Metric would be nice but here even the Volvo dealership just gets regular non-metric hose at a parts store.

I learned:
Hose from flame trap to manifold has to be fuel rated.

For all hoses:
Non metric works, but get the size that fits kinda tight rather than just slipping on pretty easily. That larger size softens up over time, and then it doesn't seal well. So the tighter fit one is better. Been working fine for me.
--
Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.

ran into that problem myself...........considering that a little leak will really mess up performance I decided to stick w/ metric

iPd has a nice kit full of reinforced vacuum hose in the right diameters and appropriate bulk lengths...it's not cheap but it's good stuff, and considering gas is $4.00 a gallon it is getting to the point where it costs too much money to run to the parts store only to find out they have the "wrong stuff"....

HTH
J
--
89 245DL M47 300K+mi no rust

...but, don't take the ID on the old hose to be correct. Assume at least a size smaller, or bring in a removable part that needs one (AC control parts, PCV nipple, etc.).