One and a half hours can be written as 1h30 or 1.5 hours. But 1.30 decimal hours means 1h18. The difference comes from the units: an hour contains 60 minutes, while the decimal part describes a fraction of one hour.
Before adding durations or filling a spreadsheet, check which format it expects. Copying numbers without checking the unit can give a plausible-looking answer that represents a different length of time.
Convert minutes to decimal hours
Divide the minutes by 60 and add the whole hours: decimal hours = hours + minutes ÷ 60. For 2h45, calculate 2 + 45 ÷ 60 = 2.75 hours.
- 15 minutes = 0.25 hours.
- 30 minutes = 0.5 hours.
- 45 minutes = 0.75 hours.
- 1h15 = 1.25 hours.
- 1h30 = 1.5 hours.
Decimal separators vary by language and software settings. Check whether a field expects a point or comma before pasting a value.
Convert back to hours and minutes
Keep the whole-number part and multiply the decimal part by 60. In 3.25 hours, the three hours remain unchanged; 0.25 × 60 gives 15 minutes. The result is 3h15.
For 1.30 hours, the decimal part is 0.30. Multiply by 60 to get 18 minutes. Writing 1.30 when you mean 1h30 therefore shortens the recorded duration by 12 minutes.
Add values in the same format
Consider two periods: 1h30 and 2h45. In minutes, they total 90 + 165 = 255 minutes, or 4h15. In decimal hours, 1.5 + 2.75 = 4.25 hours. Both methods describe the same duration.
Check the duration sum in the hours and minutes calculator, using the hours-and-minutes format requested by its fields. Decimal conversion is the calculation explained here; do not enter a decimal into a field that expects HH:MM.
Watch rounding
Twenty minutes equals one third of an hour: 0.333… hours. Rounding every small part to two decimal places before adding many parts can change the total. Keep precision during the calculation and round the final display where possible.
If a system specifies a format or rounding rule, consult that guidance. This article explains the mathematics rather than setting payment or timesheet rules.
A quick sense check
Check the unit, convert a familiar example and see whether the total makes sense. Half an hour should become 0.5, not 0.30. For breaks and overnight periods, see the guide to adding hours and minutes.
