AC Size Calculator: BTU, Tons and HP for Your Room
Work out how many BTU/h your room needs and which split unit to buy, in tons, kW and the horsepower (HP) sizes many shops use, from a quick area estimate or a detailed heat-gain calculation for hot climates. Designing the whole system? Browse our MEP Revit families.
Quick or detailed methodBTU/h, tons, kW and HPHot and very hot climate dataHeat-gain breakdown chartRecommended unit size
Quick uses load per floor area. Detailed adds up walls, windows, roof, people and equipment.
Room
Length × width of the room.
Homes use the ENERGY STAR sizing chart. Other uses include typical people and fresh air.
Adds 600 BTU/h for each person above two.
A sunlit top-floor roof can add more heat than the rest of the room.
Climate and sun
The hottest outdoor temperature you design for. Moderate: up to 35 °C. Hot: 36–40 °C. Very hot: 41–46 °C.
Large west-facing windows or single glazing? Use the detailed method for a better figure.
Room size
North and south walls run along the length; east and west walls along the width.
Walls and windows
U-values in W/m²·K.
Applies to all windows in the room.
For each wall: does it face outside or another indoor space? Shading: Blinds means curtains or blinds inside; Shutters means shutters, louvres or an overhang outside. For a diagonal facade, use the closer direction.
Roof
A sunlit top-floor roof is often the largest single gain in hot climates.
People and equipment
Lighting and appliances: the power in use at the hottest time of day, such as a TV (100 W) or a desktop computer (150 W).
Adds 4,000 BTU/h (1.17 kW), the ENERGY STAR kitchen allowance.
Design conditions
Difference between the day's high and low.
Height of the site above sea level. Sets the air pressure.
Air changes per hour: 0.3 tight, 0.5 average, 1.0 leaky or a door used often.
Added to the total; 10 % is common.
Estimated cooling load
Enter your room to see the result.
Estimate only. Quick mode uses rules of thumb; detailed mode is a simplified heat-gain method with typical values, not a full ASHRAE calculation. For design and for equipment schedules, get a full load calculation (for example Carrier HAP, Trane TRACE or the ASHRAE heat balance or RTS method) and check your local code.
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Choose a method
Use the quick estimate when you only know the floor area, or the detailed method when you know the walls, windows, roof and what is in the room.
Describe the room and climate
Enter the area or dimensions, the sun and roof exposure, and pick a climate type or enter your site’s design conditions.
Read the load and the unit
You get the load in BTU/h, tons, kW and shop HP size, the smallest standard split size that covers it, and a note when one unit is not enough.
How the cooling load is calculated
Quick estimate. Homes use the ENERGY STAR room air conditioner sizing chart (for example 6,000 BTU/h for 150–250 ft², 12,000 BTU/h for 450–550 ft²) with its adjustments: −10 % for a heavily shaded room, +10 % for a very sunny one, +600 BTU/h for each regular occupant above two and +4,000 BTU/h for a kitchen. Offices, shops, classrooms and restaurants use published square-feet-per-ton rules of thumb. A climate factor of 1.10 (hot, 36–40 °C) or 1.30 (very hot, 41–46 °C) and a top-floor roof allowance are this calculator’s own rules of thumb, taken from its detailed method.
Detailed method. A simplified heat-gain calculation in the style of the ASHRAE CLTD/SCL/CLF method adds up each gain at its peak:
Lighting and appliances count in full, a 10 % safety factor is added, and 1 W = 3.412 BTU/h. The solar allowances and solar cooling loads are representative values for about 24–32° N in summer, not ASHRAE table values.
Worked example: a 5 × 4 × 3 m room in a hot, dry climate (37.8 °C outdoor) with outside south and west walls in uninsulated brick, 2 m² of south window with blinds and 1.5 m² of unshaded west window, single glazing, 2 people, 100 W lighting and 200 W appliances: walls 817 W, window sun 960 W, window conduction 280 W, people 260 W, lights and appliances 300 W and infiltration 140 W. With 10 % safety that is 3,033 W = 10,348 BTU/h, so a 12,000 BTU/h (1.5 HP) unit. Under a bare concrete roof the same room needs 16,710 BTU/h: an 18,000 BTU/h (2.25 HP) unit.
