This Hitpoints Calculator can be used to estimate your future Hitpoints(now called Constitution) level based on any given combat stats. The formula for estimating your hitpoints level is very simple - sum up the experience of each combat skill and then divide the total experience by three. The result will become the total experience for hitpoints skill which is then converted to a level.
The accuracy of this calculator depends on the methods used while training your combat skills. For example, if you were using Pest Control for melee training or a Dwarf Cannon for ranging, calculator may not be as accurate because you weren't getting any hitpoints experience thus your actual hitpoints level will be lower compared to other players who trained the normal way.
A note about Magic - the calculator will assume that you've trained magic using combat spells from level one and right up to your current magic level. Such magic training strategy rarely the case as most people use non-combat spells such as high alchemy to train magic so it's best to enter your last magic level at which you've stopped using combat spells or you may choose to not include Magic skill at all when approximating your hitpoints level.
Just for fun, the calculator does not stop at level 99. If your hitpoints experience goes far beyond 13 million (level 99), the calculator will show your real hitpoints level as if max level cap did not exist.