rrh wrote:
Do you really want to do this? The kit will expect 125mm and by cutting that
down to 100mm you will be severely restricting the airflow. I have just
taken delivery of a new cooker hood with 125mm outlet and while I agree that
it's more difficult to get 125mm ducting stuff than 100mm - especially with
the ducting in aluminium which I wanted because it's going to be visible -
it is possible. Try:

It depends.
IIRC, the restriction is about 2.5 times for 100mm pipe.
So if you'r using 1m of 100mm pipe, it'll be similar to 2.5m of 125mm
pipe.