AC Tonnage & BTU Calculator
Find the AC size for a room — quick chart or a heat-gain estimate for hot climates.
Room
Location and weather
Outside walls
Walls to other rooms of the house are not counted. A wall that never gets direct sun can be marked shaded; its windows then get no direct sun either.
Windows and glass doors
Roof or ceiling
People, appliances and air
Running cost optional
Where the heat comes from
| Part | Watts | Btu/h | Share |
|---|
Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the AC Tonnage & BTU Calculator
Choose the right size of room air conditioner two ways. Quick uses the ENERGY STAR sizing chart for room ACs with its official adjustments for shade, sun, extra people and kitchens. Heat gain adds up what actually heats the room on a hot, clear day — sun on the walls and roof, glass, people, appliances and outside air leaking in — hour by hour, and takes the worst hour. Both give the load in tons of refrigeration, Btu/h and kW and the common AC size that covers it, and the running cost from the BEE star label at your electricity tariff.
This is an estimate to help you choose a room AC — not a professional cooling-load calculation. For large rooms, whole houses, ducted systems or offices, ask an HVAC engineer for a full calculation.
How to use it
- Enter the room’s length and width (and the ceiling height for the heat-gain method). Feet and inches such as
12' 6"work. - Quick: say whether the room is shaded or very sunny, how many people use it and whether it is a kitchen.
- Heat gain: set your latitude (or pick a city), the hottest month and your outdoor design temperature. Add each outside wall with the way it faces, each window, and say whether the roof is exposed to the sun (top floor) or there is a floor above. Choose the wall and roof construction and colour.
- Read the load and the suggested size, then compare it with the rated cooling capacity in watts on the AC you are looking at.
- Under Running cost, type the units per year from the BEE star label, the hours you use it and your tariff.
Examples
Chart 6,000 Btu/h (150 up to 250 sq ft) + 10% for sun + 600 for the third person = 7,200 Btu/h = 0.6 TR (2.11 kW) → an 8,000 Btu/h or 0.75-ton unit.
West wall 4 m with a 1.5 m² clear window, south wall 3.5 m, both 230 mm brick; bare 150 mm RCC roof in sun; 2 people; 200 W of appliances
Peak at about 2:30 pm solar time: roof 1.99 kW, walls 1.22 kW, window sun 0.77 kW, glass conduction 0.14 kW, people, appliances and air 0.63 kW → 4.75 kW = 1.35 TR → a 1.5-ton AC. With a floor above instead of the exposed roof the peak is 2.90 kW (0.83 TR, a 1-ton AC).
BEE label 1,000 units a year (for 1,600 hours); you run it 8 hours a day for 150 days = 1,200 hours → 750 kWh; at ₹8 per unit, ₹6,000 a year.
Quick method: the ENERGY STAR chart
ENERGY STAR’s room-AC chart runs from 5,000 Btu/h for 100–150 sq ft to 34,000 Btu/h for 2,000–2,500 sq ft and assumes 8 ft ceilings. Its adjustments: reduce by 10% for a heavily shaded room, add 10% for a very sunny one, add 600 Btu/h for each person beyond two, and add 4,000 Btu/h for a kitchen. The chart does not know your climate or how the room is built, so it can be much too small for a sun-baked top-floor room in a hot climate — use the heat-gain method there. Source: ENERGY STAR — Room Air Conditioners.
Heat-gain method
- Walls and roof: U × area × (outdoor − indoor + sun allowance). The sun allowance is the sol-air excess: surface absorptance × sunshine on the surface × 0.04 m²K/W (the outside surface resistance). U-values come from the layers using BEE’s Eco-Niwas Samhita 2024 method and data (§9.3, Tables 45–47): for example 2.11 W/m²K for 230 mm brick plastered both sides, 0.78 for 200 mm AAC, 3.11 for a 150 mm RCC slab and 0.55 with 50 mm of EPS insulation.
- Windows: U × area × temperature difference, plus area × SHGC × the sun on that face. “Shaded outside”, or a wall marked as shaded all day, removes the direct sun; a curtain factor reduces the rest.
