TOOL GUIDE
Cooling load & AC sizing
Estimate a room’s sensible and latent cooling loads and check equipment capacity against both components.
Purpose & scope
Estimate a room’s sensible and latent cooling loads and check equipment capacity against both components.
Single-room steady-state estimate, not hourly HB/RTS. BTU/h sizes are references; verify actual equipment sensible/latent capacities.
Inputs
Use drawings, design conditions and manufacturer data. Units and example values follow the calculator; example values are not universal design requirements.
| Parameter | Value / interpretation |
|---|---|
| Floor area · m² | Room floor area for volume, lighting load and area-based rates. |
| Room height · m | Clear room height used for room volume. |
| Indoor temperature · °C | Target room air temperature. |
| Indoor humidity · % | Relative humidity of the room air. |
| Outdoor temperature · °C | Outdoor air temperature at the design condition. |
| Outdoor humidity · % | Relative humidity of the outdoor air. |
| Atmospheric pressure · kPa | Local absolute atmospheric pressure for air properties. |
| Net opaque external wall area · m² | Outdoor-facing opaque wall area, excluding glazing. |
| Wall heat transfer coefficient · W/m²K | Lower values mean better insulation. Unit: W/m²K. |
| Wall solar equivalent rise · K | Select the material/system appropriate to the actual application; verify reference values. |
| Exposed roof area · m² | Outdoor-exposed roof area; enter 0 if this room has no exposed roof. |
| Roof heat transfer coefficient · W/m²K | Lower values mean better insulation. Unit: W/m²K. |
| Roof solar equivalent rise · K | Select the material/system appropriate to the actual application; verify reference values. |
| External glazing area · m² | Outdoor-facing glazing area. |
| Glazing heat transfer coefficient · W/m²K | Lower values mean better insulation. Unit: W/m²K. |
| Incident solar irradiance · W/m² | Solar irradiance incident on glazing at the design orientation/time. |
| Solar heat fraction through glazing | Range 0–1. A value of 0.5 means 50% of incident solar heat enters through the glazing. |
| Solar fraction remaining after shading | Range 0–1: 1 means unshaded, 0 means fully shaded in this estimate. |
| Occupants · people | Simultaneous occupants at the design condition. |
| Sensible heat per person · W | Sensible heat gain per occupant for the activity level. |
| Latent heat per person · W | Latent heat gain from each occupant’s moisture emission. |
| Lighting power density · W/m² | Lighting power per floor area, multiplied by room floor area. |
| Equipment heat gain · W | Equipment heat gain in the room before the use factor. |
| Simultaneous equipment use fraction | A value of 0.8 means 80% of equipment heat gain occurs simultaneously. |
| Outdoor air entering room · m³/h | Outdoor air supplied directly to the room, excluding infiltration. |
| Air changes per hour from infiltration · 1/h | A value of 0.3 means infiltration per hour equals 30% of room volume; excludes outdoor-air ventilation. |
| Other sensible gains · W | Additional sensible gains not included above. |
| Other latent gains · W | Additional latent gains not included above. |
| Design allowance · % | Allowance applied to both sensible and latent loads. |
| Number of AC units · units | AC unit count sharing the load equally for reference sizing. |
| Temperature-cooling / total cooling capacity ratio | A value of 0.75 means 75% temperature cooling and 25% dehumidification. Use equipment data. |
| Actual / nameplate AC capacity ratio | A value of 0.9 means 90% of nameplate capacity at design conditions. Use equipment selection data. |
Steps
- Enter room size, indoor/outdoor conditions and actual wall, roof and glazing areas.
- Enter occupants, lighting, equipment, ventilation and infiltration without double counting outdoor airflow.
- Set unit count, equipment SHR and capacity factor at design conditions; inspect load components before selection.
Worked example
This reproducible training example uses the values below. Select the appropriate mode and overwrite restored project values. Example rates, K values and equipment losses are assumptions, not mandatory design criteria.
