Is exhaust pipe size ID or OD?

Exhaust "pipe" is not water pipe It is sold OD

OT !!!! This gets complicated sometimes, eg HVAC/R vs plumbers. Copper water "piping" which except for wall thickness, is "called out" in nominal ID. The refrigeration tubing (same stuff sometimes different wall thickness, uses same fittings dimensions) is normally called out in "true" OD

I've often thought water pipe was completely stupid. You have sked 40 which is thinner walled than ske 80, but it all must be same dimensions OD.