TR to CFM and CFM to TR Calculator
How the formula works
TR (ton of refrigeration) is the cooling capacity of an air conditioning system. CFM (cubic feet per minute) is the volume of air the fan moves. HVAC designers need to convert between the two when sizing air handling units, fans, ducts and diffusers. This calculator works in both directions and gives you two methods. It can also estimate the electrical input power of a chiller, split AC or window AC.
Method 1: Rule of thumb
CFM = TR × CFM per TR
TR = CFM ÷ CFM per TR
A commonly used value is about 400 CFM per TR for comfort cooling. Many designers use a range of 350 to 450 CFM per TR.
Method 2: Air temperature difference
CFM = TR × 12000 × SHR ÷ (1.08 × ΔT)
TR = CFM × 1.08 × ΔT ÷ (12000 × SHR)
Unit conversions
1 TR = 12000 BTU/hr = 3.517 kW (thermal)
1 CFM = 1.699 m³/h = 0.4719 L/s
ΔT in °F = ΔT in °C × 1.8
Electrical power (optional)
Electrical input power (kW) = TR × kW per TR
COP = 3.517 ÷ (kW per TR)
Where
- TR = cooling capacity in tons of refrigeration
- CFM = airflow in cubic feet per minute
- ΔT = temperature difference between the return air and the supply air, in °F in the formula
- SHR = sensible heat ratio, which is the sensible load divided by the total load (typically 0.70 to 0.80 for comfort cooling)
- 1.08 = air constant for standard air at sea level (0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/hr)
- kW per TR = electrical power the equipment draws for each TR of cooling
- COP = coefficient of performance, the cooling kW produced for each electrical kW used
Typical kW per TR values
- Water cooled chiller: about 0.6
- Efficient air cooled chiller or VRF system: about 0.8
- Split AC, inverter: about 0.9
- Air cooled chiller: about 1.0
- Split AC, non inverter: about 1.1
- Packaged DX unit: about 1.2
- Window AC: about 1.3
These are typical values only. The real value depends on the model, the star rating, the load and the ambient temperature, so use the manufacturer data for the actual selection. Choose Custom in the calculator if you know the exact value.
How to use the calculator
- Select TR to CFM or CFM to TR.
- Select the method. For a quick estimate use the rule of thumb and pick the airflow per TR. For a better result use the air temperature difference with the sensible heat ratio.
- Enter the value and the other inputs that appear.
- If you want the electrical power, select the kW per TR for your equipment. Leave it on Skip if you do not need it.
- Click Calculate. You can then download the result as a PDF.
Important notes
- The result is for standard air at sea level. At high altitude the air is thinner and the airflow needed is higher.
- The rule of thumb is an estimate. The actual airflow for a space must come from the heat load calculation.
- Check the final airflow against the ventilation (fresh air) requirement of your code and project specification.
Worked example
Example 1: TR to CFM using the rule of thumb
An AHU has a cooling capacity of 10 TR. Find the airflow at 400 CFM per TR.
CFM = 10 × 400 = 4000 CFM
Airflow in m³/h = 4000 × 1.699 = 6796 m³/h
Airflow in L/s = 4000 × 0.4719 = 1888 L/s
Cooling capacity = 10 × 3.517 = 35.17 kW = 120000 BTU/hr
Example 2: TR to CFM using the air temperature difference
A 10 TR coil works with a supply to return air temperature difference of 11 °C and a sensible heat ratio of 0.75.
Step 1: ΔT in °F = 11 × 1.8 = 19.8 °F
Step 2: CFM = 10 × 12000 × 0.75 ÷ (1.08 × 19.8) = 90000 ÷ 21.384 = about 4209 CFM
Step 3: Airflow in m³/h = 4209 × 1.699 = about 7151 m³/h
This gives about 421 CFM per TR, which is inside the usual range of 350 to 450.
Example 3: CFM to TR
A fan delivers 2000 CFM with a temperature difference of 10 °C and a sensible heat ratio of 0.75.
ΔT in °F = 10 × 1.8 = 18 °F
TR = 2000 × 1.08 × 18 ÷ (12000 × 0.75) = 38880 ÷ 9000 = 4.32 TR
Cooling capacity = 4.32 × 3.517 = 15.19 kW
The airflow per TR here is about 463 CFM, which is slightly above the usual range. This shows a large airflow for the cooling capacity, so check the coil selection and the design conditions.
Example 4: Window AC and split AC (with electrical power)
A room needs a 1.5 TR air conditioner. Find the airflow and the electrical input power for a window AC and for an inverter split AC. Use 400 CFM per TR.
