The inch on the box is a mounting size, not a performance figure. In our own range a 6 inch fan moves anywhere from 150 to 1,670 m³/h depending on type — eleven times the airflow at the same nominal size. This page gives the airflow, room coverage and wall cut-out behind every size we make, so you order by the number that matters instead of the one on the carton.
Last updated: November 3, 2026
Two buyers order a “6 inch exhaust fan” and receive two completely different machines: one shifts 150 m³/h out of a bathroom, the other shifts 1,670 m³/h out of a workshop. Both are honestly described as 6 inch. This guide explains what the size refers to on each fan type, how much air each size can realistically move, and what hole you have to cut in the wall — the three things that decide whether the fan works in your space.
THE BASICSOn a bathroom or panel fan, the inch refers to the nominal fan diameter inside a square wall panel — a 6 inch panel fan has a 200×200 mm faceplate. On a duct fan, the industry quotes the duct diameter in millimetres instead (100/125/150/200/250/315 mm). On a high-airflow wall shutter fan, the inch describes the blade and housing, and the same nominal size can be built with a much deeper blade and a faster motor. Airflow is set by blade design, motor speed and static pressure — never by the inch alone.
| Fan type | 4″ (100 mm) | 6″ (150 mm) | 8″ (200 mm) | 10″ (250 mm) | 12″ (300 mm) | 18″ (450 mm) |
|---|---|---|---|---|---|---|
| Bathroom fan (quiet) | 50–100 m³/h | 100–220 m³/h | — | — | — | — |
| Panel fan (KLT) | — | 150 m³/h | 380 m³/h | 432 m³/h | 792 m³/h | 1,800 m³/h |
| Duct fan (BXF) | 290 m³/h | 620 m³/h | 1,200 m³/h | 1,800 m³/h | 2,200 m³/h (315 mm) | — |
| High-airflow wall shutter | — | 1,670 m³/h (984 CFM) | — | — | — | — |
Rated free-air figures. Read the row, not the column: at 6 inch the spread is 150–1,670 m³/h, and the fan you need depends on the room, the mounting and the pressure — not on the nominal size.
ROOM COVERAGEAbout 10 m³ of air at 8–10 changes per hour: 80–100 m³/h. A 4 inch (100 mm) quiet bathroom fan is enough — going bigger mainly buys noise.
150–200 m³/h. 6 inch bathroom fan, or a KLT-10B panel fan if the wall is masonry and you want a shutter louvre.
12–15 air changes: 400–550 m³/h. 8 inch panel fan (KLT-12B, 380 m³/h) as a minimum; use the ducted BXF-150 (620 m³/h) if the fan cannot sit on an outside wall.
6–8 air changes: 350–600 m³/h. 8 inch panel fan or a silent cabinet fan where noise matters more than volume.
650–900 m³/h. 12 inch panel fan (KLT-20B, 792 m³/h) or two smaller units on opposite walls.
8–12 air changes: 1,800–3,000 m³/h. 18 inch panel fan (KLT-30B) or two 14 inch units (KLT-25B, 1,500 m³/h each).
Every size decision reduces to one sum: room volume × air changes per hour = airflow needed. Volume is length × width × ceiling height, not floor area — the single most common mistake. Air change rates by room type:
| Room | Air changes per hour | Notes |
|---|---|---|
| Bathroom / WC | 8–10 | Higher if there is no window |
| Domestic kitchen | 12–15 | 15–20 if frying regularly |
| Living room / bedroom | 6–8 | Comfort, not moisture control |
| Office / shop | 6–8 | Add for occupancy above one person per 10 m² |
| Workshop / warehouse | 8–12 | 12–15 where there is welding or solvent |
| Dishwash / laundry | 15–20 | Moisture-driven; use a ducted fan |
Worked example: a kitchen 4 × 3 m with a 2.9 m ceiling is 34.8 m³. At 15 air changes that is 522 m³/h — between the 8 inch panel fan (380) and the 6 inch duct fan (620). Take the larger of the two if the fan is ducted or the room has a window that stays shut. Conversions: 1 m³/h = 0.589 CFM; multiply m³/h by 0.59, or divide CFM by 0.59.
