AC Size Calculator
Find the right air conditioner size for your room. Enter square footage, ceiling height, and sun exposure to get BTU and tonnage.
Last updated 2026-08-21
What Is a AC Size Calculator?
An AC size calculator helps you pick the right air conditioner size in BTU and tons for a room or small home. You enter the area, ceiling height, sun exposure, and insulation. Right-sizing avoids short cycling and poor dehumidification, which are common problems with oversized units.
How It Works
BTU = Area × 20 × (Ceiling ÷ 8) × Sun factor × Insulation factor. Tons = BTU ÷ 12,000.
Example
500 sq ft with strong sun and poor insulation: 500 × 20 × 1.2 × 1.15 = 13,800 BTU ≈ 1.2 tons.
How to Calculate
- Enter the room or home area in square feet. For 500 square feet, the base load starts at 20 BTU per square foot.
- Set the ceiling height, which raises the load for taller spaces.
- Choose sun exposure and insulation levels. Strong sun and poor insulation multiply the base load.
- For 500 square feet with strong sun and poor insulation, the tool returns about 13,800 BTU, roughly 1.2 tons.
- Divide BTU by 12,000 to get tons, since one ton equals 12,000 BTU per hour.
AC Tonnage by Area (rough guide)
| Area (sq ft) | BTU | Tonnage |
|---|---|---|
| 100 to 150 | 5,000 | 0.5 |
| 250 to 350 | 7,000 to 9,000 | 0.75 |
| 400 to 550 | 10,000 to 14,000 | 1.0 to 1.2 |
| 600 to 900 | 18,000 | 1.5 |
| 1,000 to 1,400 | 24,000 | 2.0 |
Sources
Frequently Asked Questions
What does "ton" mean in AC?
One ton equals 12,000 BTU/hour — the cooling capacity of one ton of ice melting per day.
What size AC do I need?
A quick rule is 1 ton per ~500–600 sq ft of well-insulated space; this calculator refines that.
Is bigger better?
No — an oversized unit short-cycles and fails to dehumidify. Size to the actual load.
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