What is CMH in an Exhaust Fan? Fan Size Calculator (CMH, CFM)

Ventilation fan size calculator

What is CMH? CMH means cubic metres per hour (also written m³/h): the volume of air a fan moves. Fans sold in India are rated in CMH; 1 CFM is about 1.7 CMH. How much air do you actually need to move to hold a room at a given CO₂ level? Enter the room and its peak occupancy and get the answer in m³/h and CFM, with the air-change rate and the per-person outdoor-air rate that ventilation standards are written in.

Floor area (m²)

Ceiling height (m)

People at peak occupancy Size for the full room

Activity level

CO₂ level to hold (ppm) 1,000 ppm is the usual comfort target

Outdoor CO₂ (ppm) India is typically 420–450 ppm

— m³/h of outdoor air required

In CFM—

Air changes/hour—

Total airflow—

Per person—

Room volume—

Duty cycle at full occupancy—

How this is calculated

Single well-mixed zone, transient mass balance — the same model behind the ventilation-rate arithmetic in ASHRAE Standard 62.1:

V · dC/dt = G − Q · (C − C_out)

where V is room volume in m³, C is indoor CO₂, G is CO₂ generation from the occupants in m³/s, Q is outdoor airflow in m³/s, and C_out is the outdoor concentration. With constant G and Q this solves to C(t) = C_ss + (C₀ − C_ss) · e^−Qt/V, settling at a steady state of C_ss = C_out + 10⁶ · G / Q.

Per-person CO₂ generation is scaled from metabolic rate. The reference value is 0.0052 L/s (about 19 L/h) for a sedentary adult at roughly 1.2 MET — the figure the ventilation standards are built on. Other activity levels are scaled by MET ratio: sleeping about 0.9 MET, light activity 2.4 MET, hard exercise 6–8 MET. A school-age child is scaled by body surface area to roughly 0.6× an adult.

What this model assumes: the room is well mixed (no stratification or dead corners), generation and airflow are constant over the period, and there is no CO₂ source other than the occupants. Real rooms deviate — gas cooking, combustion heaters and fermentation all add CO₂; poor air distribution creates pockets that run higher than the average. Treat the output as a well-founded estimate of what a room like yours should do, and a measurement as the truth about what yours actually does.

Sizing the fan you actually buy

The calculator gives the airflow the room needs. The fan you buy has to deliver that airflow in your installation, which is not the same as its rating.

Rated airflow is free-air airflow. Manufacturers measure with nothing attached. Add ducting, a wall grille, a bird mesh and a filter and you commonly lose 20–40%. Size up accordingly.
Air has to get in as well as out. An exhaust fan can only remove as much air as can enter the room. In a sealed room the fan will run against its own pressure and move far less than its rating. Check that there is a deliberate inlet path — a louvre, a grille, an undercut door.
Placement decides effectiveness. Inlet and exhaust at opposite ends of the occupied zone sweeps the room. Both on the same wall short-circuits: the fresh air goes straight back out without reaching anyone.
Noise matters more than people expect. A fan that is too loud for the space gets switched off, and a fan that is switched off ventilates nothing. Slightly larger and slower usually beats smaller and faster.

Why the fan should not run all the time

The airflow above is what you need at peak occupancy. Most rooms are at peak occupancy a small fraction of the time, and every hour the fan runs unnecessarily in an air-conditioned building costs you the energy to cool and dehumidify air nobody needed.

The duty-cycle figure is what proportion of the time this fan would run with the room completely full, under CO₂ control with a 1,200 ppm on / 800 ppm off band. Be clear about what that number is and is not: it is the saving available even at peak load, and it is the smallest saving you will see. The larger one comes from all the hours the room is half full or empty, when a CO₂ controller runs the fan briefly or not at all while a timer would run it flat out. How big that is depends entirely on your occupancy pattern — the reasoning, and a method for estimating it for your own building, is in demand-controlled ventilation.

Questions about fan sizing

What size exhaust fan do I need for a room?

It depends on how many people are in it and what CO2 level you want to hold, not on floor area alone. Enter the room and occupancy above and the calculator returns the required airflow in m³/h and CFM. As a rule, buy the next size up — rated fan airflow is measured with no ducting, and real installations lose 20–40% to duct, grille and filter resistance.

What is the difference between CMH and CFM?

Both are airflow. CMH is cubic metres per hour, the metric unit used on most fans sold in India. CFM is cubic feet per minute, the imperial unit. 1 CFM is about 1.699 m³/h, so a 500 CFM fan moves roughly 850 m³/h. This calculator gives both.

How many air changes per hour do I need?

Air changes per hour is an output, not a target — it falls out of how many people are in the room and what CO2 level you want. A sparsely occupied large room needs few air changes; a packed small room needs many. That is why sizing a fan on ACH alone, without reference to occupancy, so often produces a fan that is too small.

Does this account for demand-controlled ventilation?

The airflow figure is the capacity you need when the room is at full occupancy — size the fan for that. The duty-cycle figure shows roughly what fraction of the time that fan would actually run under CO2 control with a 1,200 ppm on / 800 ppm off band, which is where the running-cost saving comes from.

Ventilation fan size calculator

How this is calculated

Sizing the fan you actually buy

Why the fan should not run all the time

The fan is half the answer. This is the other half.

Questions about fan sizing