How to perform a linear interpolation in Excel

Hi,
I'm trying to do a very simple thing - but I fail...
I have an array of data (x and y).
I have a x value that does not match exact to my data point, so I would like to do a linear interpolation between the two points closest to my value (i.e. upper and lower) and return the interpolated y value.
It sounds simple to my ear - but i cannot make it work. My data are in ascending order. I have tried VLOOKUP and INDEX/MATCH just to find the relevant two y-values but are having problems to find both the lower value and the upper value (as MATCH wants the data to be en ascending order to find lower value and desending to find upper - I think).

So in short - how do I perform a simple linear interpolation in excel ?

Value 2,3

X Y
1 10
2 20
3 30
4 40
5 50

Find y corresponding to x = 2,3



Kind Regards Torben

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That's better.

I always find "manual interpolation" much easier than putting it in an excel formula. but it is doable. vlookup gets me stuck to easy, using index and match helps me to solve.

just to help understand the values a bit better I have put X and Y values in col x and y cols so you can read in the formula if x or y value is used in which part

so that resulted in

Formula:

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MATCH(AA3;$X$2:$X$6;1) finds the values closest but always less then the desired value.
MATCH(AA3;$X$2:$X$6;1)+1 simply finds the next row in the value that will be over the wanted x

the index parts in the formula helps to eighter use the X value or the Y value in the formula

if you translate each index to its value it will show..

X to find Y value for in my example is 2,25 (in cell AA3 for this example) this will result in the following outcome for each index formual above

y= 20 + ((2,25-2)*(30-20)/(3-2))

y = 20 + 0,25*10

y = 22,5



The formula could be simplified somewhat if the interval of x values is always 1 but because changes are this is not always the case this formula will work on any interval. To help understand the formula you can also use the evaluate formula option under formulas menu in the Formula Auditing section (3rd or 4th section on the ribbon)

When these questions come up, I sometimes wish that QuattroPro was still around. QuattroPro had a nice, built in @LINTERP() function that made short, easy work of linear interpolation problems.

As a different alternative, I find that I prefer to not try to do linear interpolation in a single cell formula. I prefer to spread it out. Compute slope and intercept for each segment in helper columns adjacent to the original data "lookup table". Then the linear interpolation becomes something simpler =VLOOKUP(known_x,lookup_table,intercept_column,TRUE)+x*VLOOKUP(known_x,lookup_table,slope_column,TRUE). I've illustrated this in this old thread: https://www.excelforum.com/excel-cha...e-a-curve.html