BTU Calculator

HVAC contractors use detailed Manual J calculations for proper equipment sizing — accounting for insulation, windows, orientation, infiltration, and internal gains. A common rule of thumb for quick estimates is BTU/h ≈ floor area × factor, often 20–25 BTU per square foot for moderate climates and higher for sunny rooms or poor insulation. This tool applies that shortcut so you can ballpark capacity before calling a pro.

One ton of cooling equals 12,000 BTU/h. Equipment labels and spec sheets use both units. Dividing your BTU/h estimate by 12,000 converts to tons for comparing mini-split listings, window units, and central system quotes. Round up slightly for kitchen-adjacent spaces and south-facing glass.

Oversized units short-cycle: they cool the air quickly but run too briefly to remove humidity, leaving a clammy feel. Undersized units run constantly on hot days and may never reach setpoint. Rule-of-thumb sizing is prone to both errors — use it for planning conversations, not final equipment selection or code compliance.

Worked example with defaults: 400 sq ft × 25 BTU/sq ft = 10,000 BTU/h, or about 0.83 tons. A typical 12,000 BTU/h (1-ton) unit would be slightly oversized for that room alone; a 9,000 BTU/h mini-split might suffice in a shaded, well-insulated space. A sunny, vaulted room with large west windows may need factor 30 or higher — re-run with a higher BTU-per-square-foot value.

Ceiling height above 8 feet increases volume and load even when floor area stays the same — this calculator uses floor area only. Add margin for high ceilings, crowded rooms with many occupants, or heat-generating equipment. Metric mode converts square meters to equivalent square feet internally before applying the factor. Two rooms with the same footprint but different window walls can need different factors — rerun sunny and shaded spaces separately.

Cooling load differs from heating load in most climates; do not assume the same BTU number works for a furnace or heat pump without a separate calculation. Duct losses, line-set length on splits, and altitude also change real-world performance — manufacturer derating charts apply at elevation. Compare your estimate to the nameplate of existing equipment only as a sanity check; older labels may reflect outdated sizing practice.

Limits: not Manual J, not ACCA certified, not a substitute for a licensed HVAC designer. Local codes may require documented load calcs for permits. For whole-house systems, room-by-room estimates do not replace aggregate load analysis — sum multiple rooms only as a rough upper bound, not an additive whole-home size.