AC Tonnage Calculator
How the formula works
The calculator adds up the main heat sources in the room, adds a safety margin, and converts the total to tons of refrigeration (TR) and BTU/h.
Formulas
Envelope heat = Floor area × W/m² (exposure) × Climate factor × (Height / 2.7)
Glass heat = Glass area × W/m² (sun exposure)
People heat = Number of people × 125 W
Total load = (Envelope + Glass + People + Equipment) × (1 + Margin %)
TR = Total W / 3517
BTU/h = Total W × 3.412
Where:
- 1 TR = 12,000 BTU/h = 3.517 kW of cooling
- 2.7 m is the reference ceiling height for the typical W/m² values
- Equipment is the total watts of lights and appliances that run in the room
Typical values
Room exposure (W per m² of floor): interior room 70, one outside wall 90, two or more outside walls 110, top floor with exposed roof 130. Glass (W per m²): shaded or north 120, moderate sun with blinds 250, strong direct sun 400. Climate factor: moderate 0.85, hot 1.00, very hot 1.15. These are typical planning values in line with common ASHRAE-style estimating practice, not exact figures. Use the Custom option to enter your own.
How to use the calculator: choose the unit, enter the room size and people, pick exposure and climate, add window and equipment details if you have them, then click Calculate.
Important notes: the standard sizes used are 9,000, 12,000, 18,000, 24,000, 30,000, 36,000, 48,000 and 60,000 BTU/h. For large or critical spaces, do a full heat load calculation.
Worked example
Example 1: Bedroom
4 m × 3.5 m, height 2.7 m, one outside wall, hot climate, 2 m² window with moderate sun, 2 people, 200 W equipment, 10 % margin.
Envelope = 14 × 90 = 1,260 W. Glass = 2 × 250 = 500 W. People = 250 W. Equipment = 200 W. Subtotal = 2,210 W. With margin: 2,431 W = 8,295 BTU/h = 0.69 TR. Recommended: 0.75 TR (9,000 BTU/h).
Example 2: Living room in feet
15 ft × 12 ft, height 9 ft, top floor, hot climate, 20 ft² glass in strong sun, 4 people, 400 W equipment, 15 % margin.
Floor area = 16.7 m². Envelope = 2,209 W. Glass = 743 W. People = 500 W. Equipment = 400 W. With margin: 4,430 W = 15,115 BTU/h = 1.26 TR. Recommended: 1.5 TR (18,000 BTU/h).
Example 3: Small office
8 m × 6 m, height 3 m, two outside walls, very hot climate, 6 m² glass in strong sun, 8 people, 2,000 W equipment, 20 % margin.
Envelope = 6,747 W. Glass = 2,400 W. People = 1,000 W. Equipment = 2,000 W. With margin: 14,576 W = 49,735 BTU/h = 4.14 TR. Recommended: 5 TR (60,000 BTU/h).
Example 4: Interior room
5 m × 4 m, height 2.7 m, interior room, moderate climate, no window, 3 people, no equipment, 10 % margin.
Envelope = 20 × 70 × 0.85 = 1,190 W. People = 375 W. With margin: 1,722 W = 5,874 BTU/h = 0.49 TR. Recommended: 0.75 TR (9,000 BTU/h).
Common mistakes
- Choosing the AC only by floor area and ignoring sun, roof and window heat.
- Forgetting the heat from people, computers, TVs, lights and kitchen equipment.
- Ignoring a west or east facing window with direct afternoon sun.
- Not allowing for a top-floor room under an uninsulated roof.
- Buying an oversized AC, which cools quickly but cycles often and removes less moisture.
- Buying an undersized AC because the room is “small”, then running it flat out all day.
- Entering feet in a metre field or the other way round.
- Counting open doors or connected rooms as one small room.
- Skipping a safety margin for hot climates and future use changes.
Frequently asked questions
How many TR do I need for my room?
It depends on area, sun, windows, people and equipment. Enter these in the calculator to get a size in TR and BTU/h.
How many BTU/h are in one TR?
One TR is 12,000 BTU/h, about 3.517 kW of cooling.
Is the old rule of 1 TR per 100 sq ft correct?
It is only a rough rule. Sun, roof, windows and people can change the real need a lot, so a component estimate is better.
Why does the top floor need a bigger AC?
The roof absorbs strong sun and passes heat into the room, so the load per square metre is higher.
How much heat does one person add?
About 125 W in a seated, light-activity room. Heavy activity gives more.
Why do windows matter so much?
Sunlight through glass heats the room directly. Direct sun can add several times more heat per area than a wall.
What is the safety margin for?
It covers door openings, air leakage and extra loads that were not counted. Ten to 20 % is common.
What if my room is very hot or tall?
Select a hotter climate and enter the real ceiling height. The calculator scales the envelope heat with height.
Can I use this for an office or a shop?
Yes for a rough size. Include all people and equipment. For large spaces with fresh air needs, a full heat load calculation is better.
Does it include fresh air load?
Not separately. Use a higher safety margin if the room has a lot of fresh air or frequent door opening.
Should I choose an inverter AC?
An inverter AC adjusts its power to match the load. It is a good choice, but the tonnage still has to match the room load.
Is this a final design?
No. It is a planning estimate. Final design should follow the local code and a full heat load calculation.