How many BTU does your room need (and what a BTU calculator cannot see)
The "square meters times 600" rule forgets the afternoon sun, the windows, the people and the TV. Learn how to do the full calculation and the eight blind spots of any BTU calculator.
In a two-story house in the interior of São Paulo state, the 20 m² living room has a window facing west. In the afternoon, four people gather there with the TV on. The salesperson did "20 m² times 600" and sold a 12,000 BTU/h unit. In the morning, it works. After 3 pm, with the sun hitting the glass, the room does not go below 28 °C.
The unit is fine. The calculation was incomplete.
Heat load: a boat with six holes
Heat load is how much heat enters a room per hour and has to be carried out. Picture a boat with holes: the air conditioner is the bucket and must remove at least what comes in through all the holes together. There are six: walls, ceiling and floor; sun through the glass, usually the biggest; people; equipment; lighting; and outside air.
In the southern hemisphere, the west face gets the afternoon sun, the worst of all: it hits when the air and the walls have already stored the day's heat. In the northern hemisphere, swap north for south; east and west stay the same.
The simplified calculation, step by step
For an ordinary room (ceiling height of 2.50 m to 2.80 m), the holes become a few factors:
- Area: 600 BTU/h per m².
- Sun on the most exposed wall: multiply the area portion by 1.00 (shade), 1.15 (morning), 1.33 (afternoon) or 1.40 (all day).
- Top floor, penthouse or single-story house, with no occupied floor above: +12% on the area portion.
- Windows: +5% per ordinary window; a large window or glass pane, +9%.
- People: the first is already in the area factor; add 600 BTU/h for each extra person.
- Equipment that gives off heat (TV, computer, refrigerator): add 600 BTU/h each.
In the living room above: 20 × 600 = 12,000; afternoon sun, × 1.33 = 15,960; one window, × 1.05 = 16,758; three extra people, + 1,800; the TV, + 600. Total: about 19,160 BTU/h. The first commercial size above it is 22,000 BTU/h. The 12,000 BTU/h unit covers only about 63% of the afternoon load.
The calculator gets the math right, but only sees what you type
The BTU calculator on the E-Nogueira website (in Portuguese) uses exactly these factors. With the same data, it gives the same number, apart from rounding. It works like a bakery scale: it weighs exactly what is on the plate, but knows nothing about the loaf left off it.
The eight most common blind spots:
- High ceiling: there is no height field.
- Too much or unprotected glass: it counts windows, not square meters of glass.
- Open-plan rooms: living room with an open kitchen, a shop open to the corridor.
- Door always open or a lot of outside air coming in.
- Large heat sources: every piece of equipment counts as 600 BTU/h, but a 3,000 W oven gives off about 10,000 BTU/h.
- Physical activity or crowding: a gym, a full training room.
- Real sun and roof: a tree shading the window, a dark uninsulated metal roof.
- Local climate: the factors are the same across the whole country.
A real case: a 30 m² clothing store with a mezzanine, a 4.50 m ceiling and a glass door left open all day. The calculator gave 21,420 BTU/h, a 22,000 size, and the math is right. From the height alone, as a rough estimate, the real load could be about 60% higher. And the open door has not even entered the calculation.
How to decide
- Measure with a tape and check the sun side with your phone's compass before opening the calculator.
- Do the manual calculation and compare: a difference above 2% means some field was typed differently.
- Check the eight blind spots: none, use the result with confidence; any, treat the number only as an order of magnitude.
When to call a professional
If any blind spot exists, ask for a full heat load calculation before buying, as set out in ABNT NBR 16401-1 (the Brazilian standard for air-conditioning installation design). The calculator's cable and breaker range is also just a starting point: it does not know the distance to the panel or the condition of the wiring. The electrician defines the circuit, following ABNT NBR 5410 (the Brazilian low-voltage electrical installations standard). The technical basis is Brazilian; outside Brazil, your local code applies.
To keep going
The checklist "The BTU calculator and what it doesn't see" puts the steps and the eight blind spots on one sheet. It arrives as a PDF by email.
Five minutes with a tape measure are worth more than any calculation.
Marcelo Nogueira Souza — E-Nogueira, since 2014