AC Size Calculator (BTU & Tons)
Use this tool to estimate the right central air conditioner size for your room or home area.
This is an estimate. For final equipment selection, a professional Manual J load calculation is recommended.
How to know what size AC you need
Choosing the right air conditioner size is one of the biggest decisions for comfort and energy bills. A system that is too small will run constantly and still struggle on hot days. A system that is too large may short-cycle, cool unevenly, and remove less humidity.
That is why this calculator gives you an estimated cooling load in BTU/hr and a recommended unit size in tons. It applies square footage, ceiling height, climate, insulation, sun exposure, and occupancy adjustments to produce a practical starting point.
Quick rule of thumb (before using the calculator)
- Start with roughly 20 BTU per square foot for average homes with 8-foot ceilings.
- Adjust upward in hot climates, sunny homes, and drafty/older construction.
- Adjust downward for shaded spaces and excellent insulation.
- Convert BTU to tons: 1 ton = 12,000 BTU/hr.
Common residential AC sizing guide
| AC Size (Tons) | Cooling Capacity (BTU/hr) | Typical Coverage* |
|---|---|---|
| 1.5 Ton | 18,000 BTU | 700–900 sq ft |
| 2.0 Ton | 24,000 BTU | 900–1,200 sq ft |
| 2.5 Ton | 30,000 BTU | 1,200–1,500 sq ft |
| 3.0 Ton | 36,000 BTU | 1,500–1,800 sq ft |
| 3.5 Ton | 42,000 BTU | 1,800–2,100 sq ft |
| 4.0 Ton | 48,000 BTU | 2,100–2,400 sq ft |
| 5.0 Ton | 60,000 BTU | 2,400–3,000 sq ft |
*Coverage ranges vary by climate, envelope quality, windows, and layout.
Why proper AC sizing matters
If your AC is undersized
- Long run times and higher wear on components
- Difficulty holding temperature during peak heat
- Rooms far from vents may feel warm
- Higher electric bills from nonstop operation
If your AC is oversized
- Short cycling (frequent on/off operation)
- Poor humidity control and clammy indoor air
- Uneven cooling across rooms
- Higher upfront equipment cost
Inputs used in this calculator
1) Square footage
This is your starting point. If you are replacing a unit, measure the actual conditioned area instead of assuming based on lot size.
2) Ceiling height
Taller ceilings increase room volume, which means more air to cool. We scale the base estimate from the 8-foot standard.
3) Climate zone
Hotter and more humid climates increase cooling demand. A home in Phoenix or Houston typically needs more cooling capacity than a similar home in Seattle.
4) Insulation and air sealing
Better insulation and tighter construction reduce heat gain. Older, leaky homes generally require larger equipment for comfort.
5) Sun exposure and windows
Large west-facing windows and full-sun roof exposure can significantly raise afternoon cooling loads.
6) Occupants and internal heat
People, lighting, electronics, and appliances all release heat indoors. Kitchens and equipment-heavy rooms can need extra capacity.
Frequently asked questions
Is this calculator accurate enough to buy an AC?
It is accurate enough for planning and budgeting. For final equipment selection, request a contractor-performed Manual J calculation.
What tonnage is best for a 2,000 sq ft house?
Many homes land around 3 to 4 tons, but climate and insulation can shift that range. Run the calculator and confirm with a load calculation.
Do mini-splits use the same BTU logic?
Yes, BTU logic is similar, but zoning changes sizing strategy. Mini-split designs often size each room/zone individually.
Should I size up “just in case”?
Usually no. Slight oversizing can hurt comfort and humidity performance. Correct sizing almost always performs better than “bigger is safer.”
Bottom line
If you are asking, “what size AC do I need?”, the right answer is a balanced estimate plus professional verification. Use this calculator to get a solid BTU and tonnage starting point, then confirm with an HVAC pro before you purchase or install.