Tools > Goal Seek reverses the usual order for a formula. Usually, you run a formula to get the result when certain arguments are entered. By contrast, with Goal Seek, you work with a completed formula to see what values you need in an argument to get the results that you want.
To take a simple example, imagine that the Chief Financial Officer of a company is developing sales projections for each quarter of the forthcoming year. She knows what the company’s total income must be for the year to satisfy stockholders. She also has a good idea of the company’s income in the first three quarters, because of the contracts that are already signed. For the fourth quarter, however, no definite income is available. So how much must the company earn in Q4 to reach its goal? To answer, the CFO enters the projected earnings for each of the other three quarters and the projection for the entire year. Then she runs a goal seek on the cell for Q4 sales, and receives her answer.
Other uses of goal seek may be more complicated, but the method remains the same. To run a goal seek, at least one of the values for an argument must be a referenced cell or range. Only one argument can be altered in a single goal seek. After you get the result of a goal seek, you can replace the original value in the referenced cell with the result, or record the result elsewhere for later use, possibly as a scenario.
With the help of Goal Seek you can calculate a value that, as part of a formula, leads to the result you specify for the formula. You thus define the formula with several fixed values and one variable value and the result of the formula.