PERCENTILE has been a native Excel function since XL97 but you need to try a
value inside the data range for interpolation to make sense. Try:

=PERCENTILE(B:B,PERCENTRANK(A:A,D1,20))

to interpolate for y given x, e.g. D1 = 147000 gives 3500 as in Ron's
example. For a curve fit try:

=TREND(B2:B27,A2:A27^{1,2,3,4},D1^{1,2,3,4},0)

You can interchange A and B in the formulas to interpolate for x given a y
value.

"proinwv" wrote:

All,

My data is in two columns of X and Y but is not linear. (See below)

I tried the suggestion for FORECAST but it will not return the correct value
becasue of the non-lineararity.

I tried PERCENTILE, but it is not recognized. Possibly because of my version
being older (Excel 2000)?

Is there another method? My data is as shown below. Charting it shows the
non-lineararity to be significant.

X Y
2,500 60
4,200 100
21,000 500
42,000 1,000
84,000 2,000
126,000 3,000
168,000 4,000
210,000 5,000
420,000 10,000
630,000 15,000
840,000 20,000
1,050,000 24,000
1,260,000 28,000
1,470,000 31,000
1,680,000 34,000
1,890,000 37,000
2,100,000 40,000
2,520,000 44,000
2,940,000 48,000
3,360,000 52,000
3,780,000 56,000
4,200,000 60,000
5,040,000 68,000
5,880,000 75,000
6,720,000 82,000
7,560,000 90,000