A BTU calculator estimates the heating or cooling capacity needed for a room — in British Thermal Units per hour (BTU/h), the standard rating unit for AC units and furnaces in the US. The Toolenza calculator factors in room area, ceiling height, sun exposure, climate, and occupancy.

Why getting the size right matters

• Oversized AC — cycles on and off too quickly. Cools the air without removing humidity, leaving the room cold and clammy. Wastes energy, shortens compressor life.
• Undersized AC — runs continuously, never reaches setpoint, fails on the hottest day of the year.
• Right-sized AC — long, even cycles. Removes humidity. Reaches setpoint and rests. Lasts 15+ years.

The rule of thumb

Residential AC sizing uses 20 BTU/h per square foot of conditioned area in a typical climate, plus modifiers:

• Sunny / west-facing room — add 10%.
• Shaded / east- or north-facing — subtract 10%.
• Kitchen — add 4,000 BTU/h (heat from cooking and appliances).
• Each additional person past 2 — add 600 BTU/h.
• High ceilings (> 8 ft) — multiply by ceiling-height / 8.
• Hot climate (Arizona, Texas) — multiply by 1.1–1.2.

Typical sizings

• 150 sq ft (small bedroom) — 5,000–6,000 BTU/h (a window unit).
• 300 sq ft (master bedroom) — 8,000–10,000 BTU/h.
• 500 sq ft (open living area) — 12,000–14,000 BTU/h.
• 1,000 sq ft (small house) — 24,000–30,000 BTU/h (a mini-split or central system).
• 2,000 sq ft (typical 3-bedroom) — 36,000–48,000 BTU/h, often called a "3-ton" or "4-ton" central AC.

A "ton" of cooling is 12,000 BTU/h — derived from the cooling capacity needed to melt one ton of ice in 24 hours.

Pitfall

Rule-of-thumb sizing is a starting point. For a new build or major renovation, hire an HVAC contractor to run a Manual J load calculation — accounts for insulation, window quality, infiltration, and orientation. The difference between rule-of-thumb and Manual J can be a full size class.