Size comfort from the heat the room gains and loses.

The ENERGY STAR and AHAM sizing guidance works out to about 20 BTU per hour per square foot of floor area for a room air conditioner. A 200 square foot bedroom lands near 5,000 to 6,000 BTU/h; a 500 square foot open living area near 12,000. Then you adjust: add 600 BTU/h per person beyond two, add 4,000 if the unit is serving a kitchen, add about 10% for a very sunny room and subtract about 10% for a heavily shaded one.

One ton of cooling is 12,000 BTU per hour. The name is historical: it is the rate of cooling you get from a ton of ice melting over 24 hours. Room units are labeled in BTU/h and central systems in tons, so a 24,000 BTU/h unit is 2 tons and a 36,000 BTU/h unit is 3 tons. Central residential systems in the US typically run 1.5 to 5 tons.

Because cooling and dehumidifying are two different jobs and only one of them is fast. An oversized unit drops the air temperature to the thermostat setpoint in a few minutes and shuts off, long before enough air has passed over the cold coil to condense the moisture out of it. You end up with a room that is cold and damp, a compressor that short-cycles and wears out early, and a higher bill than a correctly sized unit running longer at steady state. In humid climates this is the single most common HVAC mistake.

Manual J is the ACCA standard method for residential load calculation. It works room by room through the actual construction: wall and window U-factors, orientation, shading, infiltration rate, duct location and leakage, internal gains and the local design temperatures. For a whole-house or ducted system it is the right tool, it is required by code in many jurisdictions, and any contractor who sizes a system by square footage alone is guessing. A rule of thumb like this one is for a single room unit and for sanity-checking a quote.

The baseline here works out to about 27 BTU per hour per square foot at an 8 ft ceiling in a mixed climate, scaled by climate zone from 0.88 in the warm south to 1.19 in the far north — so roughly 24 to 32 BTU/h per square foot. Published rules of thumb for cold US climates go higher, to 40 or even 50 BTU/h per square foot in zones 6 through 8, so treat the top of this range as a floor rather than an answer if you are heating in Minnesota or Maine.

One kilowatt is 3,412 BTU per hour, one BTU per hour is 0.293 watts, and one ton of refrigeration is 12,000 BTU per hour or 3.517 kW. A 9,000 BTU/h mini-split is 2.64 kW of cooling, or 0.75 tons. Note that for electric equipment you have to be careful which kW is being quoted: the cooling output in kW and the electrical input in kW are different numbers, and the ratio between them is the efficiency.

ASHRAE 62.2 sets the intermittent local exhaust minimum at 50 CFM for a bathroom and 100 CFM for a kitchen, or 20 CFM and five air changes per hour respectively if the fan runs continuously. The volumetric method gives a similar answer for a typical bathroom: a 5 by 8 ft room with an 8 ft ceiling is 320 cubic feet, and at eight air changes per hour that is 43 CFM. Take the larger of the two and add margin for duct losses.

Because the rating is measured at or near zero static pressure, and your duct is not. Every foot of duct, every elbow, every foot of flexible duct instead of rigid, and the exterior grille all add resistance, and the fan moves less air against it. A 100 CFM fan on 20 feet of flex with two elbows and a cheap louvered cap can easily deliver 50. Look for the HVI-certified rating at 0.25 inches of water column, use rigid duct, keep the run short, and use a hood with a proper damper.

IECC 2021 wood-frame wall minimums are R-13 in climate zones 1 and 2, R-20 or R-13 cavity plus R-5 continuous in zone 3, and R-30 or equivalent in zones 4 through 8. Ceilings run R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. Local amendments are common, so check what your jurisdiction has adopted — many are still on an earlier code cycle with lower numbers.

No. A wall with R-20 batts between 2×6 studs at 16 inches on center performs closer to R-15 or R-16 as a whole assembly, because the studs themselves are only about R-1.25 per inch and they bridge the insulation across roughly 23% of the wall area. That is why the code offers continuous exterior insulation as an alternative path: an inch of continuous foam outside the studs adds its full R-value everywhere, including over the framing.

They are usually quoted as U-value or as a maximum K in W/m²K, which is the inverse of resistance. To convert: R in US units equals 1 divided by (K × 0.17611). So a maximum K of 1.2 W/m²K is only R-4.7, and 0.74 W/m²K is R-7.7. Those are the limits set by the Argentine IRAM 11605 standard, and they are far below what the US IECC requires — R-13 in the mildest US zone is nearly twice the strictest IRAM limit.

It depends on the system. Room air conditioners, window units, single-head mini-splits and individual radiators are sized per room, which is what this calculator does. A central ducted system, a multi-zone heat pump or a boiler serving several radiators is sized for the whole house, and the sum of the individual room loads will overstate it — not every room peaks at the same moment. That diversity factor is one of the reasons Manual J exists.