AC Tonnage Calculator

How to use the AC tonnage calculator

What this calculator does

This calculator estimates the cooling capacity needed for one rectangular room and expresses the result as both British thermal units per hour and tons of refrigeration. It supports two planning approaches. The Quick estimate method uses a room-size capacity band and practical adjustments for a kitchen, extra occupants, and sunlight. The Simplified load method uses room volume plus allowances for occupants, windows, and exterior doors. Neither method is a substitute for a complete residential load calculation that accounts for climate, insulation, glazing performance, orientation, infiltration, ventilation, ducts, and equipment operating conditions. ACCA describes Manual J as the recognized residential load-calculation standard.

When to use it

Use this calculator when comparing room air-conditioner sizes, checking whether an existing unit is broadly undersized or oversized, preparing a preliminary budget for a replacement, or discussing likely capacity with an HVAC professional. It is especially useful before shopping because manufacturers commonly state cooling capacity in BTU/h while central systems are often discussed in tons.

How to calculate

1. The calculator opens with a complete demonstration: a 20 ft by 10 ft sunny kitchen with an 8 ft ceiling and three occupants. Results and a validated example Excel workbook are available immediately.
2. Choose Calculation method. Use Quick estimate for a room-AC planning figure. Use Simplified load when you want the explicit room-volume, people, window, and door allowances shown in the reference model.
3. Replace Room length, Room width, and Ceiling height with interior dimensions in feet. Then enter Number of occupants.
4. For Quick estimate, choose Room type and Sunlight exposure. For Simplified load, also enter Number of windows and Exterior doors. Results update as you type.
5. Read Recommended AC tonnage and the equivalent BTU/h value, then review the supporting cards and Load breakdown table. Select Download Excel to export the current inputs and results.
6. Reset clears the demonstration and all computed content. Download Excel is then disabled until a complete valid room description is entered again.

Input guide

Calculation method is required and accepts one of two choices. Quick estimate uses square-foot capacity bands and room adjustments; Simplified load uses the stated additive formula. Room length and Room width are required decimals in feet from 1 to 500; for example, 20 and 10 produce 200 sq ft. Increasing either dimension increases area and normally increases required capacity. Do not enter inches or a combined value such as “20×10.” Ceiling height is required from 6 to 30 ft; 8 ft is the example. Taller ceilings increase room volume and, in quick mode, add 1,000 BTU/h for every foot above 8. Number of occupants is a required whole number from 0 to 100. In quick mode, the first two occupants are included and each additional person adds 600 BTU/h; in simplified load, every occupant adds 100 BTU/h.

Room type is required. Choose “Kitchen” only when cooking appliances materially affect the cooled room; quick mode then adds 4,000 BTU/h. Sunlight exposure is required and applies only to quick mode: heavily shaded reduces the subtotal by 10%, average leaves it unchanged, and very sunny raises it by 10%. The Number of windows and Exterior doors are whole-number inputs used by Simplified load. Each window and exterior door adds 1,000 BTU/h in that method. Count only exterior doors, not interior passage doors.

Output guide

Recommended AC tonnage is the estimated cooling rate divided by 12,000 BTU/h per ton. It is an estimate, not a recommendation for a specific model size. The BTU/h line shows the same load before conversion. Floor area is length multiplied by width, while Room volume multiplies that area by ceiling height. Base cooling load is the unadjusted room-size or room-volume contribution. Total adjustments is the difference between the final BTU/h and the base load; it can be negative for a shaded quick-estimate room. The summary pills repeat the active method, area, and capacity. The Load breakdown table lists each component, its rule, and its BTU/h contribution. A zero result is not produced for a valid room because positive dimensions always create a base load; unusually high values indicate that a professional load calculation is especially important.

Worked example

The startup example uses Quick estimate for a 20 ft × 10 ft room, so floor area is 200 sq ft. The room-size band contributes 6,000 BTU/h. A kitchen adds 4,000 BTU/h. With three occupants, one person is above the included first two, adding 600 BTU/h. The subtotal is therefore 10,600 BTU/h. “Very sunny” increases that subtotal by 10%, or 1,060 BTU/h, giving exactly 11,660 BTU/h. Dividing by 12,000 gives 0.9717 tons, displayed as 0.97 tons. ENERGY STAR's room air-conditioner sizing guidance explains the use of square footage, BTU/h, and adjustments for special room conditions.