AC Tonnage Calculator (Room Size to Tons)
AC Tonnage Calculator (Room Size to Tons)
Room size, people, sun, floor and climate to a cooling load in tons, kW and BTU/h — every contribution on its own line — plus the running current at your voltage and the units a month at your usage.
AC tonnage from room size
A 12 m² bedroom 3 m high, two people, some afternoon sun, not the top floor, Indian plains summer, 200 W of equipment
The rule-of-thumb cooling load
tons = Q ÷ 3,516.85 BTU/h = Q ÷ 0.29307107
Pin = capacity ÷ EER I = Pin ÷ (V × PF) kWh = Pin × hours × days × duty ÷ 1,000
- qbase
- cooling load per cubic metre of room, in W/m³ — the whole thumb rule sits in this one number
- f
- the three multipliers, applied to the shell load only: people and equipment do not get hotter because the room faces west
- qperson
- total metabolic heat, sensible plus latent, per occupant
- EER / ISEER
- watts of cooling delivered per watt of electricity drawn. ISEER is a seasonal average, EER a single-point figure
- duty
- the share of the switched-on hours the compressor actually runs — a correctly sized unit cycles
Worked example
A 12 m² bedroom 3 m high, two people, some afternoon sun, not the top floor, Indian plains summer, 200 W of equipment
Shell: 12 × 3 = 36 m³ at 100 W/m³ = 3,600 W
Sun 1.1 × floor 1.0 × climate 1.1 = 1.21, which adds 756 W
People: 2 × 120 = 240 W; equipment 200 W
Total Q = 3,600 + 756 + 240 + 200 = 4,796 W = 4.80 kW = 16,365 BTU/h
In tons: 4,796 ÷ 3,516.85 = 1.36 ton, so a 1.5 ton unit — a 1.0 ton would be short
Electrically: a 1.5 ton unit at EER 3.5 draws 1.5 × 3,516.85 ÷ 3.5 = 1.507 kW, which at 230 V and PF 0.9 is 7.281 A
Running 8 h a day for 30 days with the compressor on 70% of that: 253 kWh a month
What the thumb rule gives, by room size
| Floor area | Approx. sq ft | Cooling load | Unit you would buy |
|---|---|---|---|
| 8 m² | 86 | 0.95 ton | 1.0 ton |
| 10 m² | 108 | 1.16 ton | 1.5 ton |
| 12 m² | 129 | 1.36 ton | 1.5 ton |
| 15 m² | 161 | 1.67 ton | 2.0 ton |
| 18 m² | 194 | 1.98 ton | 2.0 ton |
| 22 m² | 237 | 2.40 ton | 2.5 ton |
| 28 m² | 301 | 3.02 ton | 4.0 ton |
| 35 m² | 377 | 3.74 ton | 4.0 ton |
Cooling capacity in every unit
| Nominal size | BTU/h | Watts | kW |
|---|---|---|---|
| 0.75 ton | 9,000 | 2,638 | 2.638 |
| 1.00 ton | 12,000 | 3,517 | 3.517 |
| 1.50 ton | 18,000 | 5,275 | 5.275 |
| 2.00 ton | 24,000 | 7,034 | 7.034 |
| 2.50 ton | 30,000 | 8,792 | 8.792 |
| 3.00 ton | 36,000 | 10,551 | 10.551 |
Choosing between a 1 ton and a 1.5 ton unit
An air conditioner has to remove heat as fast as it arrives. Heat arrives through the walls, roof and glass, through gaps with the outside air, and from everything inside the room that is warm — people, lamps, a television, a laptop charger. Add all of that up and you have the cooling load, in watts. Divide by 3,516.85 and you have it in tons, because the trade still sizes machines in a unit invented for the ice business.
This page uses the thumb rule, and says so. The shell load is taken as a fixed number of watts per cubic metre — 100 W/m³ by default, which puts the shell of a 12 m² Indian bedroom with a 3 m ceiling at about one ton on its own, the answer any installer would give before adding anything. The three multipliers then lift it for afternoon sun, an exposed roof and a hot climate, and people and equipment are added afterwards, because those do not get hotter when the room faces west. Every line is shown separately so you can see which one is moving the answer.