Estimate only. For design and equipment schedules use a full load calculation such as Carrier HAP, Trane TRACE or the ASHRAE RTS method.
Split unit sizes and design temperatures
| Shop label (HP) | BTU/h | Tons | kW cooling |
|---|---|---|---|
| 1 | 9,000 | 0.75 | 2.6 |
| 1.5 | 12,000 | 1.0 | 3.5 |
| 2.25 | 18,000 | 1.5 | 5.3 |
| 3 | 24,000 | 2.0 | 7.0 |
| 4 | 28,000–30,000 | 2.3–2.5 | 8.2–8.8 |
| 5 | 36,000 | 3.0 | 10.6 |
“HP” (حصان) is the sales label many shops use for split sizes, not the compressor’s power. Ratings vary by brand, so compare the BTU/h on the label. 1 ton of refrigeration = 12,000 BTU/h = 3.517 kW.
| Climate | Dry bulb | Wet bulb | Daily range |
|---|---|---|---|
| Warm and humid, coastal | 33 °C | 22 °C | 8 K |
| Hot and dry | 38 °C | 21 °C | 11 K |
| Very hot and dry | 44 °C | 19 °C | 12 K |
| Very hot and humid | 46 °C | 29 °C | 10 K |
Typical cooling design conditions for each kind of climate, rounded from published 0.4 % design data. For a design, enter your site’s ASHRAE 0.4 % dry bulb and coincident wet bulb, or the values your local code sets.
AC size and BTU questions
For a home in a moderate climate, the ENERGY STAR chart gives about 6,000 BTU/h for 150–250 ft² (14–23 m²) and 12,000 BTU/h for 450–550 ft² (42–51 m²). Add about 10 % in a hot climate (36–40 °C) and 30 % in a very hot one (41–46 °C), plus extra for a sunny room, more than two people, a kitchen or a roof above. The detailed mode works it out from the walls, windows and roof instead.
Many shops sell split units by “horsepower”: 1.5 HP is about 12,000 BTU/h, 2.25 HP about 18,000, 3 HP about 24,000 and 5 HP about 36,000. Calculate the load in BTU/h first (حساب قدرة التكييف) and pick the smallest size that covers it. For example, a 5 × 4 m room in a hot, dry climate with south and west windows needs about 10,300 BTU/h, so 1.5 HP; on the top floor under a bare roof it needs about 16,700 BTU/h, so 2.25 HP.
A 1.5 HP split unit is rated at about 12,000 BTU/h, which is 1 ton of refrigeration or 3.5 kW of cooling. The HP figure is a market label, not the compressor’s power: one mechanical horsepower is only 2,545 BTU/h. Always check the BTU/h rating on the unit, as brands differ, especially for 4 HP units (28,000 to 30,000 BTU/h).
Divide BTU/h by 12,000 to get tons of refrigeration, and divide by 3,412 to get kW. So 18,000 BTU/h is 1.5 tons or 5.3 kW, and 24,000 BTU/h is 2 tons or 7.0 kW.
A little margin is fine, and this calculator adds a 10 % safety factor in detailed mode. A unit that is far too large cools the room quickly, switches off before it has removed the moisture and cycles on and off, which feels clammy and wastes energy. Inverter units cope better with oversizing, but sizing close to the load is still best.
Cooling loads use a design temperature that is exceeded only a few hours a year, usually the ASHRAE 0.4 % dry bulb for your site. As a guide, warm coastal climates are around 33 °C, hot and dry ones around 38 °C, very hot and dry ones around 44 °C, and very hot and humid ones around 46 °C with a high wet bulb. Use the value in your local code if it sets one.
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