Estimate a room's cooling capacity or its heating/cooling load for a chosen temperature difference. Enter floor area (or width and length), ceiling height, insulation, and room conditions. Results are heat-transfer rates in BTU/hr and equivalent thermal watts, not the unit's electrical consumption.
Choose the right tab
Use AC for a room-area planning estimate. Enter the total floor area to cool, the ceiling height, regular occupants, room type, and the insulation, sun, and climate assumptions. Use the Square Footage Calculator to combine irregular floor areas before entering the total.
Use General for the model based on room width, length, height, and temperature change. Enter a temperature difference, not an outdoor or thermostat reading: 70°F indoors and 20°F outdoors gives a 50°F change. A 10°C difference is an 18°F difference; do not add 32 when converting it. Neither tab performs a full central-system load calculation.
ENERGY STAR cooling capacity by floor area
The current ENERGY STAR room-air-conditioner sizing guide gives this area chart for 8 ft ceilings:
| Area to be cooled (sq ft) | Capacity needed (BTU/hr) |
|---|---|
| 100 to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
| 1,000 to 1,200 | 21,000 |
| 1,200 to 1,400 | 23,000 |
| 1,400 to 1,500 | 24,000 |
| 1,500 to 2,000 | 30,000 |
| 2,000 to 2,500 | 34,000 |
Examples
150 sq ft bedroom, 9 ft ceiling
Average insulation, sun, and climate, two people. Base at 8 ft is 20 × 150 + 2,500 = 5,500. One extra foot of ceiling adds 1,000, so the load is 6,500 BTU/hr, equivalent to 1,905 W of cooling capacity. This is not an estimate that the air conditioner draws 1,905 electrical watts.
Why the planning result differs from the area chart
For 200 sq ft, an 8 ft ceiling, two people, a bedroom, and average conditions,
the model gives 20 × 200 + 2,500 = 6,500 BTU/hr. The ENERGY STAR chart
lists 6,000 BTU/hr for its 150-to-250 sq ft band. Use the published
chart and equipment guidance to assess that difference; the model does not
reproduce the chart's steps.
Kitchen with four people in a hot, sunny, poorly insulated room
Start from the same 6,500, multiply by 1.2 × 1.1 × 1.2 = 1.584, then add 4,000 for the kitchen and 1,200 for two extra people: 16,096 BTU/hr (4,717 W, 1.3 tons).
12 ft by 12 ft by 9 ft room, 50 F temperature rise
Four walls plus the floor: 2 × (12 + 12) × 9 + 144 = 576 ft². Average insulation (factor 1) at 0.4 BTU/(hr·ft²·°F) and a 50°F change is 0.4 × 576 × 50 = 11,520 BTU/hr (3,376 W, 1.0 ton). Good insulation (0.6) cuts that to 6,912; poor (2.1) raises it to 24,192.
Formula
The AC tab uses a linear planning rule, then layers ceiling height, multipliers, and add-ons. Its full formula is not an ENERGY STAR formula:
base = 20 × sqft + 2,500 + ceiling_extra
load = base × insulation × sun × climate × second_floor
+ kitchen + living + 600 × max(0, people − 2)
Insulation is 0.8 / 1 / 1.2 (good / average / poor), sun is 0.9 / 1 / 1.1
(shaded / average / sunny), climate is 0.85 / 1 / 1.2 (cold / average / hot),
and a second floor is × 1.1. A kitchen adds 4,000 BTU/hr; a living room adds
1,000. Ceiling extra is +1,000 BTU per foot from 8–10 ft; above 10 ft that
linear extra is multiplied by 0.2 + 8 / height.
The general tab is a simple UAΔT on four walls plus one floor:
Q = 0.4 × insulation × (2 × (W + L) × H + W × L) × ΔT
Insulation is 0.6 / 1 / 2.1 (good / normal / poor). ΔT is in Fahrenheit
(Celsius × 1.8). Watts use the International Table relationship 1 BTU =
1,055.05585262 J (round(BTU × 1055.05585262 / 3600)). A
refrigeration ton is 12,000 BTU/hr.
Model limits
Metric floor area and height use the international foot exactly: 1 ft =
0.3048 m. The calculator's linear 20 × sqft + 2,500 base approximates the
ENERGY STAR table but does not reproduce every step in that table. The ceiling,
insulation, climate, living-room, and second-floor factors are transparent
planning scenarios rather than ENERGY STAR or Manual J coefficients. Use the
table and selected equipment data for a room unit; use a qualified load
calculation for whole-building equipment, ducts, humidity, ventilation,
infiltration, windows, orientation, and design weather.
The NIST International Table BTU relationship supports the heat-rate conversion. A refrigeration ton is a capacity unit; the result does not estimate electrical input power or efficiency.