Window and portable air conditioners are rated in BTU per hour. Energy Star publishes a simple chart that matches capacity to room area for a typical 8 ft ceiling, then suggests a few adjustments: about 10% more for very sunny rooms, 10% less for shaded rooms, 600 BTU/h for each regular occupant beyond two and 4,000 BTU/h if the unit cools a kitchen.
How to use the Air Conditioner BTU Calculator
- Room area
- Ceiling height
- Sun exposure
- Regular occupants
- Room includes a kitchen
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the room area, ceiling height, sun exposure, the number of people who usually use the room and whether it includes a kitchen. The calculator looks up the base capacity and applies each adjustment.
Energy Star sizing chart
| Area (ft²) | BTU/h |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1000 | 18,000 |
| 1000–1200 | 21,000 |
| 1200–1400 | 23,000 |
| 1400–1500 | 24,000 |
| 1500–2000 | 30,000 |
| 2000–2500 | 34,000 |
Tips for an accurate estimate
- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer’s coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.
Common mistakes to avoid
- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.
Inputs explained
| Field | What to enter |
|---|---|
| Room area | Units: ft², m², yd², acre |
| Ceiling height | Units: ft, m, in, yd, cm |
| Sun exposure | Choose one: Normal, Very sunny (+10%), Heavily shaded (−10%) |
| Regular occupants | A number |
| Room includes a kitchen | Choose one: No, Yes |
What each result means
- Recommended capacity = ([Base capacity (Energy Star chart)]×[Ceiling height]÷2.4384×[Sun exposure] + 600×max([Regular occupants] – 2, 0) + 4000×[Room includes a kitchen])
- Tons = [Recommended capacity]÷12000
Formula
Base capacity from the Energy Star chart (5,000 BTU/h for 100–150 ft² … 34,000 for 2,000–2,500 ft²)× ceiling height ÷ 8 ft, × 1.10 sunny or 0.90 shaded+ 600 BTU/h per person beyond two, + 4,000 BTU/h for a kitchenWorked example
Inputs — Room area: 350 ft²; Ceiling height: 8 ft; Sun exposure: Normal; Regular occupants: 2; Room includes a kitchen: No.
Results — Base capacity (Energy Star chart) = 8,000 BTU/h; Recommended capacity = 8,000 BTU/h; Tons = 0.67.
Frequently asked questions
What size air conditioner for a 350 ft² room?
8,000 BTU/h by the Energy Star chart, more for a sunny room or high ceiling.
How many square feet does 12,000 BTU cool?
About 450–550 ft² in typical conditions.
Should I round up?
Choose the nearest standard size; avoid going much bigger, which leaves the room cold and damp.
What formula does the Air Conditioner BTU Calculator use?
Base capacity from the Energy Star chart (5,000 BTU/h for 100–150 ft² … 34,000 for 2,000–2,500 ft²).