From 300mm

This page groups our 300mm spiral duct reducers — fittings that take a 300mm (approx. 12″) duct down to a smaller diameter, stepping down to 8 smaller diameters, from 280mm right down to 125mm. Choose the reducer whose small end matches the next duct size in your system and join it in line with your spiral ducting. Alpha Air is a Leicester-based ventilation supplier — order stocked items by 3pm and they ship for next-day delivery.

Other starting sizes are listed under spiral duct reducers.