Quick answer: To calculate the HVAC cooling load of a space, add up every source of heat gain — heat conducted through walls, roof and glass; solar radiation through windows; heat from people, lights and equipment; and the load from fresh-air ventilation and infiltration. Sum these in BTU/hr and divide by 12,000 to get the load in tons of refrigeration (TR). For a quick estimate on typical spaces, expect roughly 1 ton per 400–600 sq ft.
What is HVAC cooling load?
Cooling load is the total amount of heat (measured in BTU/hr or kW) that an air-conditioning system must remove from a space to keep it at the desired temperature and humidity. It is the single most important number in HVAC design: undersize it and the room never cools; oversize it and you waste money, short-cycle the compressor, and lose humidity control.
The load has two parts. Sensible load changes the air temperature (walls, glass, lights, equipment, people's body heat). Latent load removes moisture (people breathing/sweating, fresh air, infiltration). Together they make up the total cooling load.
The cooling load formula
At its simplest, the total cooling load is the sum of all heat gains:
Q(total) = Q(walls+roof) + Q(glass conduction) + Q(solar) + Q(people) + Q(lights) + Q(equipment) + Q(ventilation) + Q(infiltration)
1 Ton of Refrigeration (TR) = 12,000 BTU/hr = 3.517 kWEach conduction component uses the classic heat-transfer relation Q = U × A × ΔT, where U is the material's heat-transfer coefficient, A is the area, and ΔT is the indoor–outdoor temperature difference (use CLTD values for a more accurate result).
How to calculate cooling load — step by step
- Measure the space. Record floor area, ceiling height, and the area and orientation of every external wall, window and door.
- Envelope gain. Calculate
U × A × ΔTfor walls, roof and glass. A typical design ΔT is 8–12 °C for Indian conditions. - Solar gain. Add solar heat gain through glazing (glass area × solar heat gain factor × shading factor).
- Internal gains. Add occupants (≈100–130 W sensible + latent per person), lighting (W/sq ft × area), and equipment/appliances.
- Fresh air & infiltration. Add the ventilation load from outdoor air (CFM based on occupancy) plus any infiltration.
- Add a safety factor. Apply 5–10% margin, then divide total BTU/hr by 12,000 for tonnage.
Cooling load rule-of-thumb chart
Use these only for early budgeting — always do a detailed calculation for the final design.
| Space type | Area per ton (sq ft/TR) | Approx. BTU/hr per sq ft |
|---|---|---|
| Residential bedroom / living | 500–600 | 20–24 |
| Private / small office | 250–300 | 40–48 |
| Open-plan office | 200–250 | 48–60 |
| Retail shop | 150–200 | 60–80 |
| Restaurant / kitchen | 100–150 | 80–120 |
| Server / IT room | 50–100 | 120–240 |
Worked example
Consider a 1,000 sq ft open-plan office in a hot climate:
- Envelope + glass + solar: ≈ 22,000 BTU/hr
- 10 occupants × 450 BTU/hr = 4,500 BTU/hr
- Lighting + equipment (≈3 W/sq ft): ≈ 10,200 BTU/hr
- Fresh air + infiltration: ≈ 11,300 BTU/hr
Total ≈ 48,000 BTU/hr. Divide by 12,000 → 4 TR. With a 10% margin, specify a 4.5 TR system. That matches the 200–250 sq ft/ton rule for an open office — a good sanity check.
Enter your room dimensions, occupancy and location, and get an accurate cooling load in tons in seconds. Free, no sign-up.
Open the HVAC Heat Load Calculator →Common mistakes to avoid
- Oversizing. Bigger is not better — an oversized unit short-cycles and leaves the room clammy.
- Ignoring latent load. In humid climates, moisture removal can be 25–35% of the total.
- Forgetting fresh air. Ventilation air is often the largest single load in occupied spaces.
- Using rules of thumb for the final design. They are fine for budgeting, not for equipment selection.
Related MEP calculators
Once you have the cooling load, size the rest of the system with the Chiller Tonnage Calculator, the AHU Sizing Calculator, and the Duct Size Calculator.
Frequently Asked Questions
How many tons of AC do I need per square foot?
As a rough guide, expect about 1 ton of cooling per 400-600 sq ft for residential spaces and 200-300 sq ft per ton for offices. Always confirm with a detailed heat-load calculation.
What is the difference between sensible and latent cooling load?
Sensible load changes air temperature (walls, glass, lights, equipment), while latent load removes moisture (people, fresh air, infiltration). Total cooling load is the sum of both.
How do you convert BTU to tons?
Divide the load in BTU/hr by 12,000. One ton of refrigeration equals 12,000 BTU/hr or 3.517 kW.
Why should I not oversize my air conditioner?
An oversized unit short-cycles, wastes energy and fails to remove humidity, leaving the space cold but clammy. Correct sizing gives better comfort and efficiency.