AC tonnage calculator for Indian and international homes
This AC tonnage calculator estimates the cooling load of a room in BTU per hour and tons, then recommends the next standard air-conditioner size — 0.75, 1, 1.5, 2, 2.5 tons and up — and shows what that unit will cost to run per hour and per month at your electricity tariff. It takes the room size and ceiling height, your climate, how much sun the room gets, whether it is on the top floor, the number of people and the heat from appliances.
The climate band matters more than anything else. A 180 ft² bedroom needs a 1.5-ton AC in Delhi or Nagpur where the summer peak is 45 °C and walls radiate heat all evening, but only a 0.75-ton unit in a mild climate where the rule of thumb is 20–25 BTU per square foot. The calculator uses loads of 100, 80, 60 and 40 BTU/ft² for the four bands, scaled for ceilings taller or shorter than 10 ft.
Running cost uses the capacity of the recommended unit, its ISEER or EER rating and a 70% duty cycle, which is typical for an inverter AC holding a set temperature. Treat both the size and the cost as a planning guide rather than an engineered heat-load calculation.
How to use the AC Tonnage Calculator
- Choose feet or metres and enter the room length, width and ceiling height.
- Pick the climate band that matches your summer peak, how sunny the room is, and whether it is on the top floor under the roof.
- Enter the number of people normally in the room, the wattage of appliances that run while the AC is on (a TV is 60–100 W, a desktop PC 150–250 W) and tick the kitchen option if there is cooking.
- Select the efficiency of the AC you intend to buy (5-star, 3-star, older unit or a custom EER), your tariff per kWh and the hours per day it will run.
- Read the BTU/h, tons and recommended size, the power draw and the running cost, then copy or share the summary.
AC Tonnage Calculator features
Climate-aware sizing
Four load bands from mild (40 BTU/ft²) to the Indian plains (100 BTU/ft²), matching what manufacturers recommend in each market.
Real room factors
Ceiling height, sun exposure, top-floor roof heat, occupants above two, appliance watts and kitchen heat are each added as separate lines.
Standard sizes
The load is rounded up to the capacities actually sold — 0.75, 1, 1.5, 2, 2.5, 3, 4 and 5 tons.
Running cost
Power draw from the ISEER/EER rating, cost per hour, per day and per month at your tariff in ₹, $, £ or €.
Star-rating comparison
Switch between 5-star, 3-star and older units to see how much the rating changes the bill.
Metric or imperial
Feet and square feet or metres and square metres; the room converts when you switch.
How the AC size is calculated
- Room load = floor area × BTU/ft² for the climate × ceiling factor (height ÷ 10 ft) × sun factor (0.9–1.2) × floor factor (1.0–1.2). A 15 × 12 ft Indian bedroom with a 10 ft ceiling: 180 × 100 = 18,000 BTU/h.
- People: 600 BTU/h for each person above two. Appliances: watts × 3.412. Kitchen: +4,000 BTU/h.
- Tons = total BTU/h ÷ 12,000 (1 ton of refrigeration = 12,000 BTU/h = 3.517 kW). 18,000 BTU/h = 1.5 tons.
- Recommended size = the next standard capacity at or above the load.
- Running cost = (capacity in kW ÷ ISEER) × 0.7 duty × hours × tariff. A 1.5-ton 5-star unit (ISEER 5.2) draws about 1.0 kW and costs roughly ₹5.7 per hour at ₹8/kWh.
Quick reference: room size to AC tonnage (Indian climate)
These assume average sun, two occupants and a 10 ft ceiling; a west-facing top-floor room moves up one size, a shaded ground-floor room may move down one. In a mild climate divide the BTU figures by roughly 2.5.
| Room area | Typical AC size | BTU/h |
|---|---|---|
| Up to 90 ft² (8 m²) | 0.75 ton | 9,000 |
| 90–130 ft² (8–12 m²) | 1 ton | 12,000 |
| 130–190 ft² (12–18 m²) | 1.5 ton | 18,000 |
| 190–250 ft² (18–23 m²) | 2 ton | 24,000 |
| 250–320 ft² (23–30 m²) | 2.5 ton | 30,000 |
| 320–400 ft² (30–37 m²) | 3 ton | 36,000 |
Choosing well and running cheaply
- Oversizing is not “safe”: a too-large AC short-cycles, dehumidifies poorly and wastes money. Pick the size that fits the load, and only round up when two factors push you to the border.
- A 5-star inverter costs more but uses 25–35% less electricity than a 3-star unit — at 8 hours a day through an Indian summer the difference pays back in two or three seasons. Compare with the electricity bill calculator.
- Set 24–26 °C and clean filters monthly; each degree lower adds about 6% to the running cost. The lumens calculator helps cut lighting heat, and the solar panel calculator sizes an array to offset daytime AC use.
AC Tonnage Calculator FAQ
Which AC size do I need for a 150 sq ft room?
In most of India a 1.5-ton (18,000 BTU/h) unit; 1 ton if the room is shaded and on a lower floor, 2 tons if it faces west on the top floor. In a mild climate the same room needs only about 0.75 ton. Enter your details in the AC tonnage calculator for the exact figure.
How many BTU per square foot should I use?
Around 100 BTU/ft² for the Indian plains, 80 for hot coastal or Gulf climates, 60 for warm regions like the US South and 40 for mild climates, each for a 10 ft ceiling with average sun.
What is the difference between tons and BTU?
One ton of cooling equals 12,000 BTU per hour (3.517 kW), the heat needed to melt a ton of ice in a day. A 1.5-ton AC removes 18,000 BTU/h.
How much electricity does a 1.5-ton AC use per hour?
About 1.0 kWh for a 5-star inverter (ISEER 5.2) and 1.35 kWh for a 3-star unit at full load; with a 70% duty cycle that is 0.7–0.95 units per hour, or ₹6–8 at ₹8 per unit.
Does ceiling height matter?
Yes. The loads are quoted for 10 ft ceilings; a 12 ft ceiling adds 20% to the room volume and the calculator scales the load accordingly (floor to a minimum factor of 0.8 for low ceilings).
Is this the same as a Manual J or ISHRAE heat-load calculation?
No. Those account for wall construction, insulation, window glass, orientation and infiltration in detail. This calculator is a rule-of-thumb estimate that matches typical manufacturer guidance and is good enough for choosing between standard sizes.