On a payslip, 7.75 does not mean 7 hours 75 minutes. On a project quote, 2.50 does not mean 2 hours 50 minutes. This confusion between decimal notation and traditional time format leads to billing errors, disputes over overtime, and incorrect schedules.
Converting a number into hours requires a simple operation, but it relies on a distinction that many are unaware of: an hour is not divided into 100, it is divided into 60.
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Decimal hours and hundredths: the costly confusion
When we read 6.30 on a time sheet, we instinctively think of 6 hours 30 minutes. This is incorrect. 6.30 in decimal corresponds to 6 hours 18 minutes, because 0.30 hour multiplied by 60 gives 18 minutes. The part after the decimal does not express minutes; it expresses a fraction of an hour in base 10.
This distinction between decimal hours and hundredths regularly traps payroll managers and site supervisors. A time tracking software that displays 8.50 indicates 8 hours 30 minutes, not 8 hours 50 minutes. The discrepancy seems small over a day, but accumulated over a month of work, it skews the pay calculations.
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To clarify, we apply a method to convert a number into hours that separates the whole part (the hours) from the decimal part (the fraction to be multiplied by 60). This is the only reliable operation.
Conversion formula from decimal to hours and minutes
The formula consists of two steps. We present it here with a concrete example: converting 3.75 into time format.
Step 1: isolate the whole part
The whole part of the number directly gives the hours. For 3.75, we have 3 hours. Nothing to calculate, we keep this number as is.
Step 2: multiply the decimal part by 60
We take 0.75 and multiply it by 60. Result: 45 minutes. 3.75 in decimal therefore equals 3 hours 45 minutes.
Another example: 1.20 hours. The whole part gives 1 hour. We multiply 0.20 by 60, which gives 12 minutes. Final result: 1 hour 12 minutes.
For the reverse process (converting from 2 hours 40 minutes to decimal), we divide the minutes by 60. Here, 40 divided by 60 gives 0.6667. We thus obtain approximately 2.67 hours. This transition from time format to decimal is not perfectly reversible without rounding, as the fraction must be reconverted and often rounded to the nearest minute.
Quick conversion table for payroll and billing
Rather than recalculating each time, we can refer to a table for the most common decimal values. Here are the correspondences we most often encounter on time sheets:
| Decimal | Hours and minutes |
|---|---|
| 0.25 | 0 hours 15 minutes |
| 0.50 | 0 hours 30 minutes |
| 0.75 | 0 hours 45 minutes |
| 1.00 | 1 hour 00 minutes |
| 1.25 | 1 hour 15 minutes |
| 1.50 | 1 hour 30 minutes |
| 1.75 | 1 hour 45 minutes |
| 2.00 | 2 hours 00 minutes |
| 7.75 | 7 hours 45 minutes |
| 8.50 | 8 hours 30 minutes |
This table covers the majority of daily work durations. Quarter hours (0.25 / 0.50 / 0.75) represent the most frequent cases in payroll management, because many companies round working time to the nearest quarter hour.
Conversion in Excel: why the cell format changes everything
You open a spreadsheet, type 7.75 in a cell, format it as “Time,” and Excel displays 18:00:00. The result seems absurd, but it has a technical explanation.
Excel treats hours as fractions of a day, not as decimal hours. The value 1 represents 24 hours (a full day). When you enter 7.75, Excel interprets it as 7 days and 18 hours, totaling 186 hours, and only displays the remaining hour part.
To correctly convert a decimal into hours in Excel, you must divide the value by 24 before applying the time format. The useful formula:
- Cell A1 contains 7.75 (the decimal value)
- In another cell, enter =A1/24
- Format this cell in custom format [h]:mm to get 7:45
The format [h]:mm with brackets is necessary as soon as the duration exceeds 24 hours; otherwise, Excel resets to zero and displays a truncated result. Feedback on this point varies depending on the software versions, but the principle of division by 24 remains valid everywhere.
Multiplying decimal time by an hourly rate: direct calculation
The main advantage of the decimal format is direct multiplication. When a service provider bills at an hourly rate, decimal time allows calculating the amount without intermediate conversion.
Let’s take an hourly rate of 45 euros and a working duration of 3.75 hours. The calculation is immediate: 3.75 x 45 = 168.75 euros. No need to convert 3 hours 45 minutes into minutes, then divide, then multiply.
In classic format (3 hours 45 minutes), the same calculation requires going through a conversion: 3 + (45/60) = 3.75, then 3.75 x 45. We end up with the decimal anyway. That’s why payroll software and billing tools systematically store durations in decimal, even when they display an h:min format for the user.
- For payroll: decimal overtime simplifies the calculation of surcharges
- For billing: we directly multiply the duration by the rate, without intermediate steps
- For project tracking: decimal durations add up without having to manage 60-minute deductions
Converting a number into hours is not a school exercise. It is a daily operation for anyone managing working time, a quote, or a schedule. Remembering that the decimal part is multiplied by 60 (and not by 100) is enough to avoid most calculation errors.