Hi, I'm new to SW (just started on 2010). i am trying to find out how you make one dimension dependant on another. In Inventor its just =d1*2 in the dimension box which means that if you change the dimension d1 the d2 dimension changes automatically. it's probably real somple but i cant find it in any help or on the net.

Double-click the dimension. Using the dialog box that is now shown, click the down arrow on the RH side of the box and select "ADD EQUATION". Now select any dimension shown in the sketch (in your example d1) and add the relationship ( in your case *2) and select OK. Select OK in the EQUATIONS dialog box. And there you have a dimension relative to another.

If you're using the new version of SW (2012+) They have changed how the equations can be added. If you know the actually dimension name of your d1 dimension (i.e. D1@sketch1) then you can actually just enter that into the value location now (="D1@Sketch1"*2). I'd attach a screen shot, but I'm having a little difficulty with that on this site

That's right , but In Solidoworks 2012+ you don't need select "ADD EQUATION", dimension relation can be added at one

I find that the most common uses of equations are covered by constraints (relations). You are describing what may be a relationship of symmetry. If that is the case, creating a symmetric constraint is better, because it implies that the relationship will not encompass the case where the factor is different than 2. If the factor might be 3 or more, then it might be more informative to set the segments equal, or to divide the line.