Formula used
The calculator combines envelope, solar, internal, ventilation, infiltration, duct, and fan loads. Detailed mode separates sensible and latent contributions before applying diversity and sizing adjustments.
How to use this calculator
- Select quick or detailed calculation mode.
- Choose the input unit system and climate category.
- Enter the main room dimensions and add other rooms when needed.
- Set indoor and outdoor temperature, humidity, and elevation.
- Enter construction U-values, glazing, shading, doors, and skylights.
- Add occupants, lighting, appliances, equipment, and process loads.
- Enter infiltration, ventilation, recovery, duct, and fan assumptions.
- Choose sizing adjustments and compare an existing HVAC unit.
- Review the sensible, latent, total, airflow, and equipment-size results.
Example data
| Input | Example value | Purpose |
|---|---|---|
| Room size | 20 ft × 15 ft × 9 ft | Establishes area and volume |
| Indoor / outdoor temperature | 75°F / 95°F | Creates a 20°F design difference |
| Indoor / outdoor humidity | 50% / 55% | Estimates latent outdoor-air load |
| Wall U-value | 0.08 | Controls wall conduction |
| Windows | Two, 4 ft × 4 ft, west-facing | Adds conduction and solar heat |
| Occupants | Four, light activity | Adds sensible and latent heat |
| Ventilation | 60 CFM | Adds outdoor sensible and latent load |
| Design margin | 10% | Adds limited sizing allowance |
Cooling capacity reference
| Nominal tons | BTU/h | Approximate cooling kW |
|---|---|---|
| 1.0 | 12,000 | 3.52 |
| 1.5 | 18,000 | 5.28 |
| 2.0 | 24,000 | 7.03 |
| 2.5 | 30,000 | 8.79 |
| 3.0 | 36,000 | 10.55 |
| 4.0 | 48,000 | 14.07 |
| 5.0 | 60,000 | 17.58 |
Understanding HVAC cooling load
Cooling load is the rate of heat that an HVAC system must remove to maintain selected indoor conditions. Heat enters through walls, roofs, floors, windows, doors, ventilation air, infiltration, people, lights, appliances, and operating equipment. A reliable estimate separates sensible heat from moisture-related latent heat.
Sensible load changes indoor air temperature. Latent load removes water vapor and affects indoor humidity. Equipment with insufficient latent capacity may cool the space while leaving it uncomfortable or humid.
Window orientation and shading can strongly influence peak load. West-facing glass often receives intense afternoon solar exposure. Low-emissivity glazing, exterior shading, and lower solar heat gain coefficients can reduce required capacity.
Ventilation and infiltration can become major contributors in hot or humid climates. The calculator estimates moisture load from indoor and outdoor temperature, relative humidity, and elevation. Heat-recovery and energy-recovery inputs reduce applicable outdoor-air loads.
Duct losses depend on leakage, insulation, and location. Ducts inside conditioned space create smaller penalties than ducts in hot attics or outdoors. Fan power also becomes heat that the cooling system must remove.
Oversizing should be avoided. A unit that is much larger than the real load may cycle rapidly, waste energy, create temperature swings, and provide poor humidity control. Final equipment selection should also consider manufacturer performance data at the actual entering-air and outdoor conditions.
This page provides an engineering-style estimate, not a certified design. Residential projects may require an approved Manual J calculation. Commercial work may require RTS, heat-balance, CLTD, local code, ventilation, psychrometric, and equipment-selection procedures completed by qualified professionals.
Frequently asked questions
What is one ton of cooling?
One refrigeration ton equals 12,000 BTU per hour of cooling capacity.
What is the difference between sensible and latent load?
Sensible load changes temperature, while latent load removes moisture from the air.
Does floor area alone determine AC size?
No. Climate, insulation, glass, orientation, occupancy, ventilation, humidity, equipment, and ducts also affect the load.
Why does window orientation matter?
Different orientations receive different solar exposure throughout the day, changing peak heat gain.
What is a sensible heat ratio?
It is the sensible cooling load divided by the total sensible and latent cooling load.
How much sizing margin should I add?
Small margins are usually safer than large oversizing allowances. Confirm the final choice with accepted design procedures.
How does humidity affect cooling capacity?
Higher outdoor moisture increases latent load and may require greater dehumidification capacity.
Why include duct losses?
Heat gain and leakage from ducts outside conditioned space can noticeably increase required capacity.
Can this replace Manual J or a commercial load calculation?
No. It is a planning tool and should not replace required professional design documentation.
Why can an oversized AC unit perform poorly?
Short cycling can reduce moisture removal, efficiency, comfort, and equipment life.