BTU Calculator
Estimate the BTUs needed to heat or cool a room based on size, ceiling height, insulation, and room conditions.
Results
This is a quick sizing estimate, not a Manual J load calculation. Confirm equipment sizing against project plans, local conditions, and an HVAC professional when accuracy matters.
How to use this calculator
- 01Choose heating or cooling mode.
- 02Enter room dimensions or known floor area.
- 03Enter ceiling height, insulation, climate or load level, sun exposure, occupants, and room type.
- 04Read the estimated BTU/hr, recommended range, and related result values.
Understanding the math
Cooling starts near 20 BTU/hr per square foot for a normal 8 ft ceiling room. Heating uses a wider square-foot range because climate changes the load more sharply. The calculator adjusts for ceiling height, insulation, climate or load level, sun exposure, extra occupants, and room type.
base load = area x BTU/hr per sq ft adjusted load = base load x ceiling x insulation x sun x room type extra occupants = occupants above 2 x occupant allowance BTU/hr = adjusted load + extra occupants
This is a quick sizing estimate, not a Manual J load calculation.
For a whole-home heating and cooling load estimate with Manual J-style inputs, use the HVAC Load Calculator.
For furnace-specific sizing, use the Furnace Size Calculator because it accounts for furnace efficiency and input BTU.
For AC equipment sizing, use the AC Tonnage Calculator to convert the cooling load into tons and a nominal unit size.
For a single ductless room or zone, use the Mini Split Size Calculator to turn room conditions into a common mini split BTU size.
BTU sizing quick reference
Use these ranges as a quick check. Real loads can move outside the table when insulation, glass area, air leakage, sun exposure, or local climate is unusual.
| Space | Area | Cooling | Heating |
|---|---|---|---|
| Small bedroom | 100–150 sq ft | 5,000-6,000 BTU/hr | 3,000-8,000 BTU/hr |
| Large bedroom | 180–250 sq ft | 6,000-8,000 BTU/hr | 5,500-13,500 BTU/hr |
| Living room | 300–450 sq ft | 8,000-12,000 BTU/hr | 9,000-25,000 BTU/hr |
| Kitchen / open room | 400–600 sq ft | 12,000-18,000 BTU/hr | 12,000-33,000 BTU/hr |
| Small apartment / zone | 600–900 sq ft | 18,000-24,000 BTU/hr | 18,000-50,000 BTU/hr |
Frequently asked questions
How do I calculate BTU for a room?
Start with room area, then account for ceiling height, heating or cooling mode, insulation, climate, sun exposure, occupants, and room type. This calculator applies those adjustments and rounds the result to a practical BTU/hr estimate.
How many BTUs do I need?
It depends on room size, ceiling height, insulation, climate, sun exposure, and whether the room is being heated or cooled. Larger rooms, tall ceilings, poor insulation, and harsher conditions need more BTU/hr.
How many square feet will 30,000 BTU heat?
A 30,000 BTU/hr heater might cover roughly 500 to 1,000 sq ft, depending on insulation and climate. Tight rooms in mild weather land near the high end. Poor insulation or cold weather pushes the coverage lower.
How many square feet does a 12,000 BTU AC cool?
A 12,000 BTU/hr air conditioner cools roughly 500 to 600 sq ft in many normal rooms. Tall ceilings, strong sun, poor insulation, kitchens, and hot climates can reduce that coverage.
How do I convert BTU to watts or kW?
Use watts = BTU/hr / 3.412142. Then use kW = watts / 1000. For example, 12,000 BTU/hr is about 3,517 W, or 3.52 kW.
Can I use this calculator with metric measurements?
Yes. Use the Imperial / Metric toggle. Inputs, results, and reference values follow the active unit system where applicable.