CFM vs TR: the complete HVAC sizing guide

Short answer

TR (tons of refrigeration) is a cooling capacity: the rate at which the HVAC system can remove heat from a space. CFM (cubic feet per minute) is a volumetric airflow: the rate at which air moves through a duct, diffuser, or coil. They are linked by the psychrometric equation, but they answer different engineering questions.

Use TR to select the chiller, DX unit, fan coil, or air handler capacity. Use CFM to size ducts, diffusers, exhaust fans, and to verify ventilation and air changes. The classic rule of thumb is 400 CFM per ton, but that only holds for typical comfort cooling with a sensible heat ratio around 0.75 to 0.85. Restaurants, gyms, humid climates, and data centres all break the rule of thumb in opposite directions.

What each quantity actually means

TR — tons of refrigeration

One ton of refrigeration is defined as the heat needed to melt one short ton (2000 lb) of ice at 0°C in 24 hours. In practical units:

• 1 TR = 12,000 BTU/h
• 1 TR = 3,517 W = 3.517 kW
• 1 TR = 12.66 MJ/h

TR shows up in equipment selection: a 5 TR split unit, a 400 TR chiller, a 3 TR FCU. The number describes total cooling capacity, sensible plus latent, at the manufacturer's rating condition.

CFM — cubic feet per minute

CFM is the volumetric flow rate of air. In metric it becomes cubic metres per hour or per second. CFM shows up in duct sizing, diffuser selection, exhaust rates, and ventilation:

• 1 CFM = 1.699 m³/h = 0.0004719 m³/s
• 1 L/s = 2.119 CFM

The equations that link them

Sensible cooling — the temperature side

The sensible heat carried by an airstream is:

Q_s (BTU/h) = 1.08 × CFM × ΔT (°F)

In SI:

Q_s (kW) = 1.23 × L/s × ΔT (°C) / 1000

Rearranging for airflow, if the sensible load is known and the supply-to-room temperature difference is fixed at typically 10 to 12°C for comfort cooling, the required CFM follows directly. This is why comfort cooling with a 20°F supply-to-room delta and a typical sensible heat ratio of 0.8 lands close to 400 CFM per ton.

Total cooling — the enthalpy side

Total cooling handles both sensible temperature change and latent moisture removal:

Q_total (BTU/h) = 4.5 × CFM × Δh (BTU/lb dry air)

The enthalpy difference Δh is read off the psychrometric chart between the entering coil condition and the leaving coil condition. Latent load matters because it can dominate in restaurants, laundries, natatoriums, and coastal climates.

Sensible heat ratio (SHR)

SHR = Q_sensible / Q_total. Comfort offices sit around 0.75 to 0.85. Kitchens and gyms fall to 0.6. Server rooms sit near 1.0. A lower SHR means the same TR delivers less sensible cooling, so CFM per ton drops.

When to check CFM and when to check TR

Check TR when

• Selecting a chiller, RTU, DX unit, FCU, AHU coil, or VRF outdoor unit.
• Comparing efficiency (kW/TR, EER, COP, IPLV).
• Sizing the primary chilled water flow at a fixed ΔT (typically 6 to 8°C).
• Estimating condenser or cooling tower load.

Check CFM when

• Sizing supply, return, and exhaust ducts.
• Selecting diffusers, grilles, and VAV boxes.
• Verifying air changes per hour (ACH) against code minimums.
• Sizing kitchen hood exhaust, toilet extract, mechanical ventilation, or smoke extract.
• Setting fan static pressure and motor size.

Worked example: 200 m² office

Consider a 200 m² open-plan office at 25°C indoor / 45°C outdoor, 50 occupants, LED lighting at 8 W/m², equipment load 12 W/m², no direct solar (interior zone), building envelope average.

