BTU & AC Sizing Calculator

Calculate BTU / AC Size

m
m
m

Room Area

20.0 m²

Recommended BTU

4,500

Tonnage

0.4 ton

Cooling kW

1.3 kW

How This Calculation Works

This calculator uses the industry-standard baseline of 20 BTU per square foot of floor area. It then applies multipliers for ceiling height (rooms taller than 8 ft need proportionally more cooling), sun exposure, and insulation quality.

Each additional occupant beyond two adds 600 BTU, since people generate heat. The final number is rounded to the nearest 500 BTU for practical AC unit selection.

The result is also shown in tons of cooling (1 ton = 12,000 BTU/h) and kilowatts (1 BTU/h = 0.000293 kW), which helps when comparing units across different markets and brands. Keep in mind that this is an estimate — for precise sizing, especially in commercial spaces, an HVAC professional should perform a Manual J load calculation.

Common Mistakes to Avoid

  • Oversizing the AC. A unit that is too large cools the air quickly but doesn't run long enough to remove humidity, leaving the room cold and clammy. Right-sizing is better than going bigger.
  • Ignoring heat sources. Kitchens, server rooms, and rooms with south-facing windows need extra BTU. This calculator adjusts for sun exposure, but large appliances or electronics may require manual adjustment.
  • Forgetting multiple rooms. A single unit sized for one room won't cool the next room. Calculate each room separately or size a central system based on total load.
  • Using floor area for loft-style rooms. Rooms with vaulted ceilings have more air volume. Use the ceiling-height input to account for this — a 12 ft ceiling needs about 50% more BTU than an 8 ft ceiling for the same floor area.

Worked Example

Scenario: A bedroom 4 × 3.5 m (≈ 150 ft²) with standard 2.4 m (8 ft) ceilings, average insulation, moderate sun, and 2 occupants.

Step 1 — Base BTU: 150 ft² × 20 = 3,000 BTU.

Step 2 — Ceiling adjustment: 8 ft ceiling — no adjustment needed.

Step 3 — Sun/insulation: Medium sun (×1.0) and average insulation (×1.0) — no change.

Step 4 — Occupants: 2 occupants (baseline) — no extra BTU.

Result: 3,000 BTU. A 5,000 BTU window unit (the smallest commonly sold) would be appropriate with room to spare.

Frequently Asked Questions

What does BTU stand for?
BTU stands for British Thermal Unit. It measures the amount of heat energy needed to raise the temperature of one pound of water by one degree Fahrenheit. In AC terms, BTU/h tells you how much heat the unit can remove from a room per hour.
How many BTU do I need per square metre?
A rough guideline is 200–250 BTU per square metre for standard rooms. This is equivalent to the 20 BTU per square foot rule. Adjust upward for high ceilings, sunny rooms, or poor insulation.
Is a higher BTU always better?
No. An oversized unit short-cycles — it cools the air too fast, shuts off, then restarts frequently. This wastes energy, wears out the compressor, and fails to dehumidify the room properly. Match the BTU to your room size.
What is the difference between BTU and tons of cooling?
One ton of cooling equals 12,000 BTU/h. Residential central AC units are typically 1.5 to 5 tons. Portable and window units are rated in BTU. The conversion is simple: divide BTU by 12,000 to get tons.
Do I need a professional to size my AC?
For a single room or window unit, a calculator like this one provides a good estimate. For whole-house central air, a Manual J load calculation performed by an HVAC professional is recommended. It accounts for ductwork, home orientation, insulation R-values, and local climate data.