BTU Calculator
Find the right air conditioner size in BTUs for a room, using the ENERGY STAR sizing chart with adjustments for sun, people and kitchens.
Results
| Item | Value |
|---|
Quick Answer
Air conditioner size depends mainly on room area. ENERGY STAR suggests 6,000 BTU for 150 to 250 sq ft, so a 12 × 15 ft bedroom (180 sq ft) needs about 6,000 BTU. Add 10% for a very sunny room, 600 BTU for each person beyond two, and 4,000 BTU for a kitchen.
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Table of Contents
- BTU Calculator
- Quick Answer
- Related Calculators
- How This BTU Calculator Works
- How to Use the BTU Calculator
- ENERGY STAR Sizing Chart
- Key Terms
- Worked Example
- Real-Life Examples
- BTU vs Tons vs Watts
- Other Things That Affect Size
- Common Mistakes to Avoid
- Frequently Asked Questions
- References & Sources
How This BTU Calculator Works
Enter the room size and a few details. The calculator finds the floor area, looks up the base cooling capacity in the ENERGY STAR chart, then adjusts it for sun, extra people and kitchen heat.
Quick explanation: BTU = chart size for your area × sun adjustment + 600 per extra person + 4,000 for a kitchen.
- BTU: British thermal unit, a measure of heat removed per hour.
- Right size matters: too small struggles, too big cycles on and off and leaves the room damp.
- Tons: 1 ton of cooling is 12,000 BTU per hour.
How to Use the BTU Calculator
- Measure the room length and width in feet.
- Choose how much sun the room gets.
- Enter how many people usually use the room.
- Choose whether the room is a kitchen.
- Press Calculate to see the recommended BTU size and the breakdown.
ENERGY STAR Sizing Chart
| Area to cool (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 |
- Heavily shaded room: reduce by 10%
- Very sunny room: increase by 10%
- More than 2 people: add 600 BTU per extra person
- Kitchen: add 4,000 BTU
Key Terms
- BTU: British thermal unit, the heat needed to raise 1 lb of water by 1°F.
- BTU per hour: how much heat an AC unit removes each hour.
- Ton of cooling: 12,000 BTU per hour.
- Load calculation: a detailed sizing method, such as Manual J, used by HVAC pros.
Worked Example
A sunny 20 × 18 ft living room (360 sq ft) used by 4 people.
- Base from chart (350 up to 400 sq ft) = 9,000 BTU
- Very sunny: 9,000 × 1.10 = 9,900 BTU
- 2 extra people: + 1,200 BTU
- Total = 11,100 BTU, so choose about 11,000 to 12,000 BTU
Real-Life Examples
- A small bedroom: 10 × 12 ft (120 sq ft) needs about 5,000 BTU.
- A home office in the sun: 12 × 14 ft (168 sq ft), very sunny, needs about 6,600 BTU.
- A kitchen: 12 × 16 ft (192 sq ft) needs about 10,000 BTU, because cooking adds heat.
- A large family room: 25 × 20 ft (500 sq ft) with 5 people needs about 13,800 BTU.
BTU vs Tons vs Watts
- BTU per hour: the common size for room AC units in the US.
- Tons: used for central AC. 1 ton = 12,000 BTU per hour.
- Watts of cooling: 1 BTU per hour ≈ 0.293 watts of cooling.
- Power use: the electricity a unit uses is lower than its cooling power, and depends on its efficiency rating.
Other Things That Affect Size
- Ceiling height: the chart assumes 8 ft ceilings.
- Insulation and windows: older homes and large windows need more cooling.
- Climate: very hot regions may need a larger unit.
- Open plans: count connected spaces that are not closed off by doors.
Common Mistakes to Avoid
- Buying too big. An oversized unit cools quickly but leaves the air damp.
- Buying too small. It runs constantly and still may not keep up.
- Forgetting the kitchen heat. Cooking adds a lot of heat.
- Ignoring ceiling height. Tall ceilings mean more air to cool.
- Measuring the whole floor for one window unit. Room units cool one room or open area.
Frequently Asked Questions
144 sq ft is in the 100 to 150 range, so about 5,000 BTU.
Roughly 20 to 40, depending on room size. Smaller rooms need more per square foot in the chart.
About 12,000 BTU, before adjustments.
No. A unit that is too large cycles on and off often and does not remove enough humidity.
12,000 BTU per hour.
This chart is for cooling. Heating needs depend more on climate and insulation.
Window units are usually more efficient for the same BTU rating. Portable units are easier to move but often cool less effectively.
Only if the rooms are open to each other with good airflow. Add both areas together, and size for the combined space.
The Combined Energy Efficiency Ratio shows how efficiently a room AC turns electricity into cooling. A higher number uses less power.
Yes. The chart assumes 8 ft ceilings, so taller rooms may need a larger unit.
References & Sources
| Source | Used For | Link |
|---|---|---|
| ENERGY STAR, Room air conditioners | The sizing chart and adjustments used here. | https://www.energystar.gov/products/room_air_conditioners |
| Wikipedia, British thermal unit | Definition of the BTU. | https://en.wikipedia.org/wiki/British_thermal_unit |