AC BTU & Ton
Sizing Calculator

How AC Sizing Works

Air conditioner sizing is measured in tons or BTUs (British Thermal Units). One ton equals 12,000 BTU/hour of cooling capacity. Standard residential sizes range from 1.5 to 5 tons.

The Rule of Thumb (and Why It's Misleading)

The common 20-BTU-per-square-foot rule gives a rough estimate but ignores critical factors. A 2,000 sq ft home in Phoenix needs significantly more cooling than the same home in Seattle. This calculator accounts for climate zone, ceiling height, insulation quality, sun exposure, and occupancy to refine that baseline.

Why Right-Sizing Matters

An oversized AC short-cycles — it cools the air too quickly without running long enough to dehumidify. The result is a cold, clammy home with high humidity and high energy bills. An undersized unit runs constantly, never reaches setpoint on the hottest days, and wears out years early. Manufacturers' warranties often exclude failures caused by improper sizing.

When to Get a Manual J

For new construction, additions, major renovations, or whenever you're uncertain, hire an HVAC contractor to do a full Manual J load calculation. Manual J is ACCA's industry-standard methodology — it considers every window's U-value and solar heat gain, every wall's R-value, infiltration rates, internal heat sources, and orientation. This calculator gets you close enough for replacement sizing, but Manual J is the gold standard.

Standard AC Sizes

• 1.5 ton (18,000 BTU) — small condo, apartment, or 600-1,000 sq ft
• 2.0 ton (24,000 BTU) — 1,000-1,400 sq ft
• 2.5 ton (30,000 BTU) — 1,400-1,800 sq ft (most common)
• 3.0 ton (36,000 BTU) — 1,800-2,200 sq ft
• 3.5 ton (42,000 BTU) — 2,200-2,600 sq ft
• 4.0 ton (48,000 BTU) — 2,600-3,200 sq ft
• 5.0 ton (60,000 BTU) — 3,200-4,000 sq ft

Anything larger than 5 tons typically uses commercial equipment or multiple residential units. Sizes increase in half-ton increments only — you cannot buy a 3.25 ton AC.