How many BTU do I need per square meter?

A rough estimate for a well-insulated room in a moderate climate is about 30 W/m³ (roughly 80–100 BTU/h per m² with 2.5 m ceilings). In cold climates or poorly insulated rooms, use 45–60 W/m³. In hot climates for cooling, use 18–25 W/m³.

What is a ton of air conditioning?

One ton of cooling equals 12,000 BTU/h or approximately 3.5 kW. A 1.5-ton mini-split delivers 18,000 BTU/h (≈ 5.3 kW). Tons are the unit used in the USA and some other countries for AC capacity.

What size mini-split do I need for a room?

Calculate the room's BTU/h requirement, then round up to the nearest standard mini-split capacity: 9,000, 12,000, 18,000 or 24,000 BTU/h. Mini-splits perform best when sized correctly — oversizing causes short-cycling and humidity problems.

BTU Calculator

Q: How do I convert BTU to kW?

Divide BTU/h by 3,412 to get kW. For example, 12,000 BTU/h ÷ 3,412 = 3.52 kW. Conversely, multiply kW × 3,412 to get BTU/h.

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.

How to use this calculator

Enter the room length, width and ceiling height in meters.
Enter the number of windows — each adds approximately 150 W to the load.
Select insulation quality — "poor" for pre-1980 construction with minimal insulation, "average" for standard code-compliant buildings, "good" for modern well-insulated or PassivHaus buildings.
Select the climate zone that best matches your location.
Click "Calculate BTU" to see the load in watts, BTU/h and tons, plus an equipment category recommendation.

HVAC sizing methodology — simplified vs. Manual J

This calculator uses a volume-based approximation (W/m³ × insulation factor). It is suitable for quick estimates. Professional HVAC design uses ASHRAE Manual J (USA), CIBSE Guide A (UK) or equivalent national standards, which account for:

Solar gain through glazing (orientation, shading, glass specification)
Internal heat gains from people, lighting and equipment
Air infiltration and mechanical ventilation rates
Thermal mass of building fabric
Design temperatures (outdoor dry-bulb and wet-bulb for cooling; outdoor minimum for heating)

For a single room with standard construction, the volume-based method is typically within 15–25% of a Manual J result. For whole-house systems, multi-zone HVAC or energy-efficient buildings, use professional load calculations.

Avoiding the most expensive HVAC mistake — oversizing

The instinct to "buy bigger for safety" is counterproductive with HVAC. An oversized unit:

Short-cycles: Reaches setpoint quickly and shuts off before properly dehumidifying the air — results in damp, clammy conditions even at the right temperature.
Wears out faster: Frequent start/stop cycles stress the compressor more than continuous operation.
Consumes more energy per hour of actual conditioning than a correctly sized unit running longer.

Size to the load, not to a "maximum possible" scenario. Use the insulation factor to account for building quality rather than padding the result.

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.

Technical warning

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.

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