What a real load calculation does instead. ISHRAE’s handbook, ASHRAE’s Fundamentals and ACCA’s Manual J all work surface by surface: each wall’s area, orientation, construction and U-value; each window’s area, shading coefficient and the solar gain at the design hour; the infiltration rate; and the latent load from moisture separately from the sensible load. They arrive at a different, better number. The North American trade rule of one ton per 400 to 600 square feet is notorious for oversizing modern insulated houses by two or three times, and the per-cubic-metre rule here has exactly the same weakness in the other direction: it knows nothing about your glazing or your insulation.
Oversizing is a real failure, not a safe margin. A unit with too much capacity cools the air to the set point quickly and switches off. Removing moisture takes running time, not capacity, so the room ends up cold and damp, and the compressor wears from short-cycling. Round up to the next standard size, as this page does, but be suspicious when the rounding is large — and prefer an inverter unit, which modulates instead of cycling.
The electrical side. Capacity divided by EER or ISEER gives the input power: a 1.5 ton unit at EER 3.5 draws 1.507 kW. Divide by voltage and power factor for the running current, which is what sizes the cable and the breaker: 7.281 A here, which sits comfortably on a 16 A circuit — though your local code and the unit’s own nameplate decide that, not this page. Starting current on a fixed-speed compressor is several times that for a fraction of a second, which is why air-conditioner circuits get a type C breaker rather than a type B. For the cable and the breaker use the MCB size calculator and the cable ampacity calculator; for what it costs at your own tariff, the electricity consumption calculator, which this page does not duplicate; for the unit conversions on their own, the BTU to watts converter.
Frequently asked questions
What size AC do I need for a 12 m² room?
By the thumb rule this page uses — 3 m ceiling, two people, 200 W of equipment, some afternoon sun, hot summer — about 1.36 tons, so a 1.5 ton unit. Shaded, ground floor and temperate takes it to 1.15 tons, which this rule still rounds up to 1.5; drop the base figure to 80 W/m³ for a well-insulated, well-shaded room with one person and no equipment and it falls to 0.85 tons, where a 1.0 ton unit really does fit.
How many BTU is one ton of air conditioning?
Exactly 12,000 BTU/h, which is 3,516.85 W or 3.51685 kW. The name comes from the rate of cooling that would freeze one short ton of water in 24 hours.
Is the square-feet-per-ton rule reliable?
No. It ignores glazing area and orientation, insulation, airtightness and the climate, all of which change the answer by more than the rule’s own precision. Use it to pick between two adjacent standard sizes, and commission a proper ISHRAE or Manual J calculation for anything larger than a room.
How much current does a 1.5 ton AC draw?
Capacity divided by EER gives the input power; divide that by voltage and power factor. A 1.5 ton unit at EER 3.5 draws about 1.507 kW, which is 7.281 A at 230 V and a power factor of 0.9. A more efficient unit at ISEER 5 would draw about a third less.
Is it better to oversize an air conditioner?
No. An oversized unit reaches the set temperature before it has dehumidified the room, so it feels cold and clammy, and the compressor short-cycles. Size it to the load and round up one standard step, not two.
Related calculators
References
- Wikipedia, Ton of refrigeration: “The modern definition is exactly 12,000 BTU_IT/h”, i.e. 3.516853 kW. Retrieved 24 September 2026.
- Energy Vanguard, Air conditioner sizing: load calculations vs rules of thumb: the trade rule of “1 ton of air conditioning capacity for each 400 to 600 square feet”, why it “doesn’t account for the window type, orientation, or overhang … airtightness or the insulation levels”, and ACCA Manual J as the method that does.
- Engineering ToolBox, Metabolic heat gain from persons, reproducing the ASHRAE Handbook — Fundamentals table of representative rates at 24 °C room temperature: seated at rest 100 W total (67 sensible / 33 latent); seated, very light work 120 W (70 / 50); light bench work 220 W (85 / 135).
- Indian Society of Heating, Refrigerating and Air Conditioning Engineers (ISHRAE). ISHRAE Handbook — Air Conditioning: the surface-by-surface cooling load method used for design work in India. Cited as the proper alternative to the rule of thumb on this page; not quoted here.