Free AC Tonnage calculator
Enter your floor area and adjust the cooling factor for your climate — this AC tonnage calculator returns the required BTU/hr, the calculated tons, and the nearest standard residential size, updated live, as you type.
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Estimate only. ACCA Manual J calculation is the authoritative method for final sizing. Oversizing or undersizing an AC unit wastes energy and reduces comfort — consult an HVAC professional for a full load calculation.
Results are estimates. Consult a professional.
How the AC tonnage calculator works
The calculator multiplies the conditioned floor area of your space by a cooling factor that represents how hard the air conditioner has to work per square foot. That gives the required heat-removal rate in BTU per hour. Divide by 12,000 and you have the AC tonnage — the standard unit of cooling capacity in North America.
What is a 'ton' of air conditioning?
One ton of cooling equals 12,000 BTU per hour. The definition is historical: it is the amount of heat required to melt one short ton (2,000 lb) of ice over 24 hours. That works out to 288,000 BTU per day, or 12,000 BTU per hour. The unit has been the North American standard for residential and commercial cooling capacity ever since mechanical refrigeration replaced ice delivery.
Residential systems typically range from 1.5 tons (18,000 BTU/hr) for a small apartment up to 5 tons (60,000 BTU/hr) for a large home. Standard nominal sizes are 1, 1.5, 2, 2.5, 3, 3.5, 4, and 5 tons. Note that there is no 4.5-ton standard size — the market jumps from 4 directly to 5 tons.
| Tons | BTU/hr | Typical application |
|---|---|---|
| 1.0 | 12,000 | Small room or studio apartment (up to ~600 ft²) |
| 1.5 | 18,000 | Small home or apartment (~600–900 ft²) |
| 2.0 | 24,000 | Small to medium home (~900–1,200 ft²) |
| 2.5 | 30,000 | Medium home (~1,200–1,500 ft²) |
| 3.0 | 36,000 | Medium to large home (~1,500–1,800 ft²) |
| 3.5 | 42,000 | Larger home (~1,800–2,100 ft²) |
| 4.0 | 48,000 | Large home (~2,100–2,400 ft²) |
| 5.0 | 60,000 | Very large home or hot climate (2,400+ ft²) |
Ranges assume a moderate climate at 25 BTU/ft². Actual sizing depends on climate, insulation, ceiling height, window area, and orientation.
Worked example: sizing a 1,000 sq ft space
Carlos is adding central air to his 1,000 sq ft main floor in a mid-Atlantic state (moderate climate). He uses a cooling factor of 25 BTU/ft².
Step 1 — Calculate required BTU/hr
1,000 sq ft × 25 BTU/ft² = 25,000 BTU/hr.
Step 2 — Convert to tons
25,000 ÷ 12,000 = 2.08 tons.
Step 3 — Round up to the nearest standard size
2.08 tons rounds up to the next standard size: 2.5 tons. Carlos should look for a 2.5-ton (30,000 BTU/hr) central air system.
AC tonnage by square footage — reference chart
This chart shows the estimated BTU/hr and recommended AC size for common home areas at each climate zone. Use it as a planning guide; a full Manual J accounts for additional factors.
| Area (sq ft) | Mild (20 BTU/ft²) | Moderate (25 BTU/ft²) | Hot (30 BTU/ft²) |
|---|---|---|---|
| 500 | 1.0 ton | 1.0 ton | 1.5 tons |
| 750 | 1.5 tons | 1.5 tons | 2.0 tons |
| 1,000 | 1.5 tons | 2.5 tons | 2.5 tons |
| 1,500 | 2.5 tons | 3.0 tons | 4.0 tons |
| 2,000 | 3.5 tons | 4.0 tons | 5.0 tons |
| 2,500 | 4.0 tons | 5.0 tons | 5.0 tons |
| 3,000 | 5.0 tons | 5.0 tons | 5.0 tons |
Recommended sizes rounded up to the nearest standard residential tonnage. Very large or very hot loads may require two systems or a commercial unit.
When to get an ACCA Manual J calculation
The cooling-factor method gives a useful first estimate, but it omits several variables that matter in real installations: the number, size, and orientation of windows; the quality and depth of wall and attic insulation; the tightness of the building envelope; local design temperatures; and the heat output of appliances and occupants. For these reasons, the industry standard for final HVAC sizing is ACCA Manual J.
The cost of getting it wrong
An oversized AC unit cools the air quickly, shuts off, and restarts repeatedly — a cycle called short-cycling. The unit never runs long enough to remove humidity, leaving the space cold and clammy. Compressors also wear faster under frequent start-stop cycling. Undersizing means the unit runs at full capacity all summer, uses more energy, and may never achieve the setpoint on the hottest days.
AC sizing definitions
How accurate is this AC tonnage calculator?
The formula is mathematically exact for the inputs provided: square footage multiplied by the cooling factor gives the BTU/hr, and dividing by 12,000 converts that to tons. If you enter accurate measurements and choose the right cooling factor for your climate, the estimate is a reliable starting point.
What the calculator cannot capture are the variables that a Manual J accounts for: the R-value of your insulation, the number and size of windows, their solar heat gain coefficient, the air-change rate of the building envelope, internal gains from appliances, and the local 99th-percentile outdoor design temperature. Each of those factors can shift the true load by 10–20%. Use this estimate to understand the ballpark before consulting an HVAC professional for final sizing.