Picking a ventilation fan looks simple until you see the options: ceiling fans, wall fans, inline duct fans, AC motors, EC motors, and airflow numbers in both CMH and CFM. Buy the wrong size and you get a fan that either barely moves air or is louder than it needs to be.
This guide explains how to choose a ventilation fan for your bathroom, kitchen or a room without windows, with a simple sizing method and the Astberg models that suit each case. All models listed here are from our own range, so you can check every spec on the product page.
Quick picks: which ventilation fan for which room?
| Room | Duct size | Recommended model | Airflow |
|---|---|---|---|
| Small bathroom or toilet | 100 mm (4") | AF100P | 198 CMH |
| Standard bathroom | 125 mm (5") | AF125 | 284 CMH (167 CFM) |
| Large bathroom or ensuite | 150 mm (6") | AF150 | 530 m³/h (311 CFM) |
| Bathroom where you want variable speed | 150 mm (6") | AEC150 (EC motor) | 530 CMH |
| Kitchen | 150-200 mm | AF150 or AF200 | 530-840 CMH |
| Several rooms or a commercial washroom | 200-250 mm | AF200 or AF250 | 840-1,405 CMH |
Not sure which row fits your room? The next section shows how to work it out.
Step 1: Work out the airflow you need
Every room needs a certain number of air changes per hour (ACPH), which is how many times the full volume of air is replaced each hour. The formula is:
Airflow (CMH) = Room volume (m³) × ACPH
Typical ACPH values:
- Toilet or bathroom: 10-12
- Kitchen: about 6
- Bedroom or living area: 1.5-2
Example, standard bathroom: 2.5 m × 1.8 m × 2.4 m = 10.8 m³. At 12 ACPH you need about 130 CMH. A 100 mm fan such as the AF100P (198 CMH) covers this comfortably.
Example, larger ensuite: 3 m × 2.5 m × 2.4 m = 18 m³. At 12 ACPH you need about 216 CMH, so a 125 mm fan like the AF125 (284 CMH) is a good fit.
Ducts with long runs, many bends or grilles reduce the real airflow, so pick a fan with some margin instead of matching the number exactly. To skip the maths, use the Duct Fan Size Calculator on our inline fans page. It gives you CMH and CFM from your room dimensions.
Best ventilation fan for a bathroom
Bathrooms need fast moisture removal, and that is where inline duct fans do well. The motor sits inside the duct, away from the room, so you hear less noise where you actually stand.
- Small bathroom or toilet: the AF100P or the compact AMJ100 Micro Jet (147 CMH) are enough for a 100 mm duct.
- Standard or large bathroom: the AF125 (125 mm) or AF150 (150 mm) give you room to spare.
- If the fan runs for long hours: consider the AEC150 with an EC motor. It allows variable speed, so you can run it slower and quieter when full power isn't needed.
If your bathroom already has a ceiling opening and no space for ducting, a ceiling exhaust fan may be simpler to fit. More on that below.
Best ventilation fan for a kitchen
Kitchens deal with heat, steam and cooking smoke, so they usually need more airflow than a bathroom of the same size. The standard guideline is about 6 ACPH, but heavy Indian cooking with frying and tadka often justifies going one size up.
- Choose a 150 mm or 200 mm fan with a short, straight duct to an outside wall.
- The AF150 (530 CMH) suits most home kitchens. The AF200 (840 CMH) is better for larger or open kitchens.
- Use a smooth or rigid duct where possible. Long flexible duct with sharp bends cuts airflow noticeably.
For more options, see our kitchen exhaust fans.
Ventilating a room without windows
A closed room with no windows needs both extraction and fresh air coming in. A single exhaust fan alone can leave the room under negative pressure and pull air from wherever it can.
- A fresh air system brings filtered outside air in and pushes stale air out.
- An ERV (energy recovery ventilator) does this while recovering part of the heat or cooling, which helps when the room is air-conditioned.
Read more on our fresh air systems and ERV & HRV pages.
Inline fan vs ceiling exhaust fan
| Inline duct fan | Ceiling exhaust fan | |
|---|---|---|
| Where it sits | Inside the duct, in the ceiling void or a remote spot | In the ceiling of the room |
| Noise at the room | Lower, since the motor is set back | Higher, since the motor is in the room |
| Serves | One or several rooms via ducting | One room |
| Installation | Needs ducting | Simpler, no long duct |
| Best for | Retrofits, bathrooms, kitchens, multiple rooms | Small rooms with easy ceiling access |
In short: choose an inline duct fan when you want quieter operation or one unit for more than one outlet. Choose a ceiling exhaust fan when the space is small and you want the simplest installation.
How to choose: four checks before you buy
- Airflow: match the CMH or CFM to your room using the formula above.
- Duct diameter: the fan must match your duct (100, 125, 150, 200, 250 or 315 mm). The wrong size means reducers and lost efficiency.
- Noise: look for models sold as "silent" for bedrooms and occupied spaces, and check the dB rating on the product page.
- Motor type: an AC motor is budget-friendly for simple on/off use. An EC motor costs more upfront but gives variable speed and lower running costs when the fan runs for long hours.
Our range covers 100 mm to 315 mm, and the models come with a 2-year warranty.
Frequently asked questions
What size ventilation fan do I need for a bathroom?
Multiply the room volume in cubic metres by 10-12. A small bathroom of about 10 m³ needs roughly 100-130 CMH, and an ensuite of about 18 m³ needs around 215 CMH. Pick a fan slightly above that number.
Is CMH the same as CFM?
No. CMH is cubic metres per hour and CFM is cubic feet per minute. Multiply CMH by 0.59 to get CFM. For example, 530 CMH is about 311 CFM.
Can I install an inline fan away from the bathroom?
Yes. This is one of the main advantages. The fan can sit in the ceiling void or a cupboard and connect through ducting, which reduces noise in the room.
Is an EC motor fan worth the extra cost?
If the fan runs for many hours or you want to adjust speed, yes. EC motors offer variable speed and lower running costs. For occasional on/off use, an AC motor is enough.
Which ventilation fan is best for a kitchen?
Choose a higher airflow than for a bathroom, typically a 150 mm or 200 mm inline fan with a short, straight duct to the outside.
Need help choosing?
Send us your room size and duct details and our team will recommend the right model. Contact us or browse all Astberg inline ventilation fans.
