well 50mm wide with a 10 mm overlap.

you want to cover a 1 metres of exhaust.

the circumference of the exhaust is roughly 25cm. so every 40mm in coverage you use 25cm of wrap (in length).

So to cover 1 metre of exhaust you need roughly = (25 x 25cm in length) =

6.25 metres.

so get 10 metres