Time Length Calculator
How much time passed between two times? This time length calculator answers in one click: enter a start and an end time and get the exact duration in hours, minutes and seconds, plus total minutes, total seconds and decimal hours for timesheets and billing. It reads both 24-hour and 12-hour AM/PM time, spots overnight spans that cross midnight on its own, and also runs in reverse: give it a start time plus a duration and it tells you when you finish.
| Total hours | |
|---|---|
| Total minutes | |
| Total seconds | |
| Decimal hours |
Equal start and end times give 0. Negative entries are read as 0, and blank fields count as 0 too.
Quick Answer
A span from 9:00:00 AM to 5:30:00 PM is 8 hours 30 minutes: 510 total minutes, 30,600 total seconds, and 8.50 decimal hours. The calculator above has those values pre-filled, so press Calculate duration to see every form at once. Overnight spans are detected automatically, and the second tab runs the question in reverse.
Key takeaways
- Two tools in one: duration between two times, or the clock time you get from a start plus a duration.
- Overnight spans need no checkbox: an end time earlier than the start is read as the next day.
- Seconds stay in the math, so the decimal hours, total minutes and total seconds always agree.
- Equal start and end times give 0. Blank fields and negative entries count as 0.
How to use the time length calculator
Two tabs sit at the top of the card, and results stay hidden until you calculate.
- Pick a tab. Find duration takes a start and an end time and returns the elapsed time. Find end time takes a start time plus a duration and returns the clock time you finish, which answers questions like “a meeting starts at 2:15 PM and runs 1 hour 45 minutes, when does it end?”
- Pick a format. The 24-hour / 12-hour toggle switches between entries like 13:30 and 1:30 PM. In 12-hour mode, AM/PM dropdowns appear beside each hour field; 12 AM is read as midnight and 12 PM as noon.
- Enter the times. Seconds are optional in every field: leave them blank and the tool treats them as 0. The worked example is pre-filled: 9:00:00 to 17:30:00.
- Calculate, then copy or share. The result line shows the duration as hours, minutes and seconds, with total hours, total minutes, total seconds and decimal hours in the table below it. Both result panels carry Copy my results and Share results buttons.
If you are totaling a full week of shifts rather than a single span, the time card calculator is built for that job, with breaks and overtime handling this page deliberately leaves out.
The elapsed time formula
Both tabs reduce times to seconds counted from midnight, do the arithmetic there, then convert back. A minute is 60 seconds, an hour is 3,600 seconds and a day is 86,400 seconds, the definitions NIST lists for time units used with the SI.
Duration = end seconds − start seconds
If end < start: duration = end seconds − start seconds + 86,400 (overnight rule)
End time = (start seconds + duration seconds) mod 86,400 (second tab)
The overnight rule is the only judgment call in the tool, and it is automatic: no checkbox, no “next day” toggle. A start of 10:30 PM and an end of 1:15 AM gives 2 hours 45 minutes with a note that the end falls on the next day. Equal start and end times are the one boundary this rule does not cross: they give 0, not 24 hours, because the two clock times name the same instant.
Worked example: 9:00 AM to 5:30 PM
The pre-filled values run a standard workday, 9:00:00 to 17:30:00. Here is what the tool does with them, step by step.
Run the second tab with start 9:00:00 and duration 8:30:00 and you get the mirror image: 32,400 + 30,600 = 63,000 seconds, which formats back to 17:30:00. Common spans for a quick sanity check:
| Span | Duration | Decimal hours |
|---|---|---|
| 9:00 AM – 5:00 PM | 8h 0m 0s | 8.00 |
| 8:30 AM – 5:30 PM | 9h 0m 0s | 9.00 |
| 7:00 AM – 3:30 PM | 8h 30m 0s | 8.50 |
| 10:00 PM – 6:00 AM (overnight) | 8h 0m 0s | 8.00 |
| 7:00 PM – 7:00 AM (overnight) | 12h 0m 0s | 12.00 |
| 2:15:30 PM – 3:42:15 PM | 1h 26m 45s | 1.45 |
Overnight spans and the next-day rule
Shifts that cross midnight are the classic trap in duration math, and this tool handles them without asking. Whenever the end time reads earlier than the start, 24 hours are added to the end before subtracting.
- 10:00 PM to 6:30 AM: 22:00 is 79,200 seconds, 06:30 is 23,400 seconds. The end is earlier, so it becomes 23,400 + 86,400 = 109,800; minus 79,200 gives 30,600 seconds, or 8 hours 30 minutes, with the next-day note.
- 11:59:30 PM to 12:00:15 AM: 45 seconds total, 0.01 decimal hours. Sub-minute spans like this are exactly why seconds stay in the calculation.
- What it does not do: the tool reads clock times, not dates, so a span longer than 24 hours is out of scope. Adding a 30-hour duration wraps around the clock once and reports 6:00 AM with the next-day note. For multi-day totals, compute each day here and roll the hours up with the hours to days calculator.
Decimal hours for timesheets and billing
Decimal hours turn a duration into a single number a payroll system can multiply by an hourly rate: 8 hours 30 minutes becomes 8.50, 1 hour 45 minutes becomes 1.75, 7 hours 15 minutes becomes 7.25. The tool divides total seconds by 3,600 and rounds to two decimals, so every figure in the results table agrees with every other.
Two practical notes for payroll use. First, the U.S. Department of Labor permits rounding punch times to fractions of an hour only where the practice is neutral over time: it cannot systematically shave minutes off employee pay. The widely used version is the 7-minute rule, where punches within 7 minutes of a quarter hour round down and 8 minutes or more round up. Second, this calculator reports exact time, not rounded time: feed its decimal hours into your payroll rules rather than assuming the rounded figure. If your timesheet asks for quarter-hour increments, round the tool’s exact output yourself and keep the raw figure as the audit trail.
What other time length calculators miss
Duration tools cluster into a few camps, and each leaves something out:
- Minutes-only tools (AJ Designer): clean worked math and a canonical 8.5-hour example, but no seconds and the midnight crossing needs a manual checkbox. Overnight spans are automatic here.
- 24-hour-only tools (Akyno Calculator): a swap-times button is a nice touch, but there is no AM/PM input at all, so American shift workers must convert by hand. The format toggle here keeps both notations.
- Negative-or-24-hour edge cases (HRS Calculator): equal start and end times read as a full 24 hours there, and an unchecked overnight span can show a negative duration. This tool gives 0 for equal times and never shows a negative.
- Date-range tools (Calculator Park): start/end dates plus a Now button cover multi-day spans, which is genuinely more than clock times can do, but the simple two-times question carries the weight of date pickers. Pick this page for clock times and a date tool for calendar spans.
- Break-deduction tools (TimeDurationCalculator.tools): unpaid lunch subtraction and FLSA rounding notes serve payroll directly. This page reports exact elapsed time and leaves rounding to your payroll rules, which keeps the audit trail clean.
- The gap this page fills: the reverse direction. Almost every competitor only answers “how long between these times.” The second tab answers the question people ask next: “if it starts now and runs this long, when does it end?”
One honest boundary: this tool converts between time units only as part of duration math. For standalone conversions, hours to minutes, days to weeks and the rest, the time conversion calculator is the dedicated page.
Frequently asked questions
The calculator treats it as an overnight span and adds 24 hours to the end time. A start of 10:30 PM and an end of 1:15 AM gives 2 hours 45 minutes, with a note that the end falls on the next day.
Yes. Switch the format toggle to 12-hour (AM/PM) and dropdowns appear beside each hour field. The tool converts to 24-hour time before calculating: 12 AM becomes hour 0 and 12 PM becomes hour 12, so 12:30 AM reads as 00:30 and 12:30 PM as 12:30.
Decimal hours express a duration as a fraction of one hour: 8 hours 30 minutes is 8.50 hours. Payroll and billing software multiplies an hourly rate by this single number, which is simpler and less error-prone than handling hours and minutes separately. The U.S. Department of Labor allows rounding to fractions of an hour only where the practice stays neutral over time.
Some jobs need seconds precision: timing phone calls, athletic splits, experiment intervals, and broadcast segments. Rounding to minutes first would silently corrupt those measurements, so seconds stay in the calculation from start to finish.
Zero. Equal times give 0 hours 0 minutes 0 seconds with no overnight note, because the two clock times name the same instant. Some competing tools read equal times as a full 24 hours instead; if you want to add a full day, use the find-end-time tab with a 24-hour duration.
Yes, that is the second tab. Enter a start time and a duration to add, and it returns the clock time you finish in your chosen format. A meeting starting at 2:15 PM that runs 1 hour 45 minutes ends at 4:00 PM.
Within one calculation the tool works on clock times, so a duration added in the second tab wraps around the 24-hour clock: adding 30 hours to midnight gives 6:00 AM with a next-day note. For multi-day totals, run each day separately and add the decimal hours together, or roll the total up with the hours to days calculator.
This page answers single-span questions: how long between two times, or when a span ends. A time card calculator totals a whole week of shifts, usually with breaks and overtime rules. Use this page for one interval and the time card calculator for the full pay period.
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References
- U.S. Department of Labor, Wage and Hour Division. Fact Sheet #22: Hours Worked Under the Fair Labor Standards Act (FLSA) (what counts as compensable hours worked; time rounding practices must not fail to compensate employees properly over time). Accessed October 24, 2026.
- National Institute of Standards and Technology. SP 330, Section 4: Non-SI Units Accepted for Use with the SI (minute = 60 s, hour = 60 min = 3,600 s, day = 24 h = 86,400 s, the unit definitions the tool’s arithmetic rests on). Accessed October 24, 2026.
- Encyclopaedia Britannica. 24-Hour Clock: Definition, History, Uses, and Facts (the day runs midnight to midnight in 24 hours; the 24-hour notation is the ISO 8601 international standard). Accessed October 24, 2026.
- QuickBooks (Intuit). Time Clock Rounding Rules and Guide (the 7-minute rule: 1–7 minutes round down, 8–14 round up to the quarter hour; rounding must be neutral and never systematically favor the employer). Accessed October 24, 2026.
- University of Utah, Human Resource Management. MyTime Rounding Rules for Non-Exempt Employees (a real 15-minute rounding policy: 7 minutes or less rounds back to the quarter hour, 8 or more rounds forward; punches must never be altered to game the rule). Accessed October 24, 2026.