Decimal Time Explained

What Decimal Hours Are

Decimal hours express a duration as a single number of hours, allowing fractions. 8 hours 30 minutes is 8.5 decimal hours, because 30 minutes is half an hour. Billing systems and payroll software generally store time this way so that sums and products work directly.

The 1.30 Trap

The number 1.30 is one point three hours, which is 1 hour and 18 minutes. It is not one hour and thirty minutes, which is 1.5 or 1:30. Writing 8.30 for 8 hours 30 minutes understates the time by 18 minutes and is the most common timesheet error there is.

The Colon Is the Fix

Writing 1:30 unambiguously means one hour and thirty minutes, because a colon is a separator rather than a decimal point. Adopt the rule that colon means clock notation and decimal point means decimal hours, and apply it without exception in every document.

Converting To Decimal Hours

Divide the minutes by 60 and add to the hours. So 8 hours 30 minutes is 8 plus 30 over 60, which is 8 plus 0.5, giving 8.5. The reverse converts by multiplying decimal hours by 60 to get total minutes.

Where Decimal Time Is Required

Payroll, invoicing and timesheet software usually expect it, and many contracts specify it explicitly. In those contexts the decimal form is the correct one to enter, whatever looks more natural to read.

Where It Is Dangerous

Anywhere the number is read aloud or interpreted as clock notation, which includes spreadsheets shared between people who did not write the formula. Round trip check: 8.5 decimal hours converts back to exactly 8 hours 30 minutes, whereas 8.3 converts to 8 hours 18 minutes.