Minimum raise is always the same as the big blind or the amount of the initial raise .

EG: Blinds are 5/10,and someone raises on 50 - The minimum raise is to 90 (The initial raise was 40)

The exception would be when someone has less and moves all in , but that doesn't count as a raise , players that already acted will be only able to complete instead of re-raising .

EG: Blinds are 50/100 and you open to 500 from seat 2 , seat 4 and 5 call and seat 7 moves all in for 850 . The players in seat 8,9 and 1 can make a re-raise but the rest can only complete or fold .