Extrapolation is dangerous. Interpolation is okay. You can use an algorithm
that interpolates between the two nearest points of the calibration, or an
algorithm that determines the concentration based on a best fit (regression)
of the entire calibration curve. Using the regression is somewhat easier.
The unknown Y value is given by:

=TREND(Y-range,X-range,X-value)

Where Y-range and X-range are the ranges with the fitted Y and X values, and
X-value is the value of X at which you want to calculate Y.

- Jon
-------
Jon Peltier, Microsoft Excel MVP
Tutorials and Custom Solutions
Peltier Technical Services, Inc. - http://PeltierTech.com
_______



"lucybelle" wrote in message
...
i've got a series of values (calibration standards) and have made a line
of
best fit with them os area ratio against drug concentration, now i need to
use this line to establish the concentration of unknowns from their area
ratio

thanks!