Why does Excel and Google Sheets round out the least significant digit when I hit a large number?

Spreadsheets are not designed for high precision calculations. The IEEE 754 specification for floating point mathematics requires that only the 15 most significant digits be stored. If the entire part of a number is 15 places or more, the mantissa or fractional part will be eliminated.

The solution is to perform calculations on something other than a spreadsheet, which bothers us, we have been really good at spreadsheets.