Hi guys,
I’ve already disassembled Shimano SG-X 50-34 50-F outer front chainring. Just curious, how could I pick up a compatible outer front chain ring for the crank Shimano 172.5 FC-4650 or there are not too many options for this type of the crankset? 
Thank you
Measure the distance between 2 of the mounting holes. It will probably measure 110mm. Thats called the BCD.
Then you can search online for a “50 tooth 5 bolt 110 bcd chainring”.
Anything with this specification should fit, as long as it isn’t a narrow wide chainring designed for 1x systems.
According to this Shimano Service manual the bcd is 110mm so doing what the above commenter suggested would be the way to go
Yeah, mate, just googled it lol
SHIMANO FC-4650 5 x 110 10 sp
The BCD is the diameter of the mounting hole circle - the only case where you could measure this through the distance between two mounting holes is when there is an even number of bolts and you are measuring between opposite holes. On a 5-bolt ring, the distance between two mounting holes will not be equal to the BCD.
But if you insist on measuring BCD from the distance between 2 mounting holes… it’s
BCD = L / sin (pi/n)
where n is the number of holes, and L the distance between two adjacent holes.
Of course, as the responses above have shown, searching for the crank part number is probably faster. And many chainrings have the BCD printed somewhere on them. And Shimano road is either 130 or 110 BCD, and the difference is visually obvious.
Thank you very much folks for sharing your knowledge with the community. It helps a lot
Don’t you get the urge to dust off your trigonometry once in a while? If only to show off! 
Now I know who we can ask for, mate 
