Changing from 11 - 27 to 11-30 on my road bike, will I need longer chain?
Not very clued up on road bike stuff.
Depends if it was too long, too short or just right on the 27.
Yep measure and find out, safest way really, can you go big to big without ripping the mech off?
https://www.parktool.com/blog/repair-help/chain-length-sizing
Or take a look at it on big ring and 27, make sure you have a comfortable amount if S bend in the chain around the cassette to pully interface. Shift it up to a higher gear and note how much the pully moves. Going from 27 to 30 will be about the same in the other direction. If it looks like you'll still have a nice S bend moving up to that imaginery 30 then you are probably OK. Get the cassette, try and see. If it looks like it'll tight, buy a chain as well. You can't be sure without trying, but there's a chance you won't need. It's a difference of a couple of links. I did need it going from 28 to 32 on my road bike, but got away with it moving chain rings on my 1x MTB.