Cooling load is the amount of heat energy that must be removed from a space to keep it at a desired temperature and humidity. It’s measured as a rate, typically in kilowatts (kW) or BTU per hour (BTU/h), and it represents the total thermal challenge your cooling system has to overcome at any given time. Every building has a cooling load that shifts throughout the day based on weather, occupancy, sunlight, and dozens of other factors.
Sensible Load vs. Latent Load
Total cooling load breaks down into two distinct types. Sensible cooling load is the heat that raises the air temperature, the kind you feel when you walk into a hot room. Latent cooling load is the moisture in the air that your system has to condense and remove. A packed gym full of sweating people has a high latent load. A sun-baked office with floor-to-ceiling windows has a high sensible load. Both require energy to deal with, but they stress your cooling system in different ways.
This distinction matters because a system sized only for temperature control can fail badly in humid environments. If outdoor air is more humid than indoor air, latent cooling loads exist on top of whatever sensible heat the system already handles. In hot, humid climates, latent load can account for a significant share of total cooling demand.
What Creates Cooling Load
Heat enters a building from multiple directions at once. The major sources include:
- Solar radiation: sunlight hitting walls, roofs, and especially windows
- Conduction: heat flowing through walls, roofs, and floors from warmer outdoor air
- Internal gains: people, lighting, computers, cooking equipment, and other heat-generating objects inside the space
- Ventilation: fresh outdoor air deliberately brought in for air quality
- Infiltration: uncontrolled air leaking through gaps, cracks, and openings in the building envelope
Ventilation and infiltration deserve special attention because they can be enormous contributors. Research from the National Institute of Standards and Technology found that in depressurized office buildings (where outdoor air gets pulled inward), cooling loads from infiltration alone were nearly five times higher than in pressurized buildings, jumping from about 600 BTU per square foot to 2,700 BTU per square foot. Quadrupling ventilation rates increased cooling loads from ventilation by a factor of four. In practical terms, a leaky building or one that requires large volumes of fresh air will need substantially more cooling capacity than an identical but tighter structure.
Cooling Load vs. Cooling Capacity
These two terms get confused constantly, but they describe opposite sides of the same equation. Load refers to how much cooling the building needs. Capacity refers to how much cooling the equipment can deliver. Getting a proper match between the two is the central goal of HVAC system design.
Oversizing is a common mistake. A system with far more capacity than the load demands will cycle on and off rapidly, wasting energy and doing a poor job controlling humidity. Undersizing means the system runs nonstop on the hottest days and still can’t keep up. An accurate load calculation done through a method like Manual J (for residential buildings) typically produces numbers that already include about 15 to 20 percent of built-in padding above what the building actually experiences at peak design conditions. That buffer exists because extreme outdoor temperatures are rare, and a system sized to the calculated load will already handle the vast majority of real-world conditions.
How Cooling Load Is Calculated
For simple residential estimates, many contractors use a rule of thumb: roughly one ton of cooling (12,000 BTU/h) for every 500 square feet of floor area. This gives a ballpark number but ignores window orientation, insulation quality, climate zone, ceiling height, and internal heat sources. It’s a starting point, not an engineering answer.
For commercial and institutional buildings, ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends two primary methods. The heat balance method (HBM) is the most rigorous. It models four interconnected processes: how heat interacts with the outside surface of each wall and roof, how it conducts through those materials, how it behaves on the inside surfaces, and how it ultimately reaches the room air. All of these are solved simultaneously using computer software.
The radiant time series method (RTSM) is a simplified version derived from the heat balance method. It calculates heat gain through each surface using response factors, then separates those gains into radiant and convective portions to determine how quickly they actually become cooling load. The RTSM replaced older simplified approaches like the transfer function method and the cooling load temperature difference method, both of which are now considered outdated.
Both methods require detailed inputs: construction materials and thicknesses, window types and orientations, occupancy schedules, lighting power densities, equipment heat output, ventilation rates, and local climate data. ASHRAE’s 2025 Handbook of Fundamentals includes significantly expanded climate data tables to reflect current conditions and forecasts, which directly affects the outdoor design temperatures used in these calculations.
Units and Conversions
Because cooling load is energy per unit of time, it’s expressed as a power measurement. The three units you’ll encounter most often are:
- BTU/h (British thermal units per hour): standard in U.S. residential and light commercial work
- kW (kilowatts): used internationally and in larger commercial applications. One kW equals 3,412 BTU/h.
- Tons of refrigeration: common in U.S. commercial HVAC. One ton equals 12,000 BTU/h, or about 3.5 kW.
A typical home might have a cooling load of 24,000 to 60,000 BTU/h (2 to 5 tons). A large office building could require hundreds or even thousands of tons. The number depends entirely on the building’s size, construction, location, and use.
Why Accurate Cooling Load Matters
Getting the cooling load right affects energy bills, comfort, equipment lifespan, and humidity control. A properly sized system runs longer cycles at lower intensity, which removes more moisture from indoor air and maintains steadier temperatures. It also avoids the wear and tear of constant on-off cycling that comes with oversized equipment.
Different building types have very different cooling profiles. Hospitals require 25 cubic feet per minute of outside air per person for patient rooms, compared to 20 cfm per person for offices and 15 cfm per person for classrooms and theaters. Retail stores are measured differently, at 0.2 to 0.3 cfm per square foot of floor area. Each of these ventilation requirements adds directly to the cooling load, because every cubic foot of hot, humid outdoor air introduced into a building is air that the cooling system must condition.
Climate change is also shifting cooling loads upward in many regions. Updated weather data, like the expanded datasets in ASHRAE’s 2025 edition, reflects higher design temperatures and changing humidity patterns. A building designed using climate data from 20 years ago may be under-cooled by today’s standards, which is one reason load calculations should use the most current data available.