CFM to Cmh Converter
Convert airflow between cubic feet per minute and cubic meters per hour. Enter values for instant results with step-by-step formulas.
Reviewed for accuracy by Manoj Kumar, Mathematics Educator
CFM to Cmh Converter
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Formula: CMH = CFM x 1.699010796
Worked example โ 1200 CFM = 2038.81 CMH
Formula
CMH = CFM x 1.699010796
Where CMH is cubic meters per hour and CFM is cubic feet per minute. The conversion factor 1.699010796 is derived from converting cubic feet to cubic meters (1 ft3 = 0.0283168 m3) and minutes to hours (x 60). To reverse the conversion, divide CMH by 1.699010796 to get CFM.
Worked Examples
Example 1: HVAC System Airflow Conversion
Problem:A residential air handler is rated at 1200 CFM. What is the equivalent airflow in CMH?
Solution:CMH = CFM x 1.699010796 CMH = 1200 x 1.699010796 CMH = 2038.81 cubic meters per hour Additionally: Liters per second = 1200 x 0.471947 = 566.34 L/s
Result:1200 CFM = 2038.81 CMH
Example 2: Industrial Ventilation Requirement
Problem:A factory specification requires 5000 CMH of fresh air supply. What is this in CFM?
Solution:CFM = CMH / 1.699010796 CFM = 5000 / 1.699010796 CFM = 2942.88 cubic feet per minute This would require approximately a 2.5 HP fan motor for typical duct systems.
Result:5000 CMH = 2942.88 CFM
Frequently Asked Questions
What is CFM and how is it used in HVAC systems?
CFM stands for Cubic Feet per Minute and is the standard unit of airflow measurement used primarily in the United States and other countries that follow imperial measurement systems. HVAC engineers use CFM to specify the volume of air that fans, blowers, and air handling units can move through ductwork every minute. A typical residential HVAC system moves between 400 and 2000 CFM depending on the size of the home and the capacity of the equipment. Understanding CFM ratings helps technicians properly size ductwork, select appropriate fan motors, and ensure adequate ventilation for indoor air quality requirements.
What is CMH and where is it commonly used?
CMH stands for Cubic Meters per Hour and is the metric equivalent of CFM used to measure volumetric airflow rates. This unit is standard across Europe, Asia, and most countries that use the metric system for engineering specifications. CMH is widely used in international HVAC standards, industrial ventilation design, and clean room specifications where metric measurements are required. When manufacturers produce equipment for global markets, they typically list airflow specifications in both CFM and CMH to accommodate different regional standards. CMH is also the preferred unit in ISO ventilation standards and European building codes for specifying minimum ventilation rates.
What is the exact conversion factor between CFM and CMH?
The exact conversion factor from CFM to CMH is 1.699010796, which means that one cubic foot per minute equals approximately 1.699 cubic meters per hour. This factor is derived from the relationship between cubic feet and cubic meters (1 cubic foot equals 0.0283168 cubic meters) combined with the time conversion between minutes and hours (60 minutes per hour). To convert CFM to CMH, multiply the CFM value by 1.699010796. To convert CMH back to CFM, divide the CMH value by 1.699010796 or equivalently multiply by 0.58857777. These conversion factors are precise enough for all engineering and HVAC design applications.
How do I determine the right airflow rate for a room?
Determining the correct airflow rate depends on the room size, occupancy level, and intended use of the space. The general guideline is to provide between 15 and 60 CFM per person for office spaces, while industrial environments may require much higher rates depending on contaminant generation. You can calculate the required airflow by multiplying the room volume by the desired number of air changes per hour and dividing by 60 to get CFM. For example, a 1000 square foot room with 10-foot ceilings needs about 167 CFM for one air change per hour. ASHRAE Standard 62.1 provides detailed ventilation rate procedures that account for both occupancy and floor area components.
What are air changes per hour and how do they relate to CFM?
Air changes per hour (ACH) represents how many times the entire volume of air in a room is completely replaced in one hour. To calculate ACH from CFM, multiply the CFM by 60 to get cubic feet per hour, then divide by the room volume in cubic feet. For example, if a room is 2000 cubic feet and the ventilation system delivers 500 CFM, the ACH equals (500 times 60) divided by 2000, which gives 15 air changes per hour. Typical ACH requirements vary by space type: offices need 6-10 ACH, laboratories require 6-12 ACH, operating rooms demand 15-20 ACH, and clean rooms may need 20 to over 600 ACH depending on classification.
How does altitude and temperature affect airflow conversions?
Altitude and temperature significantly affect air density, which in turn impacts the actual mass flow rate even when the volumetric flow rate in CFM or CMH remains constant. At higher altitudes, air is less dense, so the same CFM delivers less mass of air. At 5000 feet elevation, air density is approximately 17 percent lower than at sea level. Similarly, hotter air is less dense than cooler air. HVAC engineers must apply correction factors when designing systems for non-standard conditions. The standard conditions for airflow measurements are typically 70 degrees Fahrenheit at sea level with 29.92 inches of mercury barometric pressure. Always specify whether CFM values are standard CFM or actual CFM.
What is the difference between CFM and SCFM?
CFM typically refers to actual cubic feet per minute at the current operating conditions, while SCFM stands for Standard Cubic Feet per Minute measured at standardized reference conditions. Standard conditions are usually defined as 68 degrees Fahrenheit (20 degrees Celsius), 14.696 psia (one atmosphere of pressure), and 36 percent relative humidity according to the Compressed Air and Gas Institute. The distinction matters because air volume changes with temperature and pressure. A compressor rated at 100 SCFM at sea level might only deliver 83 actual CFM at 5000 feet elevation. When converting between CFM and CMH, it is important to know whether the CFM figure is actual or standard to ensure accurate engineering calculations.
How do I measure airflow in an existing duct system?
Airflow in existing duct systems can be measured using several instruments and techniques. The most common method uses a pitot tube connected to a manometer to measure velocity pressure at multiple points across the duct cross-section, then calculating average velocity and multiplying by duct area. Hot-wire anemometers provide direct velocity readings and are easier to use for quick measurements. Balometers or flow hoods are placed over supply or return grilles to directly read the CFM flowing through each opening. For the most accurate results, take measurements at a location with at least 7.5 duct diameters of straight run upstream and 2.5 diameters downstream of any fittings or transitions to ensure stable airflow patterns.
What are common airflow rates for residential HVAC equipment?
Residential HVAC systems typically operate at airflow rates between 350 and 450 CFM per ton of cooling capacity, with 400 CFM per ton being the most common design target. A standard 3-ton air conditioning system should deliver approximately 1200 CFM of conditioned air. Bathroom exhaust fans range from 50 to 150 CFM, kitchen range hoods from 100 to 1200 CFM, and whole-house fans from 3000 to 6000 CFM. In metric terms, a typical 3-ton residential system delivers about 2039 CMH, while a standard bathroom exhaust fan moves approximately 85 to 255 CMH. These values help homeowners and contractors verify that installed equipment is performing within expected parameters.
Can I use this converter for compressed air flow rates?
Yes, you can use this converter for compressed air flow rates, but you must be careful about whether the values represent actual or standard conditions. The volumetric conversion factor between CFM and CMH remains the same regardless of the gas or pressure, since both units simply measure volume per unit time. However, compressed air applications often specify flow in SCFM or normal cubic meters per hour (Nm3/h) which are referenced to standard temperature and pressure conditions. If your compressed air specifications are in SCFM, the conversion to standard CMH uses the same 1.699 factor, but ensure both values reference the same standard conditions for the comparison to be meaningful and accurate.
References
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