Create logarithmic equation from excel chart

Not much to go on. Other than using the chart to visualize the process, I would probably not do any of this in the chart. If I understand what you are trying to do:

1) Choose the desired equation form. I am not certain exactly what you want for a "logarithmic equation", but I assume you know what you are looking for. As an example, I will assume you are looking for y=m*ln(x)+b
2) Use the LINEST() function to perform the desired regression. For the example, =LINEST(known_ys,LN(known_xs)) https://support.office.com/en-us/art...a-fa7abf772b6d
3a) Compute the desired y at a given value for x.
3b) Use algebra to invert the function, then compute the desired x at the given value for y, or use Solver/Goal Seek/other numerical algorithm to find x at a given y.

Does a logarithmic trendline give a sufficiently accurate fit for what you need?

Originally Posted by 37BPBrian

i dont know. the graph is pretty smooth.

Do you know how to create a trendline?

If you do, can you not look at it, and ask yourself, "Is that line close enough to my data that I'd be happy to use that result?"

Right-click the data series on the chart, select Add Trendline, and follow your nose from there.

What trendline type (linear, polynomial, logarithmic, power, etc.) did you choose? It sounds like you chose "linear", when you should have chosen something else.

Post a workbook, Brian?