A runescape calculator is a calculator that tells you how much more xp or the number of things you need to do before you reach the level you want. You can find a runescap calculator at http://www.zybez.net/calcs.php
Once you go to the web sit it will have all different skills. Chose the one you want for example woodcutting. Put in your current xp then the level you wish to achieve, lets say u hat 1500xp u would enter 1500 where it seas current then you would type in your target level lets say 15. It will tell you how much more xp you need in this case i need 911xp more to reach my target level.
It also tells you all different types of things you can do with that skill and what level you need to be to do it the xp it gives you and how many times you need to doit to reach your goal.