Examples use a decimal point and no thousands separator.
All example inputs
| Parameter | Value / interpretation |
|---|---|
| Floor area | 30 m² |
| Room height | 3 m |
| Indoor temperature | 24 °C |
| Indoor humidity | 50 % |
| Outdoor temperature | 35 °C |
| Outdoor humidity | 60 % |
| Atmospheric pressure | 101.325 kPa |
| Net opaque external wall area | 25 m² |
| Wall heat transfer coefficient | 1.8 W/m²K |
| Wall solar equivalent rise | 5 K |
| Exposed roof area | 0 m² |
| Roof heat transfer coefficient | 1 W/m²K |
| Roof solar equivalent rise | 15 K |
| External glazing area | 4 m² |
| Glazing heat transfer coefficient | 5.7 W/m²K |
| Incident solar irradiance | 400 W/m² |
| Solar heat fraction through glazing | 0.5 |
| Solar fraction remaining after shading | 0.7 |
| Occupants | 4 people |
| Sensible heat per person | 75 W |
| Latent heat per person | 55 W |
| Lighting power density | 10 W/m² |
| Equipment heat gain | 500 W |
| Simultaneous equipment use fraction | 1 |
| Outdoor air entering room | 60 m³/h |
| Air changes per hour from infiltration | 0.3 1/h |
| Other sensible gains | 0 W |
| Other latent gains | 0 W |
| Design allowance | 10 % |
| Number of AC units | 1 units |
| Temperature-cooling / total cooling capacity ratio | 0.75 |
| Actual / nameplate AC capacity ratio | 1 |
Expected results (rounded)
| Parameter | Value / interpretation |
|---|---|
| Sensible load | 2.93 kW |
| Latent load | 1.05 kW |
| Total load with allowance | 4.38 kW |
| Reference capacity per unit | 18000 BTU/h |
Small differences in displayed rounding are normal. A result that passes the entered limits does not establish suitability for the complete system.
Method & limitations
Single-room steady-state estimate, not hourly HB/RTS. BTU/h sizes are references; verify actual equipment sensible/latent capacities.
Steady-state heat-gain estimate, not hourly HB/RTS. Walls/roof: U·A·(Tout−Tin+ΔTsolar); glazing: U·A·ΔT + A·I·SHGC·Fshade. Solar equivalent rise is user-defined and is not derived from orientation, weather or design hour.
People: count × sensible/latent gain per person. Lighting: area × W/m². Equipment: heat gain × use factor. Outdoor-air total load uses dry-air mass flow × enthalpy difference; sensible load uses heat capacity at indoor humidity ratio, with latent load as the remainder. Cooler/drier outdoor air can produce negative components; aggregate sensible and latent loads are each floored at zero for sizing.
The allowance applies to both sensible and latent loads. The smallest reference BTU/h size meeting total, sensible and latent requirements is selected using unit count, assumed SHR and capacity factor. This does not replace manufacturer selection data or peak-load calculations with weather, thermal storage, solar orientation, thermal bridges and schedules. Floor/adjacent-zone, fan/duct and recovery effects are not calculated; use other gains as appropriate.
ASHRAE — Cooling load calculationsCommon mistakes
Confusing cooling capacity with electrical input; including glazing in opaque wall area; substituting room SHR for equipment SHR; double counting ventilation.
Projects & reports
- Choose or create an engineering project and enter your actual design inputs.
- Project inputs/results are autosaved in this browser. Where a saved-list panel is available, name and add the selected section or alternative to compare multiple options.
- Review the report before exporting PDF. In the print dialog select Save as PDF. Export JSON for backup and use Open file to restore it; PDF cannot restore project data.
Saved results do not automatically recalculate when another tool changes. Review any dependency warning, recalculate and update the affected results.
References & related tools
The tool implements the methods and limits stated above; linking a standard does not establish complete compliance with that standard.