The most common reason a ventilation system underperforms is not the fan's quality — it's the size. An undersized fan never clears the room; an oversized one wastes energy and creates noise and drafts. This guide shows the standard calculation used by HVAC engineers, with tables you can apply directly.
Step 1: Measure the Room Volume
Example: a bathroom 2.5 m × 2 m × 2.7 m = 13.5 m³.
Step 2: Choose Air Changes per Hour (ACH)
ACH is how many times per hour the entire room's air is replaced. Standards by application:
| Application | Recommended ACH |
|---|---|
| Residential bathroom | 8–10 |
| Kitchen (domestic) | 15–20 |
| Commercial kitchen hood exhaust | 30–60 (by hood design) |
| Bedroom / living room | 4–6 |
| Home office / retail | 6–8 |
| Workshop / light industrial | 8–12 |
| Warehouse | 4–6 |
| Hot climates (Gulf region rooms) | 10–15 (higher for heat control) |
| Laundry room | 10–12 |
| Parking garage | 6–8 |
Step 3: Apply the Formula
Worked example — the bathroom from Step 1, at 8 ACH:
13.5 m³ × 8 = 108 m³/h required
Add a 10–20% margin for duct resistance and aging: target a fan rated 120–130 m³/h.
Step 4: Convert Between m³/h and CFM
Suppliers quote both units. Conversion:
| m³/h | CFM | Typical room served |
|---|---|---|
| 100 | 59 | Small bathroom |
| 180 | 106 | Medium bathroom / WC |
| 250 | 147 | Large bathroom or kitchen |
| 500 | 294 | Workshop / office room |
| 1,000 | 589 | Commercial space |
| 2,000 | 1,177 | Warehouse zone |
The Calculation That Most Buyers Forget: Static Pressure
Catalog airflow is measured at free air (0 Pa). Real installations have ducts, bends, grilles and shutters that create resistance:
- Direct wall/window mount (no duct): nearly full catalog airflow — see the wall vs ceiling comparison for the full loss table
- 1–2 m of duct with one grille and backdraft shutter: expect 10–20% airflow loss
- Long duct runs (5 m+) with multiple bends: 30–50% loss with axial fans — this is where inline mixed-flow duct fans become necessary
Step 5: Match the Required Airflow to a Model
The formula gives you a number in m³/h. Suppliers quote models. The table below bridges the two using the KLT panel fan range we build, with the catalogue airflow in the second column and the two loss bands from the previous section applied in the third and fourth.
| Model | Catalogue airflow | Free-air CFM | After −10% grille + shutter | After −40% 5 m duct, axial | Room volume cleared at 10 ACH |
|---|---|---|---|---|---|
| KLT-10B 6" | 150 m³/h | 88 | 135 m³/h | 90 m³/h | 15 m³ |
| KLT-12B 8" | 380 m³/h | 224 | 342 m³/h | 228 m³/h | 38 m³ |
| KLT-18B 10" | 432 m³/h | 254 | 389 m³/h | 259 m³/h | 43 m³ |
| KLT-20B 12" | 792 m³/h | 466 | 713 m³/h | 475 m³/h | 79 m³ |
| KLT-25B 14" | 1,500 m³/h | 883 | 1,350 m³/h | 900 m³/h | 150 m³ |
| KLT-30B 18" | 1,800 m³/h | 1,059 | 1,620 m³/h | 1,080 m³/h | 180 m³ |
The last three columns are arithmetic, not test results: catalogue free-air figures multiplied by the loss percentages stated above, and catalogue airflow divided by 10 ACH. They are a screening tool. If your project needs a declared performance point, ask us for the airflow at 50 / 100 / 150 Pa.
Reading the table on a real job
A workshop 5 m × 4 m × 3 m is 60 m³. At 10 ACH that is 600 m³/h required. Free air, KLT-20B at 792 m³/h looks like more fan than the job needs. Put it on a 5 m duct run instead and the −40% column gives 475 m³/h — short of 600. The honest answer on that job is not a bigger panel fan; it is a BXF inline unit, which is quoted at working pressure rather than free air.
This is the decision most quotes get wrong. A panel fan is chosen for its free-air number and then installed on ductwork, where the number no longer applies. If the run is longer than one wall thickness, price the inline range before you price the panel range — the inline versus axial comparison has the pressure ratings side by side.
Full Worked Example: Gulf-Region Apartment
A bedroom 4 m × 3.5 m × 2.8 m (39.2 m³) in a hot climate requiring 12 ACH:
- 39.2 m³ × 12 = 470 m³/h
- Plus 15% margin for duct + shutter resistance = 540 m³/h target
- Recommended: two window/wall fans of ~280 m³/h each, or one ceiling inline system
Common Sizing Mistakes
- Sizing by floor area instead of volume — ceiling height changes everything
- Ignoring the backdraft shutter — adds 5–10 Pa resistance on every cycle
- Choosing one big fan for multiple rooms without duct design — the far room gets almost nothing
- Using free-air ratings for ducted installations — see static pressure above
- Forgetting make-up air — a fan cannot extract from a sealed room; there must be an inlet path (door undercut, grille, window vent)
How to Check an Airflow Claim Before You Order
Airflow is the easiest specification to quote loosely, because the same fan produces very different numbers depending on where it is measured. Four questions separate a usable figure from a sales figure.
- Free air or at pressure? Ask for the static pressure the figure was measured at. 1,200 m³/h at 0 Pa and 1,200 m³/h at 380 Pa describe two different machines, and the second is worth far more on a ducted job.
- Which report, and which models? Ask for the test report number and check that the model series you are buying actually appears in it. A report covering one series does not transfer to another, even from the same factory.
- Convert the units yourself. One m³/h is 0.589 CFM, so a “1,000 CFM” claim is 1,699 m³/h. Suppliers quoting whichever unit flatters the comparison are usually quoting the other one on the datasheet.
- Match the nameplate to the order. Every BAER unit leaves the assembly bench with a plate carrying its model and serial, so the fan on site can be traced back to the batch it came from.
If you are comparing us against another quote, send both datasheets and we will state which figure is free air and which is at pressure. That comparison is usually more useful than the price difference. Our own documentation and entity records are listed on the verification page.
Need Help Sizing?
Send us your room dimensions, application, and installation type (direct mount or ducted). We'll run the calculation, recommend the right model, and quote factory-direct with volume pricing.
Apply the Calculation by Market
Once you have the airflow figure, these pages cover the market and climate conditions that decide the rest of the specification.
- Exhaust Fans for the Middle East Climate
- Exhaust Fan Shutter Types
- Oman Exhaust Fan Supplier
- UAE Exhaust Fan Supplier
- Inline Duct Fan
Related Guides
- Exhaust Fan CFM Calculator (instant sizing tool)
- Panel Exhaust Fan Sizing Guide
- Bathroom Exhaust Fan CFM: How Much Do You Need?
- Inline Duct Fans: Sizing, Types & Selection
Frequently Asked Questions
How do I calculate exhaust fan airflow (CFM) for a room? +
What ACH (air changes per hour) should an exhaust fan deliver? +
How do I convert CFM to m3/h (and back)? +
Should I add extra CFM for ducts, elbows and filters? +
How do I size an exhaust fan for a warehouse or factory? +
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