Estimates room sensible + latent cooling load: fabric conduction Σ(U·A)·ΔT plus solar, occupants, equipment, lighting and fresh-air ventilation.
Tender-stage only. Verify final design to project spec and CIBSE Guide A.
For design guidance only. Always verify with a qualified engineer.
About this cooling load calculator
This free cooling load calculator estimates a room's summer sensible and latent cooling load. The heat that has to be removed to hold the design indoor temperature, adding up fabric conduction, solar gain, occupants, equipment, lighting and fresh-air ventilation. It is aimed at UK M&E and building-services engineers, contractors and designers sizing comfort cooling or an air-conditioning unit at tender or feasibility stage, and reports the total in W, kW and W/m² of floor. Everything runs in your browser. Nothing is uploaded.
How the cooling load is calculated
The room load splits into sensible and latent parts, following the CIBSE Guide A internal-gains and ventilation method. The sensible load is fabric conduction Σ(U × A) × ΔT across walls, glazing and roof, plus solar (per m² of glazing), occupants, equipment, lighting (per m² of floor) and ventilation Qs = ρ · cp · V̇ · ΔT using ρ·cp = 1.2 × 1.005. The latent load is the occupants' latent output plus ventilation Ql = ρ · hfg · V̇ · Δw using ρ·hfg = 1.2 × 2450, where Δw is the outdoor − indoor moisture difference in g/kg. ΔT is the outdoor − indoor design temperature difference.
Frequently asked questions
Is this a peak or a full dynamic cooling load?
It is a steady-state peak estimate for tender and feasibility work. It does not model thermal mass, admittance or the time lag of solar gains through the day. For detailed design confirm against CIBSE Guide A or a dynamic thermal model.
Why can some gains show as negative?
If the outdoor design temperature is at or below the indoor temperature, ΔT is zero or negative, so fabric and ventilation give no sensible cooling demand (shown as negative gains). The latent load from occupants and fresh air still applies.
What sensible and latent gain per person should I use?
The defaults are CIBSE sedentary-office figures, about 75 W sensible and 55 W latent per person. And both are editable for busier or more active spaces such as gyms, kitchens or retail.