BTU Calculator
Calculate the BTU rating you need to heat or cool a room. Enter square footage, ceiling height, and insulation to size your unit.
Last updated 2026-08-21
What Is a BTU Calculator?
A BTU calculator finds the heating or cooling capacity you need for a room in British Thermal Units per hour. You enter the room size, ceiling height, insulation, and window load. The result helps you pick a correctly sized heater or air conditioner so the space feels comfortable without wasting energy.
How It Works
BTU = Area × 20 × (Ceiling height ÷ 8) × Insulation factor × Window factor.
Example
A 20 ft × 15 ft room with 8 ft ceilings, good insulation, moderate windows: 300 × 20 = 6,000 BTU.
How to Calculate
- Measure the room length and width in feet and multiply for the area. A 20 ft by 15 ft room is 300 square feet.
- Enter the ceiling height. The tool scales from a standard 8 ft ceiling.
- Select your insulation level and window exposure, which adjust the base load up or down.
- The tool multiplies the area by about 20 BTU per square foot. For 300 square feet with good insulation, that is 6,000 BTU.
- For a single unit, size to the largest zone or the total square footage the unit serves.
BTU per Square Foot by Room Type
| Room Type | BTU per Sq Ft |
|---|---|
| Bedroom | 15 to 20 |
| Living room | 20 to 25 |
| Kitchen | 20 to 30 |
| Basement | 15 to 20 |
| Sunroom / high sun | 25 to 30 |
Sources
Frequently Asked Questions
What is a BTU?
A British Thermal Unit — the heat needed to raise one pound of water 1°F. Cooling and heating are sized in BTU/hour.
Is 20 BTU/sq ft accurate?
It is a good rule of thumb for typical rooms. High ceilings, sun exposure, and poor insulation push the number up.
Should I size to the larger room?
For a single unit, size to the largest zone or use the total square footage of the space it serves.
Estimate the cost with the HVAC Cost Calculator. Also try: AC Size Calculator, Heating Calculator or browse HVAC Calculators