Generally it gives you a score range. The left side of this score range I found to be with in 10 points of what is shown. The right side is questionable. If it says 700-730 your score will be between 690-710. Thats the 10 point margin. It can be exactly accurate but takes some skill to learn how it works. But stay with 10 points in either direction with the left number and you should hit it every time.

(We are talking about MyFICO or EQ calculators correct?)