Formula to calculate sum in same cell

[AndyDavey]

Hi all

I am trying to calculate a value in the same cell the formula is in.

Basically I want to add a formula to K4 that multiplies the number added to that cell by a number in another sheet. So when I add 3 in K4 it multiplies that by D2 in the Sheet Weights

Should be easy but I cannot figure it out, as I get a circular reference error!?

Thank you

Andy