Formula Used
Base load = Area × Base BTU per sq ft × Climate factor × Height factor × Insulation factor × Sun factor × Window factor.
Added load = Extra occupant load + Appliance load + Kitchen load.
Total BTU/hr = (Base load + Added load) × (1 + Safety margin ÷ 100).
AC tons = Total BTU/hr ÷ 12,000.
Tons per square foot = AC tons ÷ Area.
Square feet per ton = Area ÷ AC tons.
Airflow = AC tons × CFM per ton.
How to Use This Calculator
Enter the cooled floor area first. Use only rooms served by the air conditioner.
Add the base BTU per square foot. Use a higher value for hotter rooms.
Select the climate, insulation, sun exposure, and kitchen option.
Enter window area, occupants, appliance watts, and safety margin.
Press the calculate button. The result appears below the header and above the form.
Use CSV or PDF buttons to save the calculation.
Example Data Table
| Space Type |
Area |
Base BTU/sq ft |
Conditions |
Approx BTU/hr |
Approx Tons |
| Bedroom |
180 sq ft |
22 |
Moderate, average insulation |
4,000 |
0.33 |
| Small office |
450 sq ft |
25 |
Computers, mixed sun |
13,500 |
1.13 |
| Apartment zone |
900 sq ft |
28 |
Hot climate, full sun |
34,000 |
2.83 |
| Open living area |
1,200 sq ft |
25 |
Humid, kitchen load |
39,000 |
3.25 |
AC Tonnage Per Square Foot Guide
Cooling size should match the building load. A small unit runs too long. A large unit starts and stops often. Both cases waste power. Both cases can reduce comfort. This calculator gives a practical first estimate. It uses floor area, room height, climate, insulation, sun exposure, window area, people, and internal heat. It then converts the required BTU per hour into tons.
Why Area Alone Is Not Enough
Many quick rules use square feet per ton. That method is easy. It is also limited. Two rooms with the same area can need different capacity. High ceilings add air volume. Large windows add solar heat. Poor insulation leaks heat faster. Kitchens and equipment add extra load. Humid climates may also need longer run time for moisture control.
How The Estimate Works
The calculator starts with a base BTU per square foot. You can adjust this value. It multiplies the base load by climate, ceiling, insulation, sun, and window factors. It adds heat from extra occupants, kitchen use, and appliances. A safety margin is then applied. The result is shown as BTU per hour, exact tons, rounded tons, tons per square foot, square feet per ton, and airflow.
Better Planning Tips
Use realistic inputs. Measure the cooled area only. Exclude garages, open porches, and rooms that are not served by the system. Enter ceiling height as an average. Use higher appliance heat for server rooms, workshops, or busy kitchens. Keep the safety margin moderate. Oversizing can hurt humidity control. For final equipment selection, compare the estimate with a professional load study.
When To Use This Tool
This page is useful during early planning. It helps compare room zones, additions, rentals, offices, and small buildings. It can also show why a simple area rule may be wrong. The result should guide budgeting and discussions. It should not replace local code, duct design, or a detailed Manual J style calculation.
Remember that local weather matters. So does building direction. A south or west facing wall may gain more heat. Clean filters, sealed ducts, and balanced vents also affect comfort. Use the final tonnage range as a planning value. Then confirm it carefully before buying equipment for your project.
FAQs
What is AC tonnage per square foot?
It is the cooling capacity in tons divided by the cooled floor area. It helps compare AC size against room area.
How many BTU are in one AC ton?
One AC ton equals 12,000 BTU per hour. The calculator divides total BTU per hour by 12,000.
Is square footage enough for AC sizing?
No. Square footage is only a starting point. Climate, windows, insulation, ceiling height, people, and appliances also change the load.
Why does ceiling height matter?
Higher ceilings increase air volume. More air volume usually needs more cooling capacity than a standard eight foot ceiling.
Should I add a large safety margin?
No. A large safety margin can oversize the unit. Oversizing may cause short cycling and weak humidity removal.
What is a good CFM per ton value?
Many systems use about 350 to 450 CFM per ton. The default value here is 400 CFM per ton.
Can this replace a professional load calculation?
No. It gives a planning estimate. Final equipment sizing should consider local codes, ductwork, building details, and professional design methods.
Why is my rounded tonnage higher?
Equipment is commonly sold in standard sizes. The calculator rounds up to the next quarter ton for simple planning.