How to use the BTU calculator
- Enter the room length and width.
- Choose the sun exposure: average, heavily shaded or very sunny.
- Enter how many people regularly use the room.
- Say whether the room is a kitchen.
The calculator is for sizing a room (window or portable) air conditioner. The ENERGY STAR chart assumes an 8 ft ceiling. Treat the result as a starting point for one room. For a whole house, Decoratoradvice recommends a load calculation from an HVAC contractor.
ENERGY STAR sizing chart
| Area (sq ft) | Capacity (BTU per hour) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
Formula and adjustments
Capacity = chart value × sun adjustment + 600 × (people − 2) + kitchen allowance
ENERGY STAR recommends these changes to the chart value:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people regularly in the room: add 600 BTU per hour for each extra person.
- Kitchen: add 4,000 BTU per hour.
Worked examples
A 20 × 16 ft bedroom, average sun, two people:
Area: 320 sq ft, in the 300–350 row
8,000 BTU per hour (about 2.34 kW of cooling)
The same room, very sunny, four regular occupants, used as a kitchen:
8,000 × 1.10 = 8,800
+ 2 extra people × 600 = 1,200
+ kitchen 4,000
14,000 BTU per hour
Tips
- An oversized unit cools the air quickly but can switch off before it removes much humidity. Sizing to the chart helps comfort.
- Insulation and air sealing reduce the load; see the insulation calculator.
- Estimate running costs with the electricity cost calculator from the unit's wattage.
- For backup power, include the unit's starting watts in the generator size calculator.
Common mistakes
- Sizing a whole house with a room chart. Central systems need a load calculation by an HVAC professional.
- Ignoring ceiling height. The chart assumes 8 ft ceilings.
- Stacking every adjustment without thinking. Use the options that genuinely describe the room.
Frequently asked questions
How many BTU do I need for a 12 × 12 room?
144 sq ft falls in the 100–150 row: 5,000 BTU per hour before adjustments.
How many BTU for 500 sq ft?
12,000 BTU per hour, from the 450–550 row, before adjustments.
Is a bigger air conditioner better?
Not necessarily. A unit that is too large for the room can cycle on and off and leave the air damp.
How do I convert BTU to kW?
Multiply BTU per hour by 0.000293. 8,000 BTU per hour is about 2.34 kW of cooling.
Can I use this for central air?
No. Whole-house systems should be sized with a room-by-room load calculation. Duct sizes come next; see the duct size calculator.
What about rooms over 2,500 sq ft?
They are beyond the room air conditioner chart. Ask an HVAC contractor about multiple units or a central system.
Key takeaways
- Find the floor area's row in the ENERGY STAR chart.
- Adjust ±10% for shade or sun, +600 BTU per extra person, +4,000 for a kitchen.
- The chart assumes 8 ft ceilings and is for room units, not central systems.
- Measure the area with the square footage calculator for odd-shaped rooms.