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Step one: measure and adjust the base load
Multiply room length by width to get square footage. As a rough planning rule, many sizing charts pair small rooms of around 150 square feet with about 5,000 BTU and larger ones with proportionally more, but check the manufacturer's chart for the unit you consider. Worked example: a 12 by 20 foot bedroom is 240 square feet, which falls near the middle of common chart ranges, so start from a base figure for that band. Write the number down before shopping, because every later adjustment builds on it.
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Step two: adjust for heat and conditions
Next, note what makes the room hotter than average. Heavy afternoon sun, a top-floor location, a kitchen opening into the space, tall ceilings or several regular occupants all raise the load, while deep shade can lower it. Continuing the example, suppose the bedroom is on the top floor with west-facing windows. A common practice is to nudge the base figure upward by a modest margin for sun and occupancy. Record each factor separately, so the final adjustment is a reasoned estimate instead of a guess, and you can explain it to a seller or technician.
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Step three: compare, verify and decide
Finally, match your adjusted number to unit specifications and ask a qualified installer when the space is unusual. An oversized unit may cool quickly yet cycle too often and handle humidity poorly, while an undersized one runs constantly. In the example, you would shortlist models whose rated capacity sits close to the adjusted figure, then compare noise, efficiency rating, window fit and installation needs. For open floor plans, older insulation or multiple rooms, request a professional load calculation. Keep the worksheet, because it simplifies quote comparison and repair conversations later.