BTU Calculator
Calculate the heating or cooling capacity (BTU/h, kW and tons) needed for a room, based on dimensions, insulation quality and climate zone.
What this calculates
Calculates the room heating or cooling load in watts, BTU/h and tons, and recommends an equipment category. Windows add 150 W each as a fixed supplement.
Formula used
Load (W) = volume × W/m³ (by climate) × insulation factor + windows × 150 W. BTU/h = W × 3.412. Tons = BTU/h ÷ 12,000.
Worked example
A 5 m × 4 m × 2.7 m room in a moderate climate with average insulation and 1 window: volume = 54 m³. Base load = 54 × 30 × 1.0 = 1,620 W + 150 W = 1,770 W ≈ 6,037 BTU/h ≈ 0.5 tons. Recommended: mini-split 9,000 BTU/h.
W/m³ reference by climate
Hot climate: 18 W/m³. Moderate: 30 W/m³. Cold: 45 W/m³. Very cold: 60 W/m³. These are starting estimates — real loads depend on orientation, glazing area, occupancy and ventilation.
When not to use this calculator
Full HVAC design for commercial buildings, multi-room systems and high-performance buildings requires a Manual J calculation or equivalent energy simulation. Use this for a first estimate only.
Frequently asked questions
How many BTU do I need per m²?
Roughly 80–100 BTU/h per m² (with 2.5 m ceiling) in a moderate climate with average insulation. Use more for cold climates or poor insulation.
What is a ton of AC?
1 ton = 12,000 BTU/h ≈ 3.5 kW. A 1.5-ton mini-split delivers 18,000 BTU/h.
How do I convert BTU to kW?
BTU/h ÷ 3,412 = kW. Or kW × 3,412 = BTU/h.
What size mini-split do I need?
Round up your BTU/h result to the nearest standard size: 9,000 / 12,000 / 18,000 / 24,000 BTU/h. Avoid oversizing — it causes humidity issues.
Assumptions
- W/m³ base values: hot 18, moderate 30, cold 45, very cold 60.
- Insulation factors: poor 1.3×, average 1.0×, good 0.75×.
- Windows: +150 W each (fixed supplement).
- Does not include solar gain, occupancy, equipment heat or ventilation loads.
This is a simplified heating/cooling estimate. HVAC equipment sizing for comfort, code compliance or energy efficiency certification requires a professional load calculation (e.g., ASHRAE Manual J or equivalent).
Common mistakes
- Oversizing — too large a unit short-cycles and fails to dehumidify properly.
- Not accounting for solar gain through south-facing glazing.
- Forgetting that open-plan spaces need the total combined area calculated.