Start with the full shift length
To calculate work hours, begin with the time you started work and the time you finished. Subtract the start time from the end time to find the full length of the shift. If you worked from 8:30 AM to 5:00 PM, the full shift length is 8 hours and 30 minutes before breaks are removed.
That first number is useful because it gives you the total time on site. From there, you can decide which parts count as paid time and which parts should be left out. This matters when you are recording hours worked on an employee timesheet or reviewing a daily shift before it goes to payroll.