Airflow: CFM = 1.5 × 400 = 600 CFM = 1019 m³/h = 283 L/s
Cooling capacity: 1.5 × 3.517 = 5.28 kW = 18000 BTU/hr
Window AC at 1.3 kW per TR: electrical power = 1.5 × 1.3 = 1.95 kW, COP = 3.517 ÷ 1.3 = 2.71
Inverter split AC at 0.9 kW per TR: electrical power = 1.5 × 0.9 = 1.35 kW, COP = 3.517 ÷ 0.9 = 3.91
For the same cooling, the inverter split AC draws about 0.6 kW less than the window AC. The actual values depend on the model and the star rating, so check the data sheet.
Common mistakes
- Using the rule of thumb for final design. 400 CFM per TR is only a quick estimate. The real airflow must come from the room heat load calculation.
- Forgetting to convert °C to °F. The factor 1.08 works with ΔT in °F. A ΔT of 11 °C is 19.8 °F.
- Using the total load as sensible load. Total cooling includes latent load. Use the sensible heat ratio, or the CFM will be too high.
- Mixing up cooling kW and electrical kW. 1 TR is 3.517 kW of cooling capacity. It is not the power the compressor draws from the supply. The electrical power is TR × kW per TR.
- Using one kW per TR value for every AC. A window AC, a split AC and a chiller have different efficiencies. Select the right type or use the data sheet value.
- Ignoring altitude. At high altitude, air is less dense, so the actual airflow needed is higher than the sea level value.
- Using too low a supply air temperature difference. A very small ΔT gives a very high CFM, which means big ducts, higher fan power and more noise.
- Ignoring the fresh air requirement. The airflow must also meet the ventilation rate required by the code for the occupancy.
- Mixing CFM and m³/h. 1 CFM is about 1.7 m³/h. Always check the unit of the fan or AHU data sheet.
Frequently asked questions
How many CFM per TR is normal?
For comfort cooling, many designers use about 350 to 450 CFM per TR, and 400 CFM per TR is the most common rule of thumb. Process cooling and special applications can be different.
How do I convert TR to CFM?
Multiply TR by the CFM per TR value. For example, 5 TR × 400 = 2000 CFM. For a better result, use the temperature difference method: CFM = TR × 12000 × SHR ÷ (1.08 × ΔT in °F).
How do I convert CFM to TR?
Divide the CFM by the CFM per TR value. For example, 2000 CFM ÷ 400 = 5 TR. Or use TR = CFM × 1.08 × ΔT ÷ (12000 × SHR).
What is 1 TR in kW and BTU/hr?
1 TR equals 12000 BTU/hr, which is 3.517 kW of cooling capacity.
How do I find the electrical kW from TR?
Multiply the TR by the kW per TR of the equipment. For example, 10 TR at 0.8 kW per TR needs about 8 kW of electrical input power. The kW per TR comes from the equipment data sheet.
What is the difference between cooling kW and electrical kW?
Cooling kW (1 TR = 3.517 kW) is the heat the system removes. Electrical kW is the power the compressor and related equipment draw from the supply. Their ratio is the COP.
How many kW does a 1.5 TR split or window AC use?
Multiply the TR by the kW per TR. For a 1.5 TR inverter split AC at 0.9 kW per TR, the input power is about 1.35 kW. For a 1.5 TR window AC at 1.3 kW per TR, it is about 1.95 kW. The actual power depends on the star rating, the room temperature and the model, so check the AC data sheet.
How many CFM does a 1 TR, 1.5 TR or 2 TR AC need?
At 400 CFM per TR, a 1 TR AC needs about 400 CFM, a 1.5 TR AC about 600 CFM, and a 2 TR AC about 800 CFM.
What is the sensible heat ratio?
It is the sensible cooling load divided by the total cooling load. Comfort cooling usually has a value of about 0.70 to 0.80. The rest of the load is latent, which removes moisture from the air.
Why is the constant 1.08 used?
It comes from the density of standard air (0.075 lb/ft³), the specific heat of air (0.24 BTU/lb·°F) and 60 minutes per hour. It is valid only for standard air at sea level.
How do I convert CFM to m³/h and L/s?
Multiply CFM by 1.699 for m³/h and by 0.4719 for L/s. The calculator gives both results.
Can I use this to size a fan or duct?
It gives the airflow needed for a given cooling capacity. For final fan and duct sizing, also consider the static pressure, the fresh air requirement, the air velocity limits and the manufacturer data.