BTU Calculator
Find the right AC or heater size for any room. Enter your dimensions and conditions to get the recommended BTU and the nearest standard unit size.
How It Works
The BTU calculation starts with a base load and then applies adjustment factors for room conditions:
Base BTU = Room area (ft²) × 25 BTU/ft²
This base figure (25 BTU/ft²) is the industry standard for a standard room at standard ceiling height with average insulation.
Adjustment factors:
1. Insulation quality
- Poor insulation (old or uninsulated walls): × 1.18
- Average insulation: × 1.00
- Good insulation (modern construction, double glazing): × 0.85
2. Sun exposure
- High sun (south/west facing, lots of glass): × 1.10
- Medium sun: × 1.00
- Low sun (north/east facing, shaded): × 0.85
3. Ceiling height (above 8 ft standard)
- Adjusted BTU = BTU × (ceiling height ÷ 8)
4. Occupancy (above 2 people standard)
- Add 600 BTU per extra person
5. Kitchen heat load
- Kitchens generate significant heat from cooking: × 1.50 additional multiplier applied after base calculation
Worked example: 15 ft × 12 ft room = 180 ft². Base BTU = 4,500. Good insulation (×0.85) = 3,825. High sun (×1.10) = 4,208. 10 ft ceiling (×10/8) = 5,260. 3 occupants (+600) = 5,860 BTU. Recommended unit: 6,000 BTU.
Standard AC sizes: 5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 14,000 / 18,000 / 24,000 BTU.
Frequently Asked Questions
What happens if I choose an AC that is too big or too small?
An oversized AC cools the room too quickly without running long enough to remove humidity, leaving the air cold but clammy. It also cycles on and off frequently, which increases wear and energy consumption. An undersized unit runs constantly without reaching the target temperature, wastes energy, and shortens the compressor's life. The right size is important for both comfort and efficiency.
What is the difference between BTU and tons?
1 ton of cooling capacity = 12,000 BTU/hr. The term "ton" comes from the amount of heat needed to melt one ton of ice in 24 hours. A typical window AC is 0.5–2 tons; central air systems are 1.5–5 tons. Divide BTU by 12,000 to get tons.
Should I get a window AC or a mini-split?
Window ACs are cheaper upfront and easier to install but are noisier, block your window, and are less efficient. Mini-splits (ductless systems) are significantly quieter, more efficient (higher SEER ratings), and can both cool and heat. They require professional installation but are the better long-term investment for rooms you use frequently.
Can I use one AC unit for multiple rooms?
A single window or portable unit is only effective in the room it is in and immediately adjacent open areas. For multiple rooms, you need either a central HVAC system (ducted) or a multi-zone mini-split system with separate indoor units per room. Add the BTU requirements of all rooms you want to condition together.
Does humidity affect BTU requirements?
Yes. In high-humidity climates, you need extra dehumidification capacity. The 25 BTU/ft² base formula already assumes an average climate. If you are in a tropical or very humid area, increase your BTU estimate by 10–15 %. Air conditioners dehumidify as they cool, so a correctly sized unit in a humid climate is especially important.