Every house has one room that never cools down: the bedroom over the garage, the office with the west window, the kitchen at dinnertime. Whether you're buying a window unit, a portable or a mini split head for it, the BTU rating decides whether it keeps up.
Too small, and it runs nonstop on hot afternoons. Too big, and it cools the room before it has pulled out the humidity, which leaves the air cold and clammy.
This calculator sizes a single room. For a whole house on central air, use the AC tonnage calculator instead.
ENERGY STAR's sizing chart puts a 5,000 BTU unit in a 100 to 150 sq ft room, 12,000 BTU in 450 to 550 sq ft, and 18,000 BTU in 700 to 1,000 sq ft, before adjustments for sun, people and cooking. The calculator below makes the same estimate with more inputs. At its default settings, a 15 × 12 ft living room in a mixed climate needs about 5,400 BTU per hour, so a 6,000 BTU unit.
BTU Calculator
Estimated cooling capacity for any room.
Share This Calculator
Help others save money on HVAC costs by sharing this free calculator tool.
Embed This Calculator
Add this calculator to your website, blog, or social media. Free to use with attribution.
ENERGY STAR's room sizing chart
ENERGY STAR publishes this chart for room air conditioners:
| Room size | Cooling capacity |
|---|---|
| 100 to 150 sq ft | 5,000 BTU/hr |
| 150 to 250 sq ft | 6,000 BTU/hr |
| 250 to 300 sq ft | 7,000 BTU/hr |
| 300 to 350 sq ft | 8,000 BTU/hr |
| 350 to 400 sq ft | 9,000 BTU/hr |
| 400 to 450 sq ft | 10,000 BTU/hr |
| 450 to 550 sq ft | 12,000 BTU/hr |
| 550 to 700 sq ft | 14,000 BTU/hr |
| 700 to 1,000 sq ft | 18,000 BTU/hr |
Its four adjustments:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- More than two people regularly: add 600 BTU for each additional person.
- Kitchen: add 4,000 BTU.
Above 1,000 sq ft you're sizing a whole floor or house, which is central-air territory: see the tonnage calculator.
How the calculator works
The calculator starts from a cooling rate per square foot for your climate, then multiplies by the room's characteristics:
| Input | Values |
|---|---|
| Climate (BTU per sq ft) | Very hot 32 · Hot-humid 30 · Hot-dry 28 · Mixed-humid 22 · Mixed-dry 20 · Cool 18 · Cold 16 · Very cold 14 |
| Room type | Bedroom 1.0 · Kitchen 1.0 · Bathroom 0.9 · Basement 0.8 · Living room 1.1 · Home office 1.2 · Garage 1.3 · Sunroom 1.5 |
| Windows | Single pane 1.3 · Double pane 1.0 · Low-E 0.9 · Triple pane 0.8 |
| Window area | +3% for every 10% of floor area that's glass |
| Insulation | Poor 1.3 · Average 1.0 · Good 0.9 · Excellent 0.85 |
| Sun | Heavy shade 0.9 · Moderate 1.0 · Direct sun 1.1 · All-day south/southwest sun 1.15 |
| Ceiling height | Height ÷ 8 ft (9 ft = 1.125, 10 ft = 1.25) |
Then it adds three flat amounts that don't depend on the room's size or insulation:
- 4,000 BTU for a kitchen, for the heat from cooking (ENERGY STAR's figure).
- 600 BTU for each person beyond two, matching ENERGY STAR's chart.
- 400 BTU for each appliance you list, such as a TV, computer or game console.
It rounds the total up to the next common size: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 24,000, 30,000 or 36,000 BTU. Above 36,000, one room unit won't cover it.
Three worked examples
1. The default: a 15 × 12 ft living room in a mixed-humid climate, double-pane windows totaling 30 sq ft, average insulation, moderate sun, 8-ft ceiling, two people, two appliances:
- 180 sq ft × 22 = 3,960 BTU/hr
- × 1.1 (living room) × 1.05 (window area) = 4,574
- Plus 800 for two appliances = 5,374 BTU/hr → a 6,000 BTU unit
2. A 12 × 10 ft home office in a hot-dry climate, 20 sq ft of west-facing glass in direct sun, one person, three appliances:
- 120 sq ft × 28 = 3,360 BTU/hr
- × 1.2 (office) × 1.05 (window area) × 1.1 (direct sun) = 4,657
- Plus 1,200 for three appliances = 5,857 BTU/hr → a 6,000 BTU unit
3. A 14 × 12 ft kitchen in a hot-humid climate, 15 sq ft of windows, three people, two appliances:
- 168 sq ft × 30 = 5,040 BTU/hr
- × 1.027 (window area) = 5,175
- Plus 4,000 for cooking, 600 for the third person and 800 for appliances = 10,575 BTU/hr → a 12,000 BTU unit
The kitchen shows why room type matters: it's 40% bigger than the office but needs twice the capacity, mostly because of the 4,000 BTU cooking allowance.
