Add up the heat gains that an air conditioner must remove from a room: envelope conduction, window solar, people, equipment, and infiltration air. A first-pass Manual-J-style estimate in BTU/h, tons, and watts.
The diagram is labeled with the same symbols as the input fields below.
The room starts hot-soaked with every gain running. An air conditioner sized at exactly the calculated load pulls it to setpoint - and shows why right-sized units run nearly continuously at design conditions.
This preview solves a handful of lumped nodes. The NovaThermal engine behind ThermalResults.com (coming soon) runs the same physics on tens of thousands of nodes - full transients with phase change, radiation, fluid loops, and Monte-Carlo design envelopes, GPU-accelerated at 400× real-solver speed - and hands you review-ready margin reports.
Every air conditioner is bought to defeat a list of heat gains, and the fastest way to a sane size is to write the list down. Conduction through walls and roof scales with area over R-value; windows bring both conduction AND the big hitter, solar gain - forty square feet of west glass at 200 BTU/h per square foot swamps everything else on a summer afternoon, which is why shading and low-SHGC glazing are the cheapest tons you will ever buy.
The living contributions matter too: people run about 450 BTU/h apiece, electronics convert watts to heat at 3.412 BTU/h per watt, and outside air sneaking in carries 1.08 BTU/h per CFM per degree of sensible load. Summing them puts you within planning range of the familiar rules of thumb - and shows exactly WHICH lever (glass, air sealing, equipment) actually moves your total.
The honest boundary: this is a peak steady snapshot. It has no thermal mass (a west room peaks hours after noon), no latent infiltration in muggy climates, and no duct losses in a hot attic. A full Manual J or an hour-by-hour simulation handles those; use this to scope, sanity check a contractor's bid, and see the breakdown before the detailed pass.
Rules of thumb run 20-40 BTU/h per square foot, but the honest answer depends on glass area and orientation more than floor area - which is exactly what this breakdown shows. Compute the list; do not guess from floor area alone.
It cools the air quickly and shuts off before the coil has run long enough to remove moisture, so the room lands clammy at setpoint. Sizing near the computed load keeps cycles long and humidity controlled.
About 450 BTU/h per person at rest, and 3.412 BTU/h per watt of electronics - a gaming PC and two occupants add roughly 2,600 BTU/h, a fifth of a small room unit all by themselves.