Figuring out exactly how many hours and minutes are between two points in time is a daily necessity—whether you are filling out a timesheet, planning a flight, or tracking how long a project took.
The Challenge of Time Math
Time math is difficult for the human brain because time is not based on a base-10 (decimal) system. We use a base-60 system for minutes and seconds, and a base-24 system for hours. You can't simply subtract 8.30 from 17.15 on a standard calculator and get the right answer.
Step-by-Step Manual Calculation
The most foolproof way to calculate time duration manually is to use the 24-hour clock (military time).
Step 1: Convert to 24-Hour Time
If your time is in PM (and not 12 PM), add 12 to the hours.
Example: 3:15 PM becomes 15:15.
Step 2: Stack the Subtraction
End Time: [HH] [MM]
- Start Time: [HH] [MM]
Step 3: Borrow (If Necessary)
If the Start minutes are greater than the End minutes, you must borrow 1 hour from the End Time's hours, and add 60 to the End Time's minutes.
Practical Examples
Example 1: Standard Shift (No Borrowing)
Start Time: 8:15 AM (08:15)
End Time: 4:45 PM (16:45)
- Minutes: 45 - 15 = 30 minutes
- Hours: 16 - 8 = 8 hours
Duration: 8 hours and 30 minutes.
Example 2: Complex Shift (Borrowing Required)
Start Time: 9:45 AM (09:45)
End Time: 5:15 PM (17:15)
We cannot subtract 45 minutes from 15 minutes.
- Borrowing: Borrow 1 hour from 17. The hour becomes 16. Add 60 minutes to 15. The minutes become 75.
- New End Time: 16:75
- Minutes: 75 - 45 = 30 minutes
- Hours: 16 - 9 = 7 hours
Duration: 7 hours and 30 minutes.
Converting to Decimal Hours for Payroll
If you are calculating time for payroll, you usually need to convert minutes into a decimal fraction of an hour.
Decimal Hours = Minutes ÷ 60
For our previous example (7 hours and 30 minutes):
- 30 ÷ 60 = 0.5
- Total Payroll Time = 7.5 hours
Common decimal conversions:
- 15 minutes = 0.25 hours
- 30 minutes = 0.50 hours
- 45 minutes = 0.75 hours
Tip: If a duration crosses midnight, the easiest manual method is to calculate the time from the start until midnight, and then add the time from midnight to the end time.