What is included in a cooling load calculation?

A cooling load calculation sums all heat gains the cooling system must remove: solar gain through windows (often the largest component), internal gains from people (90W each), lighting, and equipment, plus fabric gains through walls and roof. CIBSE Guide A provides the methodology used in UK building services design.

How much heat does each person add to a room?

Per CIBSE Guide A, a sedentary person in an office generates approximately 90W of sensible heat and 60W of latent heat (total 150W). People doing moderate physical work generate 140-210W sensible heat. This calculator uses the 90W sensible figure for typical office and commercial spaces.

What is a g-value for glazing?

The g-value (solar factor) is the fraction of solar radiation that passes through glazing. Single glazing has a g-value of about 0.85, double clear glazing 0.72, low-E double glazing 0.50, and solar control glass as low as 0.25. Lower g-values mean less solar heat gain but also less natural light.

What is a typical cooling load density for a UK office?

A typical UK office has a cooling load density of 60-100 W/m². Heavily glazed south/west-facing offices can exceed 150 W/m². Server rooms may reach 500-2000 W/m². These figures help validate detailed calculations and provide quick estimates for budget purposes.

Do I need to account for infiltration in cooling load?

Yes, in a full CIBSE calculation, infiltration and ventilation loads should be included. Air entering the building at outdoor temperature must be cooled to indoor temperature. This calculator focuses on the primary gains; add approximately 10-15% for infiltration in naturally ventilated buildings.

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Cooling Load Calculator — Total Heat Gains

Calculate total room cooling load from solar, internal, and fabric heat gains using CIBSE Guide A methodology. Size AC and cooling systems accurately.

Floor Area

Ceiling Height

Glazing Area

Glazing Type

Determines solar heat gain factor (g-value)

Primary Orientation

South and west facing = highest solar gain

Number of Occupants

90W sensible heat per sedentary person

Equipment Heat Load

Outdoor Design Temp

CIBSE summer design: 30-32°C

How We Calculate This

This calculator estimates room cooling load by summing all heat gain components per the CIBSE Guide A methodology used in UK building services design.

Heat gain components

Solar: Glazing area x peak irradiance x g-value x blind factor
Occupants: Number x 90W sensible (CIBSE Guide A)
Lighting: Floor area x W/m² (typical 10-15 W/m²)
Equipment: Sum of all equipment heat output
Fabric: Wall area x U-value x temperature difference

Peak solar irradiance (UK summer)

South: 500 W/m² peak
West: 450 W/m² peak
East: 400 W/m² peak
North: 100 W/m² (diffuse only)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.