kitchen (200 sq ft), calculate for 600 sq ft total, then add the +4,000 BTU kitchen adjustment.
For multi-story open plans, calculate each floor separately — heat rises, so upper floors need more cooling BTU.
Related BTU Calculators
More tools to complete your HVAC sizing.