BTU / Air Conditioner Size Calculator
Correctly sizing an air conditioner prevents two common and costly mistakes: undersizing (unit runs constantly without reaching target temperature) and oversizing (unit short-cycles, failing to dehumidify and wearing out faster). This calculator uses a simplified ACCA Manual J method — the industry standard for residential load calculations. Enter room area, ceiling height, sun exposure, insulation quality, climate zone, and occupant count to get calculated BTU load, rounded up to the nearest standard unit size, tonnage, and an estimated unit cost range. The base load starts at 20 BTU/sq ft and is adjusted up or down for each factor. For a full whole-house Manual J calculation prior to HVAC replacement, consult a licensed HVAC contractor — the simplified method is accurate enough for unit selection but not for ductwork design.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
How many BTUs do I need for a 500 sq ft room?
A 500 sq ft room in average conditions with 8-ft ceilings needs approximately 10,000 BTUs. In a hot, sunny climate with poor insulation, you may need up to 14,000 BTUs. A 12,000 BTU (1-ton) window unit is a common choice.
Is it better to size up on AC?
No — an oversized AC cools the space quickly without dehumidifying adequately, leading to a clammy feeling and higher energy bills. Match the BTU to the actual load as closely as possible.
What size central AC do I need for a 2,000 sq ft home?
A 2,000 sq ft home in a moderate climate with average insulation typically needs a 3–4 ton (36,000–48,000 BTU) central air conditioner. In hot and humid climates (Florida, Gulf Coast) or poorly insulated older homes, size up to 4–5 tons. In well-insulated homes in mild climates, 2.5–3 tons may be sufficient.
What is the difference between BTU and tons for AC?
One ton of air conditioning equals 12,000 BTU per hour — the amount of heat required to melt one ton of ice in 24 hours. Central AC systems are rated in tons (1.5T, 2T, 3T, 4T, 5T), while window units and mini-splits are rated in BTU. A 3-ton central AC = 36,000 BTU.