How to use the BTU calculator
Cooling load is the amount of heat that has to be removed from a room every hour to keep it comfortable. In North America it's given in BTU per hour (BTU/h) or tons; most of the rest of the world uses kW. Architects need a sense of it early: it decides whether a wall unit, a ducted split or a central system makes sense, how much space plant needs, and whether a glazing-heavy design is about to create a problem. This calculator gives a quick, defensible figure for a single room or zone.
- Enter the room size in feet or metres, including the ceiling height. For open-plan spaces, measure the whole area the unit will serve.
- Set sun exposure, climate and insulation. Choose "very sunny" for rooms with large west or south glazing (north glazing in the southern hemisphere), and "heavily shaded" for rooms under deep overhangs or trees.
- Add internal gains: the number of people who are regularly in the room, whether it contains a kitchen, and the wattage of any extra equipment such as servers, AV racks or workstations.
- Read the results: the load in BTU/h, tons and kW, the load per unit area, and the smallest standard unit size that covers it.
BTU formulas
The base load comes from the ENERGY STAR room air conditioner sizing chart, which sets a capacity for each band of floor area. The calculator then applies the ENERGY STAR adjustments (sun, occupants, kitchen) and three rule-of-thumb factors for climate, ceiling height and insulation.
Base = ENERGY STAR capacity for the floor area
Load = Base × Sun × Climate × max(1, Ceiling ÷ 8 ft) × Insulation
+ 600 × (Occupants − 2) + 4,000 if kitchen + Equipment W × 3.412
Tons = BTU/h ÷ 12,000 · kW = BTU/h × 0.000293
| Floor area (ft²) | Floor area (m²) | Base capacity (BTU/h) |
|---|---|---|
| 100–150 | 9–14 | 5,000 |
| 150–250 | 14–23 | 6,000 |
| 250–350 | 23–33 | 7,000–8,000 |
| 350–450 | 33–42 | 9,000–10,000 |
| 450–550 | 42–51 | 12,000 |
| 550–700 | 51–65 | 14,000 |
| 700–1,000 | 65–93 | 18,000 |
| 1,000–1,400 | 93–130 | 21,000–23,000 |
| 1,400–2,000 | 130–186 | 24,000–30,000 |
| 2,000–2,500 | 186–232 | 34,000 |
Worked examples
A sunny bedroom
A 14 × 12 ft bedroom is 168 ft², so the base is 6,000 BTU/h. It has a big west-facing window, so add 10% for sun: 6,600 BTU/h. Two people sleep there, so there's no occupant allowance. The nearest standard unit at or above the load is 8,000 BTU/h, though a 6,000 BTU/h unit with good blinds would be close.
Open-plan living and kitchen
A 30 × 20 ft great room is 600 ft², giving a base of 14,000 BTU/h. Four people use it (two over the baseline, so +1,200) and it includes the kitchen (+4,000). The total is 19,200 BTU/h, which is 1.6 tons or about 5.6 kW. An 18,000 BTU/h unit would be slightly small, so specify a 2-ton (24,000 BTU/h) system or split the zone.
A small office in a hot, humid city
A 7 × 5 m office (35 m², 377 ft²) has a 2.7 m ceiling. The base is 9,000 BTU/h. Hot and humid climate: × 1.2 = 10,800. The ceiling is 8.86 ft, so × 1.11 brings it to about 11,960. Three occupants add 600, and 600 W of computers and screens add 2,047. The load is about 14,600 BTU/h (4.3 kW), so a 15,000 BTU/h (4.4 kW) unit fits.
Who this calculator helps
- Architects setting out plant space, wall unit positions and condenser locations at concept stage.
- Interior designers checking that a new layout, extra glazing or a kitchen move won't overwhelm the existing cooling.
- Building services and MEP engineers doing a quick sense check before running a full heat-gain model.
- Contractors and installers sanity-checking the unit size in a quote.
- Homeowners and landlords buying a window, portable or mini-split unit for one room.
- Students learning how area, orientation and internal gains drive cooling demand.
Practical tips
- Reduce the load before you size the unit. External shading, low-e glass and roof insulation cut heat gain far more cheaply than a bigger compressor. The window-to-wall ratio calculator shows when glazing is driving the load.
- Check which facades get afternoon sun with the sun path calculator before choosing "very sunny" or "shaded".
- Get accurate floor areas for irregular rooms from the square footage calculator.
- Choose the nearest standard size at or just above the estimate. Going two sizes up usually hurts comfort through poor dehumidification.
- For whole buildings, ducted systems or permit drawings, commission a Manual J or equivalent calculation.
Planning a new layout for the room you're cooling? Upload a photo and try new furniture, finishes and blinds with Room Redesign.
