Estimate how many BTUs per hour you need to cool a typical room or house — for a window unit or central AC — or to heat or cool a space by a given temperature change. Enter floor area (or width and length), ceiling height, insulation, and the conditions that add or cut load.
Formula
The AC tab starts from a linear ENERGY STAR–style rule of thumb, then layers ceiling height, multipliers, and add-ons:
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 1 BTU = 1,055 J (round(BTU × 1055 / 3600)). A
refrigeration ton is 12,000 BTU/hr.
ENERGY STAR cooling capacity by floor area
Area-only rule of thumb (8 ft ceilings, typical conditions) from EnergyStar.gov, as published on calculator.net:
| 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 (1,905 W) — a typical 6,000–8,000 BTU window unit.
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.