The bigger unit is not the safer choice

An air conditioner does two jobs at once: it lowers the temperature and it removes moisture. This section is explanation rather than a sourced figure: an oversized unit can reach the thermostat setpoint quickly and stop before it has run long enough to remove much moisture, which can leave a room cool but humid. The thermostat measures the temperature, not the humidity.

Many ductless units can slow down rather than switch off, which can soften this, but every model has a lowest output. Below it a unit cycles on and off, so ask the installer how the specific model you are quoted behaves at low load.

On this site’s own numbers, much of the overshoot comes from rounding up to the next standard size. The worksheet rounds to our own list of common nominal sizes, 6,000, 9,000, 12,000, 15,000, 18,000, 24,000, 30,000, 36,000 BTU per hour (our list, not a sourced market survey), so a room that computes to something between two sizes gets the larger one. The table below uses the same calculation as the sizing tool across a range of room areas at otherwise ordinary settings, and shows how far the rounding goes.

The pattern is uneven, and that is the point. A room that lands just above a rung gets a near-perfect match; a room that lands just below one gets a substantial overshoot through no fault of anybody’s. A 250 square foot room asks for 7,000 BTU per hour on the published table^[1] and there is no 7,000 size in our list, so it steps up. A 450 square foot room asks for 12,000^[2] and lands almost exactly on a rung.

Two further things push people up the ladder unnecessarily. The first is a leaky-envelope setting used defensively: our own leaky multiplier is 1.2 ^[3], and applying it to a house that is merely average, on top of rounding up, can push the head up one size. The second is sizing for the worst hour of the worst day: equipment sized to the single most extreme afternoon of the year is oversized for nearly every other hour you use it.

When your room lands between two sizes, look at the gap before choosing the larger one. A small shortfall shows up on the hottest afternoons; a large overshoot can affect humidity control across the season. Ask the installer to explain the choice against a load calculation.

If the gap is large, a different layout can be a better answer than a different box — combining two small rooms onto one correctly sized head, or moving a head to where the air can actually reach both spaces. That is a zoning decision, and it is why the zone worksheet exists as a separate tool rather than a checkbox here.

And if a contractor proposes a size well above the number, ask what load calculation they ran. A room-by-room Manual J load calculation with your window areas and orientations in it is a better guide than any table on this site, including the one the tools are built on.