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AC Tonnage Calculator
Use this free AC tonnage calculator to size an air conditioner for a room or a whole home. Enter the floor area, pick your climate zone, and add the ceiling height, insulation quality, sun exposure, and number of people — the calculator works out the cooling load in BTU per hour and converts it to tons. Getting the size right matters: the right unit cools evenly, controls humidity, and lasts longer, while a unit that is too big or too small wastes energy and comfort.
How it works
An air conditioner is sized by its cooling load, measured in BTU per hour. One BTU is the heat needed to raise one pound of water by one degree Fahrenheit, and an air conditioner rated at 30,000 BTU per hour removes that much heat from the air every hour. Air conditioners are also sold by the ton, where one ton of cooling equals 12,000 BTU per hour — a name left over from the days when cooling was measured by melting a ton of ice per day. Divide BTU per hour by 12,000 and you have tons.
The starting point is floor area times a climate factor. Hotter climates push more heat through walls, windows, and roofs, so they need more cooling per square foot. The table below shows the factors this calculator uses. A moderate climate at 20 BTU per square foot is the middle of the range; a hot climate at 30 needs half as much again for the same floor area. Pick the zone that matches your summers, or choose the custom option and enter your own figure if a contractor or energy audit has given you one.
| Climate zone | BTU per sq ft | Typical summers |
|---|---|---|
| Hot | 30 | Long, very hot summers, highs often above 95°F |
| Warm | 25 | Hot summers with regular days above 90°F |
| Moderate | 20 | Warm summers, highs mostly in the 80s |
| Cool | 15 | Short, mild summers, highs rarely above 80°F |
Area and climate alone are only a rough estimate, so the calculator applies four adjustments. Ceiling height scales the load by height divided by 8 feet, because a 10-foot ceiling holds 25 percent more air than an 8-foot ceiling over the same floor. Insulation multiplies the load by 0.90 for excellent insulation, 1.00 for average, and 1.15 for poor insulation — drafty windows and thin walls let heat in fast. Sun exposure multiplies by 0.95 for a mostly shaded space, 1.00 for average sun, and 1.10 for very sunny rooms with large south- or west-facing windows. Finally, each person beyond the first two adds 600 BTU per hour, because people give off heat, and a crowded living room genuinely needs more cooling than an empty one.
The full formula is: BTU per hour = floor area × BTU per square foot × (ceiling height ÷ 8) × insulation factor × sun factor + 600 × (people − 2, if more than 2). Then tons = BTU per hour ÷ 12,000. Air conditioners are sold in half-ton steps — 1.5, 2, 2.5, 3, 3.5, 4, and 5 tons — so round your result to the nearest available size. If you land exactly between two sizes, the smaller one is usually the better pick for humidity control, unless the space has unusually large windows or poor insulation.
This is a planning estimate, not a full Manual J load calculation. A professional load calculation also measures window sizes and directions, wall and roof construction, air leakage, appliances, and local design temperatures, and it is worth getting one before buying a whole-home system. This calculator is accurate enough to check a contractor's recommendation, to size a single room or a small addition, and to compare quotes — if a quote is a full ton away from this result, ask why.
Two common mistakes are worth avoiding. First, do not size by the old unit: if the existing air conditioner never kept the house cool, it may have been undersized, but it may also have been low on refrigerant, badly ducted, or simply old and worn out — replacing like-for-like repeats the problem or the wrong size. Second, do not add a safety margin on top of a safety margin. The climate factor already includes a margin, and rounding up a half ton on top of that is how homes end up with oversized units that short-cycle, leave humidity high, and wear out early.
Worked example
A living area measures 1,500 square feet with 8-foot ceilings in a moderate climate. Insulation is average, sun exposure is average, and two people use the space regularly.
Step 1: Base load — 1,500 × 20 BTU per sq ft = 30,000 BTU per hour.
Step 2: Ceiling factor — 8 ÷ 8 = 1.00, so no change.
Step 3: Insulation and sun — both 1.00, so no change.
Step 4: People — only 2 people, so no extra heat added.
Step 5: Convert to tons — 30,000 ÷ 12,000 = 2.50 tons.
The space needs a 2.5-ton air conditioner. If the same 1,500 square feet were in a hot climate at 30 BTU per square foot, the load would be 45,000 BTU per hour, or 3.75 tons — rounded to a 4-ton unit — which shows how much climate matters to the answer.
FAQ
How many BTU per square foot do I need for air conditioning?
It depends on your climate. A common rule is about 30 BTU per square foot in hot climates, 25 in warm climates, 20 in moderate climates, and 15 in cool climates. A 1,500 square foot space in a moderate climate therefore needs about 30,000 BTU per hour before adjustments for ceiling height, insulation, and sun exposure.
How many tons is 30,000 BTU?
One ton of cooling equals 12,000 BTU per hour, so 30,000 BTU per hour is 2.5 tons. Divide any BTU per hour figure by 12,000 to get tons. Common home sizes are 1.5, 2, 2.5, 3, 4, and 5 tons.
Is a bigger air conditioner better?
No. An oversized air conditioner cools the air quickly and shuts off before it removes enough humidity, leaving the room cool but clammy, and it cycles on and off more often, which wears parts out faster. An undersized unit runs constantly and never reaches the set temperature on hot days. The right size matches the calculated load as closely as possible.
Does ceiling height change the AC size I need?
Yes. Taller ceilings mean more air volume to cool. This calculator scales the load by ceiling height divided by 8 feet, so a 10-foot ceiling needs about 25 percent more cooling than the same floor area with an 8-foot ceiling, all else being equal.
Does insulation affect AC tonnage?
Yes. Good insulation and sealed windows slow heat entering the home, so less cooling is needed. This calculator applies a factor of 0.90 for excellent insulation, 1.00 for average insulation, and 1.15 for poor insulation, which can shift the result by a quarter of a ton or more on a large home.