My ring size is a 8 would a ring that is 8.5 be to big for my finger?

My boyfriend wanted me to measure my finger for a ring. It came out to be 8 or 8.5 . Should i get the 8.5 just to be safe, or would the ring be to big ? I don't want it to fall off, or lose it. Btw I don't have time to go to the jeweler, Its closed tomorrow and he needs the ring size for Tuesday :( .