Use the relationship between these functions and functions for which Excel
does have inverses.
ERF(x) = GAMMADIST(x^2,0.5,1,TRUE)
ERFC(x) = 2*NORMSDIST(-x*SQRT(2)) = CHIDIST(2*x^2,1)
so
invERF(p) = SQRT(GAMMAINV(p,0.5,1))
invERFC(p) = -NORMSINV(p/2)/SQRT(2) = SQRT(CHIINV(p,1)/2)

For Excel 2003 or later, NORMSDIST and NORMSINV are much more accurate than
ERFC for large values of x. For earlier versions of Excel, CHIDIST is more
accurate than ERFC, but neither inverse function performs well for large x
(small p).

Jerry

"amit saraf" wrote:

How to find inverse of complementary error and error function