MEASURINGThree dimensions decide whether a replacement fits, and only one of them is the size printed on the box. The panel is the visible faceplate that covers the wall. The cut-out is the hole you actually make. The outlet is the spigot diameter, which matters if a duct connects to it.
| Model | Nominal size | Panel (mm) | Wall cut-out (mm) | Outlet (Φ mm) | Airflow (m³/h) | Noise (dB) | Coverage |
|---|---|---|---|---|---|---|---|
| KLT-10B | 6″ | 200×200 | 160×160 | 85 | 150 | 35 | 5–8 m² |
| KLT-12B | 8″ | 250×250 | 205×205 | 100 | 380 | 38 | 12–15 m² |
| KLT-18B | 10″ | 310×310 | 245×245 | 100 | 432 | 42 | 14–18 m² |
| KLT-20B | 12″ | 340×340 | 290×290 | 100 | 792 | 45 | 26–32 m² |
| KLT-25B | 14″ | 400×400 | 330×330 | 150 | 1,500 | 48 | 50–60 m² |
| KLT-30B | 18″ | 600×600 | 480×480 | 150 | 1,800 | 48 | 60–72 m² |
Coverage assumes a 2.7–3.0 m ceiling at 8–10 air changes. Note that the KLT-10B is called 6 inch but needs a 160×160 mm hole — the panel covers the rest. Send a photo of the opening with a tape measure across it and we will confirm the model before you cut anything.
SAME SIZE, THREE FANSQuiet, low power, short duct. Built to shift 50–220 m³/h with an internal backdraught flap for a duct of 100–150 mm. The right choice when the limit is noise, not airflow.
Square wall fan with an automatic shutter louvre. Same nominal size, several times the airflow because the blade is deeper and the motor is bigger: 150 m³/h at 6 inch, 1,800 m³/h at 18 inch.
Mixed-flow impeller for pressure, not just volume: 290–2,200 m³/h and up to 693 Pa. Choose it when the fan sits away from the wall or the run has bends.
Our 6 inch industrial shutter fan is built for workshops: 984 CFM / 1,670 m³/h at up to 180 Pa. Same 6 inch label, industrial blade and motor.
1. Using floor area instead of volume. A 3 m ceiling holds 20% more air than a 2.5 m one. Multiply by height, always.
2. Buying one size up instead of fixing the duct. A fan with a long, narrow duct loses more airflow than the next size up would gain. 100 mm duct above 290 m³/h runs at 10 m/s and turns airflow into noise.
3. Forgetting make-up air. Every cubic metre out needs a way back in. A sealed room with a 1,000 m³/h fan can lose a third of that airflow and pull combustion gases down the flue.
4. Oversizing a bathroom. Above about 10 air changes you get a draught across the room, more noise, and a fan that pulls against the door seal. Match the size to the volume.
5. Ordering by inch across borders. A 6 inch fan is 150 mm in a duct, 200×200 mm on a panel and 152 mm on a US spec sheet. Quote both the inch size and the cut-out when you compare suppliers.
RELATEDCE is standard — certificate XH2601E0321945GC (Xihua International Testing & Certification, Shenzhen), valid to 2031-01-28. The factory holds ISO 9001 (04625Q15304R1M) and CCC (2024010702612682, covering KLT-10B to 18B at 220V/50Hz). ETL can be arranged on request; SAA is renewed per order. All numbers are published for you to verify on the certificate verification page.
BUYER FAQTell us the room dimensions, the ceiling height, what the space is used for, and the wall or window opening you have. We reply with the airflow you need, the size and model that meets it, and a formal quote — within 24 hours.