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BTU Calculator for Room Air Conditioners

Enter the room area, how sunny it is, how many people use it and whether it is a kitchen. The calculator reads the ENERGY STAR room air-conditioner chart and applies its adjustments, giving BTU/h, kW and tons. A 200 ft² room with normal sun and two people needs about 6,000 BTU/h.

Rule-of-thumb estimate only. A room-by-room load calculation by a professional (Manual J in the US, EN 12831 in Europe) is the right way to size heating and cooling equipment.

Measurements

Units

Results

Cooling capacity

7,000 BTU/h

Cooling capacity (kW)

2.05 kW

Tons of cooling
0.58 tons

Based on the ENERGY STAR room air-conditioner chart (8 ft ceilings). Whole-house rules of thumb of 400–600 ft² per ton tend to oversize: in one sample of load calculations the average was about 1,200 ft² per ton. Ask a professional for a Manual J or EN 12831 calculation.

Show the working
  1. ENERGY STAR chart250 ft² → 7,000 BTU/h
  2. Adjustmentsnormal, 2 people → 7,000 BTU/h
  3. Rule of thumb vs load calculations400–600 ft²/ton vs ~1200 ft²/ton (Manual J sample)
Paper and orientation
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How to use the BTU calculator

  1. Choose Imperial (ft²) or Metric (m²); metric areas are converted to square feet for the chart.
  2. Enter the room area.
  3. Choose the sun exposure: normal, heavily shaded or very sunny.
  4. Enter how many people regularly use the room.
  5. Say whether the room is used as a kitchen.

How the sizing works

The calculator looks up the room area in the ENERGY STAR room air-conditioner sizing chart, which assumes 8 ft ceilings. It then applies the chart’s adjustments: −10% for heavy shade, +10% for strong sun, +600 BTU/h per person beyond two, and +4,000 BTU/h for a kitchen. The result is converted to kW (÷ 3,412.14) and tons (÷ 12,000).

Worked examples

Bedroom. 200 ft², normal sun, two people: 6,000 BTU/h (1.76 kW).

Sunny living room. 200 ft², very sunny, four people: 6,000 × 1.1 + 2 × 600 = 7,800 BTU/h.

Shaded kitchen-diner. 600 ft², heavily shaded, two people, kitchen: 14,000 × 0.9 + 4,000 = 16,600 BTU/h.

How to interpret the results

Choose the unit size closest to the result. If you are between sizes, consider insulation, ceiling height and window area. High ceilings, poor insulation or large west-facing windows push the need up; good shading and insulation bring it down.

Tips and common mistakes

  • Don’t oversize: a unit that is too large controls humidity poorly.
  • Measure the actual cooled space, including open-plan areas that share air.
  • Reduce the load first: shading, sealing and insulation often cost less than a bigger unit.
  • Check the electrical supply for larger units.

Assumptions and limitations

This is a rule-of-thumb estimate based on the ENERGY STAR chart, which covers rooms from 100 ft² to under 2,500 ft². For whole homes or unusual rooms, get a professional load calculation.

Frequently asked questions

How many BTU do I need for my room?

The ENERGY STAR chart sizes room units by area for 8 ft ceilings: for example 6,000 BTU/h for 150 to under 250 ft² and 12,000 BTU/h for 450 to under 550 ft². The calculator applies the chart and its adjustments.

What adjustments does ENERGY STAR recommend?

Reduce capacity by 10% for a heavily shaded room, increase it by 10% for a very sunny room, add 600 BTU/h for each person beyond two, and add 4,000 BTU/h if the unit is in a kitchen.

How do I convert BTU/h to kW or tons?

One kW is about 3,412 BTU/h, and one ton of cooling is 12,000 BTU/h. A 12,000 BTU/h unit is about 3.5 kW.

Is 1 ton per 500 square feet right?

Whole-house rules of thumb of 400–600 ft² per ton tend to oversize equipment. In one sample of professional load calculations reported by Energy Vanguard, the average was about 1,200 ft² per ton, while the installed systems averaged 856 ft² per ton.

Why not just buy a bigger unit?

An oversized unit cools the air quickly and switches off before removing much humidity, leaving the room clammy, and it cycles more often.

Does it work for whole houses?

No. The chart is for single rooms up to about 2,500 ft². Central systems should be sized with a room-by-room load calculation such as ACCA Manual J (US) or EN 12831 (Europe).

Sources