Excel - logarithms back to numbers in Excel?

Asked By Poll on 22-May-07 09:46 AM
How do I convert logarithms back to numbers in Excel.  Log base 10 and natural.




Bernard Liengme replied on 22-May-07 09:58 AM
The definition of log (base 10) is:
If x = 10^y  then we say  LOG(x) =y
Example: 100 = 10^2 so LOG(100)=2
It follows that if I know y and want x I use x = 10^y
If I know the log of x is 2, then x = 10^2 = 100

Likewise:
The definition of LN (base e) is:
If x = e^y (which can also be written as x=EXP(y))  then we say LN(x) =y
Example: 3 = e^1.096 so LOG(3)=1.096
It follows that if I know y and want x I use x = e^y
If I know LN(x) = 1.096 then x = EXP(1.906) which works out to be 3

best wishes
--
Bernard V Liengme
www.stfx.ca/people/bliengme
remove caps from email
Poll replied on 22-May-07 10:24 AM
Very helpful, thank you Bernard.  But is there a function in Excel that I can
use for a column of many figures?


Kind regards
Polly
David Biddulph replied on 22-May-07 10:28 AM
=10^A1 or =POWER(10,A1)
=EXP(A1)
--
David Biddulph
Stan Brown replied on 22-May-07 09:01 PM
Tue, 22 May 2007 06:46:01 -0700 from Polly

=10^A1

=EXP(A1)

--
Stan Brown, Oak Road Systems, Tompkins County, New York, USA
http://OakRoadSystems.com/
Roland Orlie replied to Poll on 31-Oct-10 01:49 PM
i need to know the reverse of a log10 funtion to get back to the number.
Ron Rosenfeld replied to Roland Orlie on 31-Oct-10 02:51 PM
First tell us what happened when you tried the solution in this
message that you quoted, since that should have worked.
joeu2004 replied to Roland Orlie on 31-Oct-10 04:43 PM
If A1 contains the result of your LOG10 formula (e.g., =3DLOG10(1.234)),
then the antilog is =3D10^A1.

Although that does return exactly 1.234 in that case, in general do
not expect the antilog to exactly equal the parameter of LOG10.  For
example, if A1 is =3DLOG10(PI()), =3D10^A1-PI()=3D0 is FALSE(!) [1].

Also, do not expect the antilog to exactly match mathematical
equalities.  For example, if A1 is =3DLOG10(4.5)+LOG10(2) [2], =3D10^A1=3D9
is FALSE(!).

Infinitesimal differences are due to the limitations of computer
arithmetic as well as to the fact that generally LOG10 and the power
operator (^) use generating functions or algorithms to approximate
their results (when the exponent is non-integer in the case of the
power operator).

-----
Endnotes

[1] But =3D10^A1=3DPI() is TRUE.  The difference is due to Excel
heuristics which try to hide inequalities when the difference is

[2] We expect LOG10(4.5)+LOG10(2) =3D LOG10(4.5*2) =3D LOG10(9) based on
mathematical equalities.
joeu2004 replied to joeu2004 on 31-Oct-10 04:44 PM
PS:  For broader participation, you might want to post future
inquiries using the MS Answers Forums at
http://social.answers.microsoft.com/Forums/en-US/category/officeexcel.
it is not that I like that forum.  it is just that MS has ceased to
support the Usenet newsgroups.  Hence, participation here is limited
to the sites that share a common newsgroup mirror, which is no longer
centralized at MS.