AC Size Calculator — What Size Air Conditioner Does Your Room Need?
Enter your room's dimensions and conditions to get the right air conditioner size in BTU/hr, tons, kW and PK — with oversize / undersize warnings, in metric or imperial units.
Room details
Start with a preset or enter your own measurements.
Rooms with ceilings above 3.5 m get a 5% stratification penalty.
+600 BTU/hr per person beyond the first two.
Computers, servers, fridges… converted at 3.412 BTU/hr per watt.
Recommended size
—
BTU/hr cooling capacity (rounded up)
— tons
— kW
— PK
Recommended standard unit
- Buy this standard size
- —
- In tons / kW / PK
- —
How the number was calculated
| Step | Value |
|---|
Rounded up to the nearest 500 BTU/hr, then matched to the smallest standard unit size. An undersized unit is the worse failure — it runs non-stop and never reaches the setpoint.
How the math works
Transparent, published rule-of-thumb model — every number above comes from this formula:
BTU/hr = volume(m³) × climate factor × adjustments
+ 600 × max(0, people − 2)
+ 4,000 (kitchen) + watts × 3.412
Climate factors (BTU/hr per m³)
| Zone | Factor | Example: 54 m³ room |
|---|---|---|
| Cool / temperate | 90 | 4,860 → 5,000 BTU/hr |
| Moderate | 115 | 6,210 → 6,500 BTU/hr |
| Warm | 135 | 7,290 → 7,500 BTU/hr |
| Tropical (hot & humid) | 160 | 8,640 → 9,000 BTU/hr |
| Desert (extreme heat) | 200 | 10,800 → 11,000 BTU/hr |
The cool-zone factor (≈90) is anchored to the published ENERGY STAR rule of 20 BTU/hr per ft² at an 8 ft ceiling (20 ÷ 0.2265 m³ = 88.3). The tropical factor (160) matches Southeast Asian practice: 1 PK (9,000 BTU/hr) per ±18 m² bedroom with a 3 m ceiling.
Adjustment multipliers
| Factor | Options |
|---|---|
| Insulation | Poor ×1.20 · Average ×1.10 · Good ×1.00 · Excellent ×0.92 |
| Sun exposure | Heavy ×1.12 · Partial ×1.00 · Shaded ×0.92 |
| Windows | Large single-glazed ×1.10 · Standard ×1.00 · Small/double-glazed ×0.95 |
| Ceiling > 3.5 m | ×1.05 stratification penalty (volume already counts the extra air) |
Worked examples
| Room | Conditions | Result |
|---|---|---|
| Bedroom 4×5×2.7 m | Tropical, good insulation | 9,000 BTU/hr (1 PK) |
| Bedroom 4×5×2.7 m | Desert climate | 12,000 BTU/hr (1½ PK) |
| Bedroom 4×5×2.7 m | Cool climate | 5,000 BTU/hr (½ PK) |
| Loft 4×5×4 m | Tropical, high ceiling | 14,000 BTU/hr rec. |
| Bedroom 12×15×8 ft | Moderate, average insulation | 6,000 BTU/hr rec. |
| Kitchen 5×6×3 m | Tropical, 4 people, 300 W appliances | 30,000 BTU/hr rec. |
Examples computed with the same engine as the calculator above — change any input and re-run to verify.
Frequently asked questions
How many BTU do I need for a 4×5 meter bedroom?
For a 4×5 m bedroom with a 2.7 m ceiling (54 m³) in a tropical climate, the base cooling load is 54 × 160 = 8,640 BTU/hr, which rounds up to a 9,000 BTU/hr unit — that's 1 PK, 0.75 ton, or about 2.64 kW of cooling. This is the classic Indonesian rule of thumb: one 1-PK AC per ±18 m² bedroom. If the room has poor insulation, heavy sun, or more than two occupants, the calculator above will push the recommendation to the next standard size.
What is the rule of thumb for air conditioner size per square foot?
The widely used HVAC rule of thumb is 20 BTU/hr of cooling per square foot of living space, assuming a standard 8 ft ceiling — so a 300 sq ft room needs about 6,000 BTU/hr as a starting point. In metric terms that is roughly 215 BTU/hr per m² of floor area. This rule assumes average insulation and moderate climate; add 10–15% for heavy sun exposure, 600 BTU/hr per extra person beyond two, and 4,000 BTU/hr for a kitchen (per ENERGY STAR guidance).
How do you calculate BTU from room volume?
Multiply the room's volume by a climate factor: BTU/hr = length × width × height (in m³) × climate factor, then apply adjustment multipliers. The climate factors used here are 90 (cool/temperate), 115 (moderate), 135 (warm), 160 (tropical), and 200 (desert) BTU/hr per m³. Example: a 54 m³ room in a tropical climate gives 54 × 160 = 8,640 BTU/hr before adjustments for insulation, sun, windows, and occupants.