- Sun: a clear-sky model (air mass after Kasten & Young; beam 1,353 × 0.7^(AM^0.678) W/m²; diffuse about 10% of the beam; ground reflection 20%) at your latitude on the 21st of the month, every half hour from 6 am to 7 pm solar time.
- People: about 70 W sensible + 50 W latent each when seated doing very light work, 80 W + 80 W for moderate work (figures for a 24 °C room, after ISO and ASHRAE data as tabulated by the Engineering ToolBox).
- Air leakage: air changes per hour × room volume; sensible 1.2 kg/m³ × 1.006 kJ/kg·K × ΔT, latent from the outdoor and indoor humidity ratios × 2,501 kJ/kg.
The largest half-hourly total is the design load. Using the peak outdoor temperature all day and ignoring the delay of heavy walls and roofs both err on the generous side.
Sizes, tons and the BEE label
One ton of refrigeration is 12,000 Btu/h = 3.517 kW. Air conditioners are sold in nominal sizes (0.75, 1, 1.5, 2 tons; or 5,000–36,000 Btu/h), but the cooling capacity printed on the label can differ from the nominal size, so compare the required watts with the rated capacity. Choosing far bigger than needed is not better: an oversized unit cycles on and off and removes less moisture.
The BEE star label gives the electricity consumption in units (kWh) per year based on 1,600 hours of cooling a year (BEE room-AC labelling regulations, 2017, using ISO 16358 with Indian temperature bins). Running cost = label units × your hours ÷ 1,600 × tariff.
Sources
- ENERGY STAR room AC sizing chart
- BEE Eco-Niwas Samhita 2024 — U-value method, surface resistances and material conductivities
- PVEducation — air mass and clear-sky intensity and azimuth angle
- BEE labelling regulations for room air conditioners, 2017 — 1,600 cooling hours a year
- Engineering ToolBox — metabolic heat gain from persons
- ASHRAE Handbook—Fundamentals (2021), ch. 1 (psychrometrics) and ch. 18 (cooling load components)
Limitations
- A screening estimate for one room: it ignores the delay of heavy walls and roofs, uses a clear-sky model rather than local weather data, and treats walls to other rooms as neutral.
- Use your city’s design temperature for the hottest month (for example from ISHRAE climatic data or your weather service), not the record high.
- Window SHGC, U-values for other constructions, surface absorptance and the label consumption are product figures you can enter; the defaults (such as about 0.3 for white and 0.9 for dark surfaces) are approximate starting points.
- Rooms over 2,500 sq ft, open-plan homes and ducted or multi-split systems need a professional load calculation.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
How many square feet does a 1.5-ton AC cool?
It depends on the climate, sun and construction more than on the floor area. On the ENERGY STAR chart, 18,000 Btu/h (1.5 tons) is listed for 700–1,000 sq ft — but that chart is for US conditions. A top-floor room of about 150 sq ft in a 40 °C summer with a west window can need 1.5 tons, as the heat-gain example shows. Use the heat-gain method for hot climates.
What is 1 ton of AC in watts and BTU?
One ton of refrigeration is 12,000 Btu/h, which is 3.517 kW of cooling. It is the heat needed to melt one short ton of ice in a day.
Why does the top floor need a bigger AC?
A sunny concrete roof gets far hotter than the air, and that heat comes through the ceiling. In the example the roof alone adds about 2 kW; painting it white brings the peak from 4.75 to 3.92 kW, and 50 mm of EPS insulation under a screed to 3.19 kW. Try the roof colour and construction in the calculator.
Should I add a safety margin?
Only a small one, if any. The heat-gain method already uses the peak outdoor temperature all day. A much larger unit than needed short-cycles and leaves the room clammy. The Margin box lets you add up to 50% if you know of extra loads.
How do I work out the yearly running cost?
Take the “units per year” from the BEE star label — it is for 1,600 hours of use — scale it to your hours and multiply by your tariff per unit. Without a label, the power input in watts × hours gives an upper limit.