Cooling Load
Calculator
Estimate the design cooling load by summing the major gains — envelope conduction, window solar, occupants, lighting & equipment, and ventilation. Result in BTU/h and tons (÷12,000) for equipment sizing.
Who it's for: MEP engineers, mechanical contractors, and architects sizing cooling equipment at schematic design — you'll need the envelope UA, window area and SHGC, occupant count, and lighting/equipment wattage. This is a simplified component-sum tool, not a full ACCA Manual J or ASHRAE RTS analysis.
How to use this calculator
The formula
Qtotal = (UA · ΔT) + (Aw · SHGC · E) + (N · 245 + N · 155) + (W · 3.412) + (CFM · 1.08 · ΔT)
UA = envelope conductance sum (BTU/h·°F)
ΔT = outdoor design dry-bulb minus indoor setpoint (°F)
Aw = window area (ft²) · SHGC = solar heat gain coefficient · E = peak irradiance (BTU/h·ft²)
N · 245 = sensible occupant heat (office activity, ASHRAE Fundamentals Table 1)
N · 155 = latent occupant heat (office activity)
W · 3.412 = lighting and equipment power converted from watts to BTU/h
CFM · 1.08 · ΔT = sensible ventilation load (1.08 = ρ·cp for standard air)
Tons = Qtotal ÷ 12,000
Worked example
Envelope conduction: 180 × 20 = 3,600 BTU/h
Window solar: 120 × 0.40 × 120 = 5,760 BTU/h
Occupants (sensible): 15 × 245 = 3,675 BTU/h; latent: 15 × 155 = 2,325 BTU/h
Lighting + equipment: 4,000 × 3.412 = 13,648 BTU/h
Ventilation: 300 × 1.08 × 20 = 6,480 BTU/h
Total = 35,488 BTU/h → 2.96 tons (≈ 507 ft²/ton)
When this estimate will be off
- Uses fixed occupant heat rates of 245 sensible / 155 latent BTU/h (office sedentary activity). Assembly halls, restaurants, gyms, and retail spaces have significantly higher latent loads — use the appropriate ASHRAE Fundamentals Table 1 values.
- Peak-coincident component sum only — does not apply the Radiant Time Series (RTS) time-delay correction for thermal mass. Heavyweight concrete and masonry construction stores and re-releases radiant heat over several hours; the result will read high for those assemblies.
- A single average irradiance value covers all glazing. If windows face multiple orientations, split them by orientation and sum separately using orientation-specific peak irradiance values from ASHRAE or local climate data.
- Latent load covers occupants only. Infiltration, kitchen or lab equipment, and process loads are not modeled. A full cooling load analysis by a licensed mechanical engineer is required for final equipment selection and permit.
Frequently asked questions
A common mistake: Uses fixed occupant heat rates of 245 sensible / 155 latent BTU/h (office sedentary activity). Assembly halls, restaurants, gyms, and retail spaces have significantly higher latent loads — use the appropriate ASHRAE Fundamentals Table 1 values.
Solar heat gain coefficient is the fraction of incident solar radiation that enters as heat. A lower SHGC means less solar cooling load. Values appear on the NFRC label and in product spec sheets. Triple-pane low-e glass typically runs 0.25–0.35; clear double-pane runs 0.55–0.65. ASHRAE 90.1 sets maximum SHGC limits by climate zone and orientation.
One ton of refrigeration equals 12,000 BTU/h — the rate at which one short ton of ice melts per day. HVAC equipment (chillers, rooftop units, mini-splits) is rated in tons, so dividing by 12,000 gives the equipment size directly. A typical small office runs 300–500 ft²/ton.
This simplified component sum omits radiant time delays, diversity factors, and infiltration. Expect results within 15–25% of a Manual J for a straightforward office. Use it for feasibility checks and early equipment budgeting; size final equipment from a full ACCA Manual J or ASHRAE RTS analysis by a licensed engineer.
1.08 is a combined constant equal to air density × specific heat in customary units (approximately 0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/h). Multiplying CFM × 1.08 × ΔT converts volumetric airflow and temperature difference directly into sensible BTU/h — a standard HVAC shortcut.
Sources
- ASHRAE Fundamentals Handbook (2021) Ch. 18, Table 1 — sensible and latent heat rates per occupant by activity level for cooling load calculations
- ASHRAE Standard 62.1 — minimum outdoor air ventilation rates used to size the ventilation sensible load
This free Cooling Load Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.