To calculate CFM for a factory, multiply the building volume in cubic feet by the air changes per hour (ACH) you need, then divide by 60. The answer is the total air delivery, in cubic feet per minute, that your ventilation fans must move. Picking the right ACH for your process, and then checking each fan's air delivery at real static pressure, is what turns that number into a correct fan selection.
How to calculate CFM for a factory: the basic formula
The formula engineers use is simple:
CFM = (Length × Width × Height in feet × ACH) ÷ 60
Air changes per hour is the number of times the entire volume of air inside the building is replaced in one hour. Dividing by 60 converts that hourly figure into a per-minute air delivery, which is how most fan performance data is published.
If you work in metric, as most Indian plant drawings are, the steps are the same:
- Volume in cubic metres × ACH = required airflow in m³/h
- 1 cubic metre = 35.31 cubic feet
- 1 m³/h is roughly 0.589 CFM
Step-by-step worked example
Take a single-bay production shed in an industrial estate outside Mumbai, measuring 40 m long, 25 m wide, with an average height of 8 m to the roof.
- Find the volume. 40 × 25 × 8 = 8,000 m³, which is about 282,500 cubic feet.
- Choose an ACH. For this example we use 20 air changes per hour. This is an illustration only; your figure depends on the heat, fumes and occupancy in your building (covered below).
- Multiply. 282,500 × 20 = 5,650,000 cubic feet per hour.
- Divide by 60. 5,650,000 ÷ 60 = about 94,000 CFM.
- Cross-check in metric. 8,000 m³ × 20 = 160,000 m³/h, which converts to roughly 94,000 CFM. The two methods agree.
That 94,000 CFM is the total air your exhaust and supply fans must move together. It is not yet a fan count. To get there you need the air delivery of the specific fans you are considering, at the static pressure they will actually work against.
Choosing the right air changes per hour for an industrial facility
There is no single correct ACH for every factory. The figure you choose matters more than any other part of the calculation, because doubling the ACH doubles the CFM.
In India, the National Building Code of India 2016, Part 8 (Building Services) sets out recommended air changes for different types of occupancy, and the Factories Act, 1948, Section 13 requires every factory to maintain adequate ventilation by circulation of fresh air and a reasonable working temperature. For occupied spaces, ASHRAE Standard 62.1 takes a different route, setting outdoor air rates per person and per unit floor area rather than as air changes. Use these as your starting reference, then adjust for the realities of your shop floor:
- Heat sources: furnaces, ovens, moulding machines, compressors and welding stations all push the requirement up.
- Fumes, dust and moisture: processes that release vapour or fine particles need more air changes, and often local extraction at the source as well.
- Occupancy: a packed assembly line needs more fresh air than a mostly automated store.
- Roof and walls: an uninsulated metal roof in a coastal Maharashtra summer adds a large solar heat load.
- Climate: hot and humid conditions reduce how much cooling each air change delivers, so more airflow is usually needed for the same comfort.
For a deeper look at how these two measures relate, read our guide to understanding CFM and air changes per hour.
When heat is the real problem: the heat-load method
In foundries, forging shops and plants with large ovens, sizing by ACH alone can undershoot. HVAC engineers instead size ventilation to remove a known amount of heat using the standard sensible heat equation:
CFM = Sensible heat gain (BTU/h) ÷ (1.08 × allowable temperature rise in °F)
The heat gain comes from equipment ratings, people and solar load; the temperature rise is how much warmer you will allow the inside air to get than the outside air. A smaller allowable rise means a much larger CFM. Run both methods and design to the higher figure. Our factory fan capacity calculation guide walks through this in more detail.
Industrial fan sizing: floor area and ceiling height
Floor area alone is misleading. Two sheds with the same footprint can need very different CFM if one has a 6 m roof and the other a 12 m roof, because the volume, and therefore the air to be exchanged, is doubled. Always size on volume.
Height also affects where fans go. Hot air stratifies under the roof, so exhaust fans are most effective mounted high on the walls or roof, with fresh air inlets low on the opposite side. This draws cool air across the working zone and pushes hot air out at the top.
