BTU Heating Calculator
Estimate heating load in BTU per hour from floor area, climate preset, and insulation quality. Optional ceiling height reports room volume. Rule of thumb only. Not Manual J.
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How to estimate heating BTUs
- Enter room length and width. Ceiling height defaults to 8 and is used for volume only.
- Pick a climate preset or type a custom BTU per square foot value.
- Choose insulation quality. Poor multiplies load up; excellent multiplies it down.
- Read BTU/h, approximate tons (BTU/h ÷ 12000), floor area, and the factors used.
Heating load estimate
Rule-of-thumb BTU/h from area and climate
| BTU/h | British thermal units per hour of heat delivery |
|---|---|
| Floor area | Length × width converted to square feet |
| Climate factor | Preset BTU per square foot |
| Insulation factor | Multiplier for envelope quality |
| Ton | 12000 BTU/h |
| Manual J | Professional load calculation this page does not replace |
What this estimate computes
Heating load here is floor area times a BTU-per-square-foot factor times an insulation multiplier. A 20 ft by 15 ft room in a moderate climate with average insulation is 10,500 BTU/h.
BTU, area, climate, and insulation
A BTU is a traditional heat energy unit. Area sets how much envelope you heat. Climate presets stand in for outdoor design severity. Insulation adjusts for leaky versus tight construction.
How the factors combine
Dimensions convert to feet. Area multiplies the climate (or custom) BTU/ft² value, then the insulation factor. Tons equal BTU/h divided by 12,000. Volume is reported for context and does not change the area rule.
Limits
Rule of thumb only. Not Manual J, not duct design, not a permit document. Confirm equipment with a qualified contractor. See the disclaimer.