I’ve found myself having to convert time in Excel stored as hours, minutes, seconds (e.g. 3:42:09) into hours in decimal format (e.g. 3.70) for invoicing my time and every month I end up googling this.

The method I use is a formula in Excel using the Excel Time functions (HOUR, MINUTE, and SECOND).

=HOUR(CELL) + MINUTE(CELL) / 60 + SECOND(CELL) / 3600

Replace CELL with the appropriate cell in your Excel workbook (e.g. A2).