In another location (column H, for instance) you could use
=FORECAST(E15,E14:E16,F14:F16)
to interpolate the y value corresponding to E15 from its surrounding x-y
pairs. Obviously, this assumes that that F15 is empty. It will not
work from F15, since that would involve circular references.

Jerry

Pal wrote:

I have 2 columns of numbers from a load-deformation curve.
X values in range E14..E74
Y Values in Range F14..F74
The range is not always completely filled with values.

I need to interpolate the value of Y for any given X value.
Since my data points may not be an exact value of X I need to look at the
value before and after and get the slope
of the line then solve Y for my desired X value.

I have been doing this with multiple cells looking up the X Value Before and
After my desired X.
Then get the corresponding Y values. I then use the equation to solve.

Is there an easier way to do this? I assume a straight line between each
point.