How to Calculate Exhaust Fan Airflow: CFM & m³/h Sizing Guide

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

Room volume (m³) = Length (m) × Width (m) × Height (m)

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

Required airflow (m³/h) = Room volume (m³) × ACH

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:

CFM = m³/h ÷ 1.699 | m³/h = CFM × 1.699

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

Rule of thumb: If your installation involves real ducting, either size up one fan class or switch from axial to a mixed-flow inline fan. Request the fan's performance table showing airflow at 50/100/150 Pa, not just the free-air headline number.

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)

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How to Calculate Exhaust Fan Airflow: The Complete CFM & m³/h Sizing Guide