AC Tonnage Calculator: Find the Right Air Conditioner Size
An undersized AC runs nonstop and never quite cools the room. An oversized one short-cycles, leaves humidity behind, and costs more to install. This AC tonnage calculator works out the cooling load from the room’s dimensions and conditions, then recommends the closest standard tonnage above the result. Enter length, width, and ceiling height, note the sun exposure and insulation, and the tool returns BTU per hour and a ton size you can actually buy.
Every input shifts the load. A west-facing window adds BTU. A poorly insulated wall adds more. The occupants themselves count. Two people barely register. Five people push the total up by 1,200 BTU before you’ve added anything else. Getting the number right means the unit runs efficiently instead of fighting the room.
How to Use the AC Tonnage Calculator
Pick your currency from the top dropdown.
Enter the room length, width, and ceiling height, then set the unit for each.
Type how many people regularly use the room.
Choose the sun exposure: shaded, partial, full, or very high.
Pick the insulation quality, from new construction to uninsulated.
Add the number of windows in the room.
Set the appliance load: none, light, medium, or heavy.
Adjust the safety margin, then enter your installed price per ton.
The Summary tab shows total BTU and recommended tonnage. Detailed Breakdown lists every multiplier and adjustment behind the result.
How the AC Tonnage Calculator Formula Works
The calculation starts with square footage and applies multipliers for the room’s real-world conditions. Base cooling load is 20 BTU per square foot per hour, then it climbs or drops based on the rest of the inputs.
Formula: Base BTU = Room area (ft²) × 20
Formula: Adjusted BTU = Base BTU × Sun multiplier × Insulation multiplier
Formula: Total BTU = (Adjusted BTU + Occupants + Windows + Appliances) × (1 + Safety %)
Formula: Required tons = Total BTU ÷ 12,000
Sun and insulation are multipliers, so they compound. A very-high-sun room with poor insulation multiplies the base load by 1.25 × 1.35, which is roughly a 69 percent increase before any other input counts.
The add-on loads are fixed. Each occupant beyond the first two adds 400 BTU. Every window adds 1,000 BTU. Appliances range from zero for a sparse room up to 4,000 BTU for a kitchen or server closet. Those are industry rules of thumb, not exact measurements.
One ton of cooling equals 12,000 BTU per hour. That’s the standard unit used in North America, and it comes from the old ice-melting equivalent, not from the weight of the machine. The tool then rounds the required tonnage up to the nearest standard residential size: 0.75, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7.5, 8, 10, 12.5, 15, or 20 tons.
Worked Example: A 15 ft × 12 ft Bedroom
Say you’re sizing an AC for a 15 ft × 12 ft bedroom with 9 ft ceilings. Two occupants. Partial sun exposure. Well-insulated. Two windows. No heavy appliances. Safety margin is 10%.
Room area: 15 × 12 = 180 ft²
Room volume: 180 × 9 = 1,620 ft³
Base BTU: 180 × 20 = 3,600 BTU/hr
Sun multiplier (partial): 1.00
Insulation multiplier (good): 1.00
Adjusted base: 3,600 × 1.00 × 1.00 = 3,600 BTU/hr
Occupants beyond 2: 0 → 0 BTU
Windows: 2 × 1,000 = 2,000 BTU
Appliances: none → 0 BTU
Subtotal: 3,600 + 0 + 2,000 + 0 = 5,600 BTU/hr
With 10% safety: 5,600 × 1.10 = 6,160 BTU/hr
Required tons: 6,160 ÷ 12,000 = 0.513 tons
Recommended size: 0.75 ton (smallest standard)
A bedroom at that size needs just over half a ton. Since nothing smaller than 0.75 ton is standard, the tool recommends the 0.75 ton unit. That’s a window or mini-split unit, not a central air system.
