What is a decimal hour?
A decimal hour is the same span of time written as a base-10 number: one hour and thirty minutes is 1.5, not 1.30. The distinction matters because clock time runs in base 60 and arithmetic runs in base 10, and nothing warns you when the two get mixed. Add 7:45 and 6:30 in a spreadsheet that reads them as decimals and you get 13.75 when the true total is 14.25, half an hour lost across two entries. Payroll and billing software avoids the problem by converting every duration to a decimal fraction of an hour before any multiplication happens. That is why North Carolina's Department of Labor publishes a minutes-to-decimal chart, and why most university payroll offices publish one too.
How do you convert minutes to decimal hours?
Divide the minutes by 60, then add that to the whole hours. Forty-five minutes is 45 divided by 60, which is 0.75, so 7 hours and 45 minutes is 7.75. Twenty minutes is 20 divided by 60, which is 0.3333, so 2 hours and 20 minutes is 2.33 to two places. Going the other way, multiply the decimal part by 60: 0.4 hours is 0.4 times 60, which is 24 minutes. The only values that land exactly are the ones whose minutes divide into 60 cleanly, which is why 15, 30 and 45 minutes feel natural and 10, 20 and 50 do not.
| Minutes | Exact decimal | Two places | Nearest tenth |
|---|---|---|---|
| 5 | 0.0833 | 0.08 | 0.1 |
| 10 | 0.1667 | 0.17 | 0.2 |
| 15 | 0.25 | 0.25 | 0.3 |
| 20 | 0.3333 | 0.33 | 0.3 |
| 25 | 0.4167 | 0.42 | 0.4 |
| 30 | 0.5 | 0.50 | 0.5 |
| 35 | 0.5833 | 0.58 | 0.6 |
| 40 | 0.6667 | 0.67 | 0.7 |
| 45 | 0.75 | 0.75 | 0.8 |
| 50 | 0.8333 | 0.83 | 0.8 |
| 55 | 0.9167 | 0.92 | 0.9 |
| 60 | 1.0 | 1.00 | 1.0 |
Two decimal places, or one?
Both conventions are in live use and they disagree. The North Carolina Department of Labor chart runs to two places, so 25 minutes is 0.42. Duke University's payroll chart rounds to the nearest tenth, so 25 minutes is 0.4 and a single minute is 0.0. The Northern District of California, which requires attorney time in tenths, instead rounds whole six-minute bands upward, so anything from 1 to 6 minutes is 0.1 and anything from 19 to 24 minutes is 0.4. Three official charts, three answers for the same 20 minutes. Pick the convention your payer uses and apply it to every entry, rather than switching between them inside one week.
Where decimal conversion goes wrong
Two failures account for most of the drift. The first is rounding each entry and then summing the rounded numbers: six entries rounded up to the nearest tenth can add nearly half an hour that nobody worked. Keep the raw minutes, sum them, and round once at the end. The second is silent unit mixing, where a duration typed as 7.45 means 7 hours 45 minutes to the person typing and 7 hours 27 minutes to the software. In the US, 29 CFR 785.48 accepts recording start and stop times to the nearest 5 minutes, tenth of an hour or quarter hour, on the condition that the practice does not, over time, undercount what was actually worked. Rules differ elsewhere.
In short
- Divide minutes by 60 to get decimal hours. Multiply the decimal part by 60 to go back.
- 7:45 is 7.75, not 7.45. Treating clock time as a decimal is the single most common timesheet error.
- Official charts disagree on rounding: two decimal places, nearest tenth, and six-minute bands rounded up are all in use.
- Sum the raw minutes and round once at the end, never entry by entry.
- US rounding practice is accepted under 29 CFR 785.48 only if it does not undercount actual time over the long run. Rules differ elsewhere.
Related guides
- How to Calculate Work Hours: Subtract the start time from the end time, subtract any unpaid break, convert what is left to decimal hours, and total the days.
- What Are Billable Hours?: Billable hours are the hours a client is charged for, recorded against a specific matter or project and usually logged in tenths of an hour.
- What Makes a Good Timesheet: A good timesheet carries a date, a start time, an end time, any unpaid break, a daily total and a period total, plus whatever the work is attributed to.