Excel's numeric display limit is on significant figures, not decimal places.
Excel (like almost all software) follows the IEEE standard for double
precision binary representation of numbers.
http://www.cpearson.com/excel/rounding.htm
In particular, all 15 digit and most 16 digit integers can be exactly
represented. But rather than explain why some 16 digit numbers unavoidably
change value from what you enter, MS chose to display only 15 digits (See
Help for "specifications").
and It requires 17 decimal digits to uniquely specify a double precision
binary number, and An exact conversion from binary to decimal of a floating
point number may require many more than 17 decimal digits
http://groups.google.com/group/micro...06871cf92f8465

If you want to write a routine that will handle more precision than Excel
natively gives, you might find the VBA code at that last link instructive.
There are some Excel add-ins like
http://digilander.libero.it/foxes/index.htm
http://precisioncalc.com/
that already implement higher precision.

Also there are commercial packages like Maple, Mathematica,
MacSyma and open source packages like Maxima
http://maxima.sourceforge.net/
that implement algebraic math and user-specified numeric precision.

Jerry

"Spaz" wrote:

How many decimal places can be displayed in a cell? I'm running a brute
force VBA procedure of finding fractions that will approximate pi to as many
decimal places as Excel will display, but I don't know how many decimal
places Excel will display accurately. Anybody know? I guess this is also a
matter of how many decimal places VBA will calculate accurately as well.

Sub PiFractions()
Dim dividend As Integer, divisor As Integer, quotient As Double
Dim rowpointer As Byte

rowpointer = 1

For dividend = 22 To 10000
For divisor = 7 To dividend \ 3
quotient = dividend / divisor
If quotient 3.14159 And quotient < 3.1416 Then
Cells(rowpointer, 1) = dividend
Cells(rowpointer, 2) = divisor
Cells(rowpointer, 3) = quotient
rowpointer = rowpointer + 1
End If
Next
Next

End Sub