RTU / Packaged Unit Sizing Calculator
Translate building design loads into preliminary packaged rooftop-unit cooling tons, airflow and heating-capacity targets.
Design cooling load
5 tons
Load + margin
5.5 tons
Next 0.5-ton step
5.5 tons
Screening only — not a model selection
Screening supply airflow
2,200 CFM
Heating capacity
80 MBH
Heating capacity
23.4 kW
Use manufacturer performance at project indoor/outdoor design conditions before selecting a packaged unit. Nominal tonnage alone is not enough.
Turn the calculation into a complete design workflow
Use the free result as a quick engineering check, then continue into MEPFlow for connected system design, sizing and project calculations.
How the RTU / Packaged Unit Sizing Calculator Works
A ton of cooling equals 12,000 Btu/hr. The calculator converts the design cooling load to tons, applies an optional sizing margin and estimates screening airflow from a user-selected CFM-per-ton value.
Heating load is reported directly in MBH and converted to kW for comparison with electric or heat-pump equipment. The next common nominal half-ton step is shown only as a screening value, not as a final equipment recommendation.
Real RTU selection must use manufacturer performance at the project outdoor and indoor design conditions. Sensible heat ratio, ventilation air, latent capacity, fan static pressure, altitude, heating derates and part-load performance can change the required model.
Calculation basis
Cooling Tons = Cooling Load (BTU/hr) ÷ 12,000
When to Use This Calculator
- Early rooftop-unit budgeting
- Load-to-tonnage checks
- Preliminary airflow estimates
- Mechanical schedule planning
Engineering Checks Before Final Design
- Do not select equipment from nominal tons alone.
- Use manufacturer performance at design conditions.
- Outdoor-air and latent loads must already be included in the design load if applicable.
Frequently Asked Questions
How many BTU/hr are in one ton of cooling?
One refrigeration ton is 12,000 Btu/hr.
Is 400 CFM per ton always correct?
No. It is a common screening value, but actual airflow depends on sensible heat ratio, coil selection, supply-air temperature and equipment performance.
Should I oversize an RTU?
Use only a justified project margin. Excess oversizing can reduce humidity control and part-load efficiency.
Does this replace a manufacturer selection?
No. It establishes preliminary duty targets; final selection must use manufacturer data at project design conditions.