How does climate change the required air conditioner size?
A lot — the same 4×5×2.7 m room (54 m³) needs 4,860 BTU/hr (rounded to 5,000) in a cool temperate climate but 10,800 BTU/hr (rounded to 11,000) in a desert climate, more than double. That is why climate presets matter: a calculator that assumes temperate weather will undersize an AC for Jakarta, Bangkok, or Riyadh by 40–70%. Pick the climate zone closest to your hottest month.
Do I need extra BTU for each person in the room?
Yes. Each person gives off roughly 600 BTU/hr of body heat, so add 600 BTU/hr for every occupant beyond the first two (ENERGY STAR guidance). Example: a 6×5×2.7 m tropical room with 6 people needs 15,360 BTU/hr, which rounds to 15,500 — about 2,400 BTU/hr more than the same room with 2 people. For living rooms, offices, and classrooms, occupant count is often the biggest adjustment.
Does a kitchen need a bigger air conditioner?
Yes — add 4,000 BTU/hr for a kitchen because ovens, stoves, and fridges dump heat into the room (ENERGY STAR guidance). Example: a 5×6×3 m tropical kitchen/living area with 4 people and 300 W of appliances needs 25,739 BTU/hr, rounded to 26,000, so the recommended standard size is 30,000 BTU/hr (≈ 3.3 PK). Never size a kitchen with the bedroom rule alone.
Does ceiling height affect what size AC I need?
Yes, because there is more air to cool — that is why this calculator uses room volume (length × width × height) instead of floor area alone. Rooms with ceilings above 3.5 m (about 11.5 ft) also get a 5% stratification penalty since hot air pools near the ceiling. Example: a 4×5 m room with a 4 m ceiling needs 13,440 BTU/hr (rounded to 13,500) versus 8,640 BTU/hr with a 2.7 m ceiling — nearly 56% more.
What size air conditioner do I need for a 12×15 ft bedroom?
A 12×15 ft bedroom with an 8 ft ceiling is 1,440 ft³ (40.78 m³). In a moderate climate with average insulation, the load is 5,158 BTU/hr, which rounds up to 5,500 — so a 6,000 BTU/hr unit is the right standard size. The old 20-BTU-per-square-foot rule would give 3,600 BTU/hr here, which ignores ceiling height and insulation; the volume-based result is the safer pick.
How many tons and kW is 12,000 BTU/hr?
12,000 BTU/hr equals exactly 1 ton of refrigeration and 3.52 kW of cooling capacity (12,000 ÷ 3,412.14). Likewise 24,000 BTU/hr is 2 tons or 7.03 kW, and 9,000 BTU/hr is 0.75 ton. Note these are cooling capacities, not electricity consumption — a 12,000 BTU/hr inverter unit typically draws only 1.0–1.2 kW of electrical power. For pure unit conversion, see our ton to kW converter.
What does PK mean on an air conditioner?
PK (paardekracht, Dutch for horsepower) is the standard AC size unit in Indonesia and much of Southeast Asia: 1 PK = 9,000 BTU/hr. The common ladder is ½ PK (5,000 BTU/hr), ¾ PK (7,000), 1 PK (9,000), 1½ PK (12,000), 2 PK (18,000), and 2½ PK (24,000). So when this calculator recommends 18,000 BTU/hr, shop for a 2 PK unit.
What happens if my air conditioner is oversized?
An oversized AC cools the air so fast that it shuts off before running long enough to dehumidify — leaving the room cold and clammy — and the compressor wears out from constant short-cycling. This calculator flags any unit more than 30% above your calculated load as oversized. Bigger is not safer; the right size runs in longer, steadier cycles that actually remove moisture.
What happens if my air conditioner is undersized?
An undersized unit runs non-stop on hot days, never reaches the thermostat setpoint, and still leaves you uncomfortable while consuming maximum electricity. It also wears out faster from continuous operation. If your calculated load is, say, 8,640 BTU/hr, a 7,000 BTU/hr unit will struggle — round up to the 9,000 BTU/hr standard size instead. When in doubt between two sizes, pick the larger one.
Is this calculator a substitute for a professional Manual J load calculation?
No. This is a planning estimate based on published rules of thumb (volume × climate factor plus standard adjustments) — accurate enough to shortlist unit sizes, but not a replacement for an ACCA Manual J calculation by an HVAC professional, which accounts for wall construction, orientation, duct losses, and exact window specs. Always confirm the final size with a technician before buying a large or multi-zone system. For the money side of the decision — repairing your old unit versus replacing it — see our HVAC repair vs replace calculator.
Related tools
Boundary note: this tool sizes an air conditioner from room dimensions.
For pure unit conversion (tons ↔ kW ↔ BTU/hr) use the ton–kW converter; for the
repair or replace money decision on an existing unit, use the HVAC economics tool.
Free tool by NRTechStudio