❄️ BTU Calculator
Find the right air conditioner size in BTU for any room from its square feet, sun exposure, people and kitchen use, based on the ENERGY STAR sizing chart.
| Chart capacity for 210 sq ft | 6,000 BTU |
|---|---|
| Total | 6,000 BTU |
Round up to the next size you can buy. Oversized units cool the air quickly but shut off before removing humidity, leaving the room cold and clammy; undersized units run constantly and never catch up on hot days.
What BTU Calculator Does
This BTU calculator tells you what size room air conditioner you need. It starts from the capacity chart ENERGY STAR publishes for room air conditioners, which matches floor area to cooling capacity, then applies the chart’s adjustments: less for a heavily shaded room, more for a very sunny one, more for every regular occupant beyond two, and a large addition for kitchens. Each adjustment is shown so you can see where the number comes from.
BTU per hour measures how much heat an air conditioner can remove. The result is also converted to watts of cooling and tons, the units used on mini-split and central air equipment. Round up to the nearest size sold — but not far beyond it, because an oversized unit cools unevenly and leaves the room damp.
How to Use BTU Calculator
- Enter the room’s length and width, or its square footage
- Choose how sunny or shaded the room is
- Enter how many people regularly use the room
- Tick the kitchen box if the unit will cool a kitchen
- Read the recommended BTU and round up to an available model
Formula Used by BTU Calculator
Sizing with adjustments
BTU = chart capacity × (1 ± 10% for sun or shade) + 600 × (people − 2) + 4,000 if kitchen
Worked example
A 15 × 14 ft (210 sq ft) sunny bedroom used by 3 people.
- Chart for 150–250 sq ft: 6,000 BTU
- Very sunny: 6,000 × 1.10 = 6,600
- One extra person: + 600
Result: 7,200 BTU — an 8,000 BTU unit is the nearest size up.
Unit conversions
1 ton = 12,000 BTU/h · 1 W ≈ 3.412 BTU/h
Worked example
12,000 BTU/h
- 12,000 ÷ 12,000
- 12,000 ÷ 3.412
Result: 1 ton, or about 3,517 W of cooling.
Room Air Conditioner Sizing Chart
Capacity by floor area before adjustments, as published by ENERGY STAR for room air conditioners.
| Area (sq ft) | Capacity (BTU/h) | Area (sq ft) | Capacity (BTU/h) |
|---|---|---|---|
| 100–150 | 5,000 | 550–700 | 14,000 |
| 150–250 | 6,000 | 700–1,000 | 18,000 |
| 250–300 | 7,000 | 1,000–1,200 | 21,000 |
| 300–350 | 8,000 | 1,200–1,400 | 23,000 |
| 350–400 | 9,000 | 1,400–1,500 | 24,000 |
| 400–450 | 10,000 | 1,500–2,000 | 30,000 |
| 450–550 | 12,000 | 2,000–2,500 | 34,000 |
How to Read Your Result
When the chart is not enough
High ceilings, poor insulation, a top-floor room under a hot roof, large west-facing windows or a hot climate all raise the load. If the room has several of these, lean toward the next size up. For whole-house central air or heat pumps, a contractor should run a Manual J load calculation instead of using square footage alone.
Efficiency matters too
Two units with the same BTU rating can use very different amounts of electricity. Compare the combined energy efficiency ratio (CEER) on the label — higher is better — and check the unit’s rated watts to estimate running cost.
Limitations & Accuracy Notes
- The chart covers rooms from 100 to 2,500 square feet and assumes standard 8-foot ceilings.
- It sizes cooling only; heating needs are calculated differently.
- Climate, insulation and window area are not modeled beyond the sun adjustment.