What size AC for a 500 sq ft room?
12,000 BTU, per ENERGY STAR's chart (450 to 550 sq ft). The calculator agrees for a quiet room: a 25 × 20 ft bedroom with 40 sq ft of windows and no appliances comes to 11,264 BTU/hr, a 12,000 BTU unit. Enter the same room as a living room with a TV and a game console and it comes to 13,190 BTU/hr, a 14,000 BTU unit, because of the living-room factor and the 400 BTU per appliance.
How big a room does a 5,000 BTU air conditioner cool?
100 to 150 sq ft, per ENERGY STAR. In the calculator, a 12.5 × 12 ft (150 sq ft) bedroom in a mixed-humid climate with 15 sq ft of windows and two devices needs 4,199 BTU/hr, so a 5,000 BTU unit fits.
A 15 × 12 ft bedroom still comes in at 4,879; much bigger and it steps up to 6,000. A hot climate, direct sun or a crowded room shrinks the area a 5,000 BTU unit can handle.
How many BTU per square foot do you need?
It depends on room size as much as climate. ENERGY STAR's chart works out to:
- 33 to 50 BTU per sq ft for small rooms (100 to 150 sq ft), because people, lights and electronics add the same heat to a small room as to a large one.
- About 22 to 27 BTU per sq ft for mid-size rooms (350 to 550 sq ft).
- 18 to 26 BTU per sq ft for large rooms (700 to 1,000 sq ft).
The calculator's climate rates run from 14 BTU per sq ft in very cold climates to 32 in very hot ones, before the room, window, insulation, sun and height adjustments. For a whole house, the tonnage calculator starts from 20 BTU per sq ft and adjusts by climate.
What the calculator can't see
A few conditions push a room's real load above the estimate:
- A room on the top floor, directly under a hot attic.
- High indoor humidity, which the air conditioner has to remove on top of cooling.
- Drafts and air leaks around old windows and doors.
- An open floor plan, where the room shares air with the spaces around it.
If two or more apply, size toward the high end of your result. For whole-house equipment, a contractor's Manual J load calculation measures these properly.
Which type of air conditioner?
- One room: a window, through-the-wall or portable unit, or a single mini split head, sized with this calculator. Portable units need the same BTU rating but lose some of it to how they vent; see single-hose vs. dual-hose portable ACs.
- Several rooms with separate heads: the mini split sizing calculator sizes each head.
- The whole house through ducts: the tonnage calculator. Central systems are sold in tons; one ton is 12,000 BTU per hour.
For how the types compare, see types of air conditioners.
Frequently asked questions
How many BTU do I need per square foot?
For a mid-size room, about 22 to 27 BTU per square foot; small rooms need more per square foot, and large rooms slightly less. ENERGY STAR's chart puts 12,000 BTU in a 450 to 550 sq ft room. Hot climates, sunny rooms and kitchens need more.
What size AC do I need for 500 square feet?
About 12,000 BTU, per ENERGY STAR. A living room with electronics or a sunny exposure may need 14,000.
How big a room will a 5,000 BTU air conditioner cool?
About 100 to 150 square feet, per ENERGY STAR. In a mild climate with modest windows, a 5,000 BTU unit can stretch to a slightly larger bedroom.
Is a bigger air conditioner better?
No. ENERGY STAR notes that an oversized unit cools the room before it removes the humidity, leaving it damp and clammy, and it's less efficient. Size to the room.
Can I use this calculator for my whole house?
It's built for single rooms up to 36,000 BTU. For whole-house central air, use the AC tonnage calculator, and ask your contractor for a Manual J load calculation before you buy.
Sources & References
Related Articles
How Many Mini Splits Do I Need? (Rooms, Zones, BTUs) 2026
data-guide • 8 min read
Water Heater Sizing Calculator: Tank vs Tankless Sizing (2026)
calculator-guide • 6 min read
AC Tonnage Calculator Per Square Foot (+Sizing Chart) 2026
calculator-guide • 12 min read
Furnace Size Calculator: What Size Furnace Do I Need? (2026)
calculator-guide • 10 min read