Keep ventilation and cooling separate in your plan. Exhaust fans and supply fans deliver air changes. Air circulators and man coolers do not add fresh air to the building; they provide spot cooling by moving air over workers. Alfa Fans air circulators are designed to throw air up to 10 metres, which is the figure to use when spacing them along a production line. For layout guidance by roof height, see our article on air circulator selection for ceiling height and layout.
How many industrial fans does a warehouse require?
Once you have the total CFM, the fan count follows:
Number of fans = Total required CFM ÷ Air delivery of one fan at the system's static pressure
Using our example, if you shortlist a hypothetical exhaust fan rated at 10,000 CFM at your working static pressure, 94,000 ÷ 10,000 = 9.4, so you install 10 fans. Always round up, and split the fans across walls or roof bays so no corner of the building is left stagnant.
Warehouses with high racking need extra attention. Tall racks block airflow, so you may need more, smaller fans spread out rather than a few large ones at one end.
Why static pressure changes the answer
A fan's catalogue air delivery at zero static pressure (free air) is always higher than what it delivers once you add louvres, bird mesh, ductwork, filters or weather cowls. Each of these adds resistance. Read the fan's performance curve at your estimated static pressure, not the headline figure, or the installed system will fall short.
For wall and roof ventilation with low resistance, propeller-type heavy duty exhaust fans are usually the efficient choice. Where air must travel through ducts, tube axial fans handle moderate static pressure, and centrifugal blowers with an aerofoil impeller are used where resistance is high, such as dust collection or long duct runs.
Common mistakes when sizing factory ventilation
- Using floor area instead of volume.
- Choosing fans from free-air ratings and ignoring static pressure.
- Counting air circulators and man coolers towards air changes.
- Installing exhaust fans with no planned inlet, so the building runs under negative pressure and airflow drops.
- Placing exhaust and inlet on the same wall, which short-circuits the air and leaves the floor untouched.
- Ignoring future expansion, such as an extra oven or a second shift.
What to send a manufacturer for an accurate fan selection
When you approach industrial fan manufacturers in India for a quotation, the more of this you share, the more accurate the selection:
- Building length, width and height (eave and ridge)
- Main heat sources and their ratings
- Any fumes, dust, moisture or corrosive gases
- Number of workers per shift
- Ducting, louvres or filters planned
- Available power supply and preferred mounting positions
Alfa Fans has manufactured heavy duty industrial fans in Mumbai since 1981, and our Research and Development team uses exactly this information to match air delivery and static pressure to your building. Where a standard model does not fit, we build custom made fans to your specification, backed by accredited test facilities and timely delivery. Kompass featured Alfa Fans among its Top 10 Industrial Fan Manufacturers in India in 2024. Learn more about us as a Manufacturer, Wholesaler and Exporter of industrial fans in India, or send your building details through our contact page for a fan selection and quotation.
Frequently asked questions
What is the formula to calculate CFM for a factory?
Multiply the building volume in cubic feet by the required air changes per hour, then divide by 60. For example, a 282,500 cubic foot shed at 20 air changes per hour needs about 94,000 CFM. In metric, multiply volume in cubic metres by ACH to get m³/h.
How many air changes per hour does an industrial facility need?
It depends on heat load, fumes, dust and occupancy, so there is no single figure. The National Building Code of India 2016, Part 8, gives recommended air changes by occupancy type, and hot processes often need more. For heavy heat sources, also size using the heat-load method and design to the higher result.
Do air circulators and man coolers count towards air changes per hour?
No. Air circulators and man coolers move air within the building for spot cooling but do not bring in fresh air. Air changes come from exhaust and supply fans, so plan the two separately.
Why does my installed fan move less air than the catalogue says?
Catalogue figures are often given at free air with no resistance. Louvres, mesh, ducts and filters add static pressure, which reduces air delivery. Always select fans using the performance curve at your actual static pressure.
Can Alfa Fans help size fans for my factory?
Yes. Share your building dimensions, heat sources, process details and any ducting, and the Alfa Fans Research and Development team will recommend suitable fans. Custom made fans can be built where standard models do not match your requirement.