1. Cooling load estimate. A block-load estimator returns roughly 130 W/m² total, so the total cooling load is 200 × 130 = 26,000 W = 26 kW = 7.4 TR.
2. Round up to equipment. Select an 8 TR AHU or split system.
3. Sensible split. With SHR ≈ 0.82, sensible load = 26 × 0.82 = 21.3 kW.
4. Supply airflow. At an 11°C supply-to-room ΔT:
   L/s = (Q_s × 1000) / (1.23 × ΔT) = 21,300 / (1.23 × 11) ≈ 1,575 L/s ≈ 3,340 CFM.
5. Ventilation cross-check. ASHRAE 62.1 gives roughly 8.5 L/s per person + 0.3 L/s/m² area: (50 × 8.5) + (200 × 0.3) = 485 L/s of fresh air. This must sit inside the 1,575 L/s supply, and dictates the outdoor air damper position, not the total supply CFM.
6. Ducts. Size the main supply duct at around 4 to 6 m/s using the friction-rate method or 0.8 to 1.0 Pa/m equal-friction method, then step down toward the diffusers.

The same building at high humidity (SHR dropping to 0.7) would still be 26 kW total, but sensible would be 18.2 kW, supply air would drop to about 2,850 CFM, and the coil would need lower entering water temperature or a deeper coil to hit dew-point.

Common mistakes to avoid

• Assuming 400 CFM/ton universally. Works for typical offices, fails badly in kitchens, gyms, natatoriums, and data centres.
• Sizing ventilation air by TR. Fresh air is set by ASHRAE 62.1 or the local ventilation code, driven by people and area, independent of cooling load.
• Ignoring the SHR. Two spaces with the same total kW can need very different CFM depending on latent share.
• Confusing capacity and airflow in equipment schedules. A schedule that lists "5 TR / 2,000 CFM" is stating two things — do not add them or divide them without understanding the underlying load.
• Ignoring altitude. Air density falls with altitude, so mass flow at a given volumetric CFM drops. Above 1,500 m the correction becomes non-trivial.
• Skipping the ACH check. Even after the sensible calculation gives a CFM, verify the room air-change rate against ASHRAE 62.1 or the local code minimum.

How this connects to the calculators on this site

Start with the Cooling Load Estimator to get a block TR for the space. Cross-check the resulting airflow with the TR to CFM Calculator using the SHR appropriate to the space type. Size the ducts with the Airflow & CFM Calculator and Duct Sizing Calculator. Verify kitchen hoods with the Kitchen Hood CFM Calculator and exhaust fans with the Exhaust Fan Sizing Calculator.

Frequently asked questions

Can I use CFM per square foot instead of a cooling load calculation?

For very early planning, yes: 1 CFM/ft² for offices, 2 CFM/ft² for retail, 4 CFM/ft² for restaurant dining, 6 to 8 CFM/ft² for kitchens. This is coarse. For construction-issue drawings, always run the proper cooling-load calculation.

What supply air ΔT should I use?

Comfort cooling: 10 to 12°C between supply and room. Low-temperature systems (cold air distribution): 15 to 18°C. Displacement ventilation: 3 to 5°C. The choice affects both CFM and diffuser selection.

How does duct velocity relate to CFM?

Velocity = CFM / duct area. Main supply ducts sit at 4 to 6 m/s in commercial applications, branches at 3 to 4 m/s, VAV runouts at 2 to 3 m/s. Higher velocities save space but increase static pressure and noise.

Do I need to consider air density for VRF or DX systems?

For most low-rise commercial installations under 1,000 m altitude, standard density is fine. For high-altitude sites, resorts in mountain regions, or specialty industrial applications, apply a density correction to both the airflow and the equipment capacity.

Use these tools next

• TR to CFM Calculator
• Airflow & CFM Calculator
• Cooling Load Estimator
• Duct Sizing Calculator
• kW to BTU Calculator
• HVAC Worked Example PDF

Verification note

This guide is planning support only. Final HVAC design must be verified against project specifications, ASHRAE 62.1 for ventilation, ASHRAE 90.1 for efficiency, SMACNA duct standards, and the applicable local mechanical code. Consult a licensed HVAC engineer for construction-issue calculations.