Sooner or later you will find Excel complaining about ‘Circular References’. This is often caused by a typing error (I do it all the time!) so I'll take a moment to tell you what it means and how to fix it.

The example below shows the problem in its simplest form. I’ve put the formula ‘=C3’ in ‘D3’ and ‘=D3’ in 'C3'.

The value in ‘C3’ depends on the value in ‘D3’ and the value in ‘D3’ depends on the value in ‘C3’, there is no solution (in this case any value would satisfy the conditions).

Circular references can be easy to spot, or it can take hours to puzzle out the problem. You might have to trace back through several links in the chain until you find the loop. It may be an obvious mistake, or it may be quite a subtle problem with your logic.

You can follow the blue 'precedence' or 'dependence' arrows to help figure out the problem. Read the menu that comes up when the problem arises to find out exactly how to do this.