Forecast function calculation method?

[Kayote]

Hi,

Im thinking of incorporating the Forecast function into my budget
spreadsheet but I just cannot figure out how Excel calculates the
forecast figure.

I understand how to set it up but if I was to try to calculate that on
paper, I'd be stumped. If I cannot understand how the function works
then how can I incorporate it into spreadsheets right?

So the question is, does anyone understand how Excel calculates the
answer please?

 

[Gary''s Student]

FORECAST() just does a linear extrapolation. It also does interpolation if
the desired point is internal. See Excel Help ( the picture is helpful )