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3/8-16 as RnR said. I use alot of SS hardware and can throw out a few tips...

I would not use button head bolts for things that get tightened down "good-n-tight" and may see multiple tightening and loosening sequences for adjustments, especially the common grades of SS (18/304 and 316). Consider using the SS hex head bolts for your hinges OR at least get them set in the right place with regular hex bolts first and then swap them out with the button heads. 18/304 and 316 SS is pretty soft and the hex socket tends to get damaged when torqued down "good-n-tight". The last thing you want is a rounded off hex socket which means grinding the bolt head off to extract the remaining threaded portion.

SS is notorious for galling at the threads. All it takes is a micro-booger to cause the threads to seize...always use anti-seize on the threads. This will save you a lot of trouble during install and down the road.

Make sure you chase all the female connections with a tap prior to putting the SS hardware in. That old rust and crud are the micro-boogers that will make your day a PITA.

Since the common SS grades are softer than normal, washers that look to be the same thickness as the original steel ones will deform, so get some that are at least half again as thick to avoid cupping the washers. Especially where they are going over the slotted holes in your hinges.

I have the SS hex bolts in my hinges...I would not use button heads there! If something goes wrong with the hex heads...at least you have something to get a wrench on and that may not really be enough. SS bolts that have galled will usually need to be broken before the galled threads let go.