How do I know what size bed or diameter cranks I have?

So been in the shed and found a old triple set of deore cranks, been asked to put a single chain ring on them for a bike build but, how do I work out what size chainring will fit e.g 104 or 96 ?
Yours confused

Measure centre to centre but it’s slightly easier to measure from a hole edge so go from the inside of one to the outside of another.

Old triple will almost undoubtedly be a 68/104.

Just measure from the centre of 1 hole to the opposite for the bcd or measure to the centre of the next, square it, double it, square root the answer. (should be 73mm for 104 bcd)

dangeourbrain

Old triple will almost undoubtedly be a 68/104.

You’d think, but I’v found what I thought were old Deore cranks that had odd BCDs, Shimano started mucking about with that 5/6 years back

Is there a model number stamped on them anywhere OP?

They did some triples with 30T middle rings to drop the gearing for 29” wheels to make closer to 32T on a 650b wheel. Not sure anyone ever thanked them for it.
They used 96mm BCD but in a symmetrical pattern as opposed to 104mm, or the later 1x/2x offset 96mm BCDs.
If your Deore crank is FC-M612, then that’s what you have and you can use one of these to go 1x :-

https://uk.oneupcomponents.com/collections/chainrings-1/products/94-96-bcd-narrow-wide-chainrings

if they’re shimano they’ll probably have a part number stamped on them somewhere, then google it if your tape-measure-fu is lacking.

Part number FTW