Rounding to n significant digits
Excel includes three functions (ROUND, ROUNDUP, and ROUNDDOWN) that round values to a specified number of digits. In some cases, however, you may need to round a value to a specified number of significant digits.
For example, you might want to express the value 1,432,187 in terms of two significant digits (that is, as 1,400,000). Here's an elegant solution. The formula below rounds the value in cell A1 to the number of significant digits specified in cell A2: