Time Duration Calculator: Hours Between Times and Decimal Conversion
This calculator works out how long lies between two clock times, deducts an unpaid break, applies whatever rounding your payroll uses and gives the answer both as hours and minutes and as decimal hours. Shifts that run past midnight are handled without needing a date. There is also a second mode for adding or subtracting a duration from a time, which is what you need when working backwards from a finish time or forwards from a start.
Table of Contents
Decimal Hours Are Not Clock Time
Eight hours thirty minutes is 8.5, not 8.30. This single confusion is responsible for more payroll disputes than any other arithmetic error, because 8.30 looks plausible and is quietly two percent short.
Divide the minutes by sixty. Fifteen minutes is 0.25, twenty minutes is 0.333, forty five minutes is 0.75. Only the quarter hours give tidy decimals, which is part of why quarter hour rounding is so widespread.
Payroll and invoicing systems want decimals, because multiplying by an hourly rate is trivial in decimal and awkward in hours and minutes. The clock format is for humans; the decimal is for the calculation.
Watch the recurring decimals. Twenty minutes is 0.3333 and ten minutes is 0.1667, so rounding to two places loses a fraction of a second per entry. Over a month of daily entries that is still under a minute, but it is why some systems keep minutes internally and convert only at the end.
How to Use Both Modes
Between two times takes a start and an end, an optional unpaid break in minutes and a rounding rule. It reports the gross duration, the break, the net figure before rounding and the final answer, so you can see exactly where the time went rather than trusting one number.
Add or subtract takes a time and a duration in hours and minutes. It tells you the resulting clock time and whether the answer landed on a different day, which matters when planning night shifts or long processes.
Order of operations is deliberate. The break is deducted first, then rounding is applied to the net figure. Most payroll systems work that way, and reversing the two can move the answer by a whole rounding increment.
Shifts That Cross Midnight
An end time earlier than the start means the clock passed midnight. From 22:00 to 06:00 is eight hours. Subtracting naively gives minus sixteen, which is the classic night shift bug in hand built spreadsheets.
This tool adds a day automatically and tells you it has done so, rather than silently producing a number you have to sanity check.
Very long shifts need dates instead. Anything over twenty four hours cannot be represented by two clock times alone, since the tool cannot know how many midnights passed. For those, use the date calculator with full dates.
Daylight saving nights are the exception. Twice a year one night is twenty three or twenty five hours long. Clock arithmetic alone cannot see that, so a shift spanning a changeover needs checking against the actual dates.
Paid and Unpaid Breaks
Only unpaid breaks come off the total. A paid rest break stays in the worked time, so it should not be entered here. Getting this backwards is a common source of underpayment.
Rules vary widely. In many jurisdictions a break of twenty minutes or more is unpaid while shorter rest breaks are paid, but the thresholds and the entitlements differ by country and often by sector or collective agreement.
Interrupted breaks usually count as work. If someone is called back during an unpaid break, most rules treat that time as worked, which is worth recording rather than averaging away.
Rounding Rules in Payroll
Quarter hour rounding is the most common. Each entry is rounded to the nearest fifteen minutes, so seven minutes either side of the mark moves the figure. Over time it should average out, which is the justification for allowing it at all.
Six minute increments suit professional billing, because a tenth of an hour is exactly six minutes and produces clean decimals. Law firms and consultancies use this almost universally.
Rounding that only ever favours the employer is generally unlawful. Always rounding down, or rounding start times up and end times down, is a well known compliance problem. Rounding must be neutral over time.
When in doubt, do not round. Modern systems record exact minutes, and the historical reason for rounding, which was manual timesheets, has largely gone.
Billing Time to Clients
Agree the increment before the work, not after. Six minute billing and fifteen minute billing produce noticeably different invoices for the same work when there are many short tasks.
Minimum charges compound quickly. A fifteen minute minimum applied to twelve two minute emails bills three hours for twenty four minutes of work. Defensible if agreed, indefensible if discovered.
Record actual minutes and round at invoicing. Keeping the raw figure means you can always show your working, and it lets you change the rounding policy without losing the underlying data.
Time in Spreadsheets
Spreadsheets store time as a fraction of a day. Six hours is 0.25 internally. Subtracting two times therefore gives a fraction, and multiplying by 24 converts it into decimal hours, which is the step people forget.
Totals above 24 hours need a bracketed format. Formatting a cell as [h]:mm rather than h:mm stops a weekly total of 42 hours displaying as 18:00, which is one of the most confusing default behaviours in Excel.
Overnight shifts need a correction. The usual fix is adding one when the end is less than the start, which is exactly what this calculator does for you.
Where Overtime Starts
Daily and weekly thresholds are different systems. Some jurisdictions pay overtime after eight hours in a day, others only after forty in a week, and some apply both. The same timesheet can produce different pay under each.
Averaging arrangements change the answer again. Where hours are averaged over several weeks, a long week may attract no premium at all provided the average holds.
This tool measures a single span. For a full week with regular and overtime split out, the time card calculator is the right tool.
Common Mistakes to Avoid
Writing 8.30 for eight hours thirty. It means eight hours eighteen minutes. The correct decimal is 8.5.
Deducting a paid break. Only unpaid time comes off. Check which category the break falls into before entering it.
Rounding before deducting the break. The order changes the answer, and payroll convention is to deduct first.
Assuming every night is twenty four hours. Twice a year one is not, and a shift spanning the changeover needs checking against dates rather than clock times.
Frequently Asked Questions
On pay and entitlements: Break rules, rounding practices and overtime thresholds are set by law and by your contract, and they differ by country and sector. Use this for the arithmetic and check the entitlement against the rules that apply to you.