Use FORECAST(). So in your example, say your x values are in the range
A1:A20 and your y values are in the range B1:B20:

=FORECAST(x_to_predict,B1:B20,A1:A20)

--
Regards,
Dave


"Steve J. Vaughan" wrote:

Approximating a curve

I have a set of X and Y values for example

X Y
690 0.82
1400 1.72
2075 2.75
2760 3.93
3450 5.31
4150 6.68
4800 8.27
5500 8.34
7600 12.48
11750 8.27
14000 10.68
20000 19.51
24000 25.86
26500 28.89
28500 31.37
32500 33.3
34500 30.3


I need to calculate a value of Y for any value of X.

Is there a F(x) that will allow me to do this.

VLOOKUP is no good as a value of X from 20,001 to 23,999 will give the same
value of Y 19.51