"brianpo" wrote:
Multiplying 111111111 by 111111111 in Excel 2007 results in
12345678987654300, which is obviously wrong.
Is there a way to improve the accuracy of results over 15 digits?

The short answer is "no", at least not with the standard product. There are
add-ins available that do multi-precision arithmetic. I know nothing about
them.

Although Excel displays only the first 15 significant digits, arithmetic is
performed to highest degree permitted by the standard binary format that it
uses internally (i.e. IEEE 754). For example, the product of your numbers
results in exactly 12345678987654320. That still does not match hand
calculations; "missed it by t-h-a-t much" ;-) (It is off by only one in
this case.) But it is "correct" insofar as it is consistent with the binary
result that most applications will yield. The next binary representation is
12345678987654322.