180 ft² bedroom, all averages
- Input
- Area = 180 ft², Ceiling = 8 ft, Insulation = average, Sun = average, People = 2, Kitchen = no
- Working
- Base = 180 × 20 = 3,600 BTU. No factors or additions apply.
- Result
- 3,600 BTU
BTU Calculator
Estimate the BTU size for heating or cooling a room from floor area with published rule-of-thumb adjustments for ceiling height, insulation, sun and occupants. Runs entirely in your browser — your numbers never leave this device.
Insulation
Sun exposure
Kitchen
Cooling needed (BTU/h)
3,600
Pick the closest standard unit size at or above this figure.
- Floor area (ft²)
- 180
- Base BTU (area × 20)
- 3,600
- Final BTU/h
- 3,600
This is an estimate from published rules of thumb — a Manual J load calculation is the sizing standard for a real installation. Oversizing short-cycles and dehumidifies poorly; undersizing never catches up on the hottest days.
How it works
Base = floor area × 20 BTU per ft² (8 ft ceiling) | × (ceiling height in ft ÷ 8) | × insulation factor (poor 1.15 / average 1.0 / good 0.9) | × sun factor (shaded 0.9 / average 1.0 / sunny 1.1) | + 600 BTU per person beyond 2 | + 4,000 BTU for a kitchen
| Adjustment | Factor |
|---|---|
| Ceiling per ft over/under 8 | × (height ÷ 8) |
| Insulation poor | 1.15 |
| Insulation good | 0.9 |
| Very sunny room | 1.1 |
| Shaded room | 0.9 |
| Extra occupants | +600 BTU each beyond 2 |
| Kitchen | +4,000 BTU |
The sizing rule of thumb starts from floor area: 20 BTU per square foot for a room with an 8 ft ceiling. Every adjustment that follows is a published factor applied to that base, and each one is printed in the table so the final figure can be decomposed back into its parts. Ceiling height scales the result by height over 8, because the unit heats or cools a volume, not a floor plan. Insulation and sun exposure apply their factors — poor insulation multiplies by 1.15, good insulation by 0.9, a very sunny room by 1.1 and a shaded room by 0.9 — and occupancy and kitchens add fixed amounts: 600 BTU for each person beyond two, and 4,000 BTU for a kitchen's appliances.
The worked examples isolate the adjustments. The first is the bare base with no factors engaged: 180 ft² gives 3,600 BTU. The second switches on sun, extra occupants and the kitchen addition on the same room and nearly triples the figure to 9,160 BTU — a demonstration that the adjustments, not the base rate, dominate real rooms. The third shows ceiling height and shade working in opposite directions on a larger space.
This is a sizing estimate from published rules of thumb, stated plainly: it is not a Manual J load calculation. Manual J, the professional standard, models the building's construction, windows, orientation, infiltration and local climate in detail, and equipment selection for an installation belongs to that calculation. The rule-of-thumb figure is for comparing unit sizes and sanity-checking quotes, in the full knowledge of what it leaves out.
Worked examples
Same room, sunny, 4 people, kitchen
- Input
- Area = 180 ft², Sun = sunny (×1.1), People = 4, Kitchen = yes
- Working
- 3,600 × 1.1 = 3,960. Extra occupants: 2 × 600 = 1,200. Kitchen: +4,000. Total = 3,960 + 1,200 + 4,000 = 9,160 BTU.
- Result
- 9,160 BTU
400 ft² room, 9 ft ceiling, shaded
- Input
- Area = 400 ft², Ceiling = 9 ft, Sun = shaded (×0.9)
- Working
- Base = 400 × 20 = 8,000. Ceiling: 8,000 × (9 ÷ 8) = 8,000 × 1.125 = 9,000. Shaded: 9,000 × 0.9 = 8,100 BTU.
- Result
- 8,100 BTU
Frequently asked questions
Is this a Manual J calculation?
No, and the page says so directly. This is the published rule-of-thumb method — area times 20 BTU with the stated factors. Manual J is the detailed professional load calculation that models construction, glazing, orientation and climate, and it governs actual equipment installation decisions.
Why does a kitchen add 4,000 BTU?
Cooking appliances release heat into the room, and the rule of thumb accounts for it as a fixed 4,000 BTU addition rather than a factor. It applies to sizing a unit that cools or heats a kitchen space, as the second worked example shows.
Why do occupants only count beyond two?
The base rate already assumes a normally occupied room. The rule adds 600 BTU — roughly a resting person's heat output — for each person beyond the first two regular occupants, such as in a room that regularly holds a family or a meeting group.
What if the ceiling is vaulted or the room is open plan?
The ceiling factor uses the height entered, so a vaulted ceiling can be approximated with its average height. Open-plan spaces break the one-room assumption of the rule — the honest approach is to size for the whole connected volume's floor area rather than one zone of it, and to treat the result as the rough estimate it is.
Are room details stored?
No. The calculation runs entirely in the browser and nothing entered is uploaded, stored or tracked.
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