Step 1: Use a 24-hour clock
Mixing AM and PM is where most hand calculations go wrong. Convert every time to a 24-hour clock first. Add 12 to any PM hour except 12 PM, so 5:30 PM becomes 17:30 and 1:15 PM becomes 13:15. A 12 AM time becomes 00.
Now each shift is a plain subtraction. 17:30 minus 08:00 is 9 hours 30 minutes, with no guessing about which half of the day you're in.
Step 2: Subtract, borrowing 60 minutes when needed
Subtract hours from hours and minutes from minutes. When the end minutes are smaller than the start minutes, borrow an hour and add 60 to the minutes.
Take 08:45 to 17:15. You can't take 45 from 15, so borrow: 17:15 becomes 16:75. Then 16 - 8 = 8 hours and 75 - 45 = 30 minutes. The shift is 8 hours 30 minutes. An easier method is to convert both times to minutes after midnight first: 525 and 1,035, which are 510 minutes apart, or 8.5 hours.
Step 3: Take off unpaid breaks
Subtract only the break time that isn't paid. In the U.S., short rest breaks are generally counted as work time, while a genuine meal break where you're fully off duty can be unpaid. If you take a 30-minute unpaid lunch in that 8 hour 30 minute shift, you worked 8 hours.
Check your own time card and policy. If you're called back to your desk during lunch, that time may need to count.
Step 4: Handle shifts that cross midnight
If the end time is smaller than the start time, the shift ended the next day. Add 24 hours to the end time before you subtract. A 22:00 to 06:00 shift becomes 22:00 to 30:00, which is 8 hours.
Our work hours calculator does this automatically and flags each overnight row, which saves a lot of second-guessing on a rotating roster.
Step 5: Convert to decimal and total the week
Payroll uses decimal hours. Divide the leftover minutes by 60: 15 minutes is 0.25, 30 is 0.5, 45 is 0.75. Then add the days.
Total the week before you split out overtime. Under U.S. federal rules overtime is based on hours in the workweek, so a 9-hour Thursday doesn't create overtime by itself if the week ends at 40 or less. A week of 8, 8.5, 8, 9 and 9 hours is 42.5 hours, so 2.5 of them are overtime.
Common time card mistakes
Small slips add up across a pay period. Writing 8.30 for 8 hours 30 minutes costs only 12 minutes on one day, but do it on every shift for a month and you've lost hours of pay. Keep a copy of your own times each week so you can compare them with your pay stub. These are the slips worth checking for.
- Writing 7 hours 45 minutes as 7.45 instead of 7.75.
- Subtracting a lunch break that was actually paid, or forgetting one that wasn't.
- Reading a PM end time as AM and getting a negative or tiny shift.
- Rounding each day, then rounding the week again.
- Splitting overtime by day when your rule is weekly, or the other way round.
Frequently asked questions
What is 12 AM in 24-hour time?
12 AM is midnight, written as 00:00. 12 PM is noon, written as 12:00.
How do I add up hours and minutes for a week?
Add all the hours, then all the minutes. Turn every 60 minutes into an hour. Or convert each day to decimal first and simply add the decimals.
Is 7.30 hours the same as 7 hours 30 minutes?
No. 7 hours 30 minutes is 7.5 decimal hours. 7.30 decimal hours is 7 hours 18 minutes.
Should I round my hours to the nearest quarter hour?
Only if your employer's policy says so. Use the exact minutes for your own records, then compare with how payroll rounds.
Do I need to count travel between job sites?
Often yes. U.S. Department of Labor guidance treats travel between work sites during the workday as work time, but rules vary, so check yours.
References
These sources support the method or guidance used for Work Hours Calculator. Verify time-sensitive rules at the source.
Try the calculator
Open Work Hours Calculator, enter your scenario, and compare its supporting rows with this guide's method and checks.