Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate a room's BTU/hr requirement from area, adjusted for insulation, sun exposure, and ceiling height.
4000
This is a more granular, room-level BTU estimate than the HVAC Size Calculator's whole-home tonnage rule of thumb -- useful for sizing a window/portable AC unit or a space heater for a specific room, adjusting a base per-square-foot rate for real factors that change a room's actual heating/cooling need.
Enter your room's area, a base BTU-per-sqft rate (20 is a common starting point for cooling estimates), an insulation quality factor (1.0 for average, higher for poor insulation, lower for excellent insulation), a sun exposure factor (higher for a room with large south/west-facing windows), and ceiling height (rooms taller than the standard 8ft have proportionally more air volume to condition).
BTU/hr = Area × Base Rate × Insulation Factor × Sun Factor × (Ceiling Height ÷ 8)
Use this to size a window air conditioner, portable AC unit, or space heater for one specific room, where a whole-home tonnage calculation isn't the relevant question.
Each factor multiplies the result, so small changes compound. Rather than guessing, pick values from what you can observe in the room.
Hypothetical example: a 300 sq ft west-facing room with older insulation and a 9 ft ceiling. Base rate 20, insulation factor 1.15, sun factor 1.1, ceiling factor 9 / 8 = 1.125. BTU/hr = 300 x 20 x 1.15 x 1.1 x 1.125 = about 8,538. This suggests looking at units labeled around 9,000 BTU, not the 6,000 BTU that a flat 20-per-square-foot estimate would have implied.
Window and portable units are sold at fixed capacities, so compare the result to the label and choose one that is close. A unit that is slightly under your result may run constantly on the hottest days, while one far above may cool too fast and leave the air clammy. If your result lands between two sizes, consider how often the room is used on peak days and whether you can add shading or a curtain to reduce load.
People and equipment add heat. A widely used approach adds capacity if more than a couple of people regularly occupy the room, or for a kitchen, but the tool has no field for that, so add a margin yourself, or raise the base rate. Rooms that connect to unconditioned spaces, or that have a lot of glass, may need more than the factors capture.
You can use the same estimate as a rough guide to the space heater capacity of a small room, but heaters are usually rated in watts. Multiply watts by 3.412 to compare with BTU per hour. Since heating needs depend on outdoor temperature differences, the heating load calculator on this site is a better model for cold climates.
A taller ceiling means more actual air volume per square foot of floor, so a room with a vaulted or unusually tall ceiling needs proportionally more cooling/heating capacity than its floor area alone would suggest.
A room with large, unshaded south- or west-facing windows in a sunny climate should use a higher factor (e.g., 1.1-1.2), while a shaded or north-facing room can use a lower one (e.g., 0.9).
Multiply watts by 3.412 to get BTU per hour. For example, a 1,500 watt heater delivers about 5,118 BTU/hr.
This tool has no occupancy input, so consider adding a margin yourself if several people or heat-producing equipment are regularly in the space.
No. Oversized units cycle off quickly and remove less humidity, so match capacity closely.
It is designed for rooms. For a whole home, the HVAC size calculator or a professional load calculation is more appropriate.