How to calculate hours worked with a lunch break deducted
If you clocked in at 9:00 and out at 17:30, you did not work 8.5 hours unless your lunch was paid. Most are not. Subtract the break first, and the arithmetic that looks simple starts producing numbers that are off by a few minutes almost every time, because the two obvious ways of doing it by hand both have a trap built in.
The calculation, done correctly
- Work out the raw span from clock in to clock out.
- Subtract the unpaid break, in minutes.
- Convert the result to decimal hours only at the very end, not before.
9:00 to 17:30 is 8 hours 30 minutes. Subtract a 30 minute lunch and you get 8 hours 0 minutes, which is exactly 8.0 in decimal. Change the lunch to 45 minutes and the raw arithmetic is 8:30 minus 0:45, which is 7 hours 45 minutes, or 7.75 decimal hours. Not 7.45. That one mix-up, treating minutes as if they were hundredths, is the single most common timesheet error there is, and it understates a shift by a few minutes every time it happens. Across two weeks of shifts that adds up to real, if small, underpaid money.
Should you subtract the break at all?
This is the part people actually get wrong, more often than the arithmetic itself.
Subtract it if it was unpaid. An uninterrupted meal break where you clock out, leave your desk, and are not expected to answer anything, is unpaid time in most jobs, and it has to come off the total.
Do not subtract it if you were paid through it, or if it was a short rest break rather than a meal break. In the US, breaks under about twenty minutes are typically paid time and should stay in the total. A ten minute coffee break is not a lunch break, and treating it like one underreports hours you are legally owed for.
The rule of thumb: ask whether the time was genuinely yours to spend, with no expectation of being reachable. If yes, it is unpaid and comes out. If you were still on the clock in any real sense, it stays in.
Shifts that cross midnight
A night shift breaks the simple subtraction, because the end time is numerically smaller than the start time. 22:00 to 06:00 is eight hours, but 06:00 minus 22:00 as plain clock arithmetic gives a negative number, which is obviously wrong and is exactly the kind of thing that makes people distrust a spreadsheet formula they built themselves. The fix is to read an end time at or before the start time as belonging to the next day, then do the subtraction as normal: treat 06:00 as 30:00 on the same clock as the 22:00 start, subtract, and you get the correct eight hours.
Decimal hours versus hours and minutes
Payroll systems almost always want decimal hours, because that is what multiplies cleanly against an hourly rate. A reader wants hours and minutes, because that is how a day actually feels. The two do not round the same way, which is why showing only one of them is how the 8.15-versus-8.25 mistake keeps happening: fifteen minutes is a quarter of an hour, 0.25, not 0.15. The safest approach is to calculate once, in minutes, and then display both forms from that same number, rather than ever typing a decimal value in by hand.
A worked example across a full week
Say a five day week looked like this, with a 30 minute unpaid lunch every day:
| Day | In | Out | Raw span | Minus 30 min lunch |
|---|---|---|---|---|
| Mon | 9:00 | 17:30 | 8:30 | 8:00 |
| Tue | 9:00 | 17:45 | 8:45 | 8:15 |
| Wed | 8:50 | 17:30 | 8:40 | 8:10 |
| Thu | 9:05 | 18:00 | 8:55 | 8:25 |
| Fri | 9:00 | 16:30 | 7:30 | 7:00 |
Add the right-hand column in minutes, not in decimal, and you get 39 hours 50 minutes for the week, which is 39.83 decimal hours. Done the other way, by converting each day to a rounded decimal first and then adding, the total comes out slightly different, because each day’s rounding error compounds across five rows instead of happening once at the end.
Rounding to the nearest quarter hour
Some employers round each punch, not the daily or weekly total, to the nearest 15 minutes. If yours does, a clock-in at 8:53 becomes 9:00, and one at 8:52 becomes 8:45. In the US this practice is permitted under 29 CFR 785.48, and it is where the “seven minute rule” comes from: seven minutes past the quarter hour rounds back, eight minutes past rounds forward. The important part is that rounding is applied to each individual punch before the subtraction, not to the finished weekly number. Rounding a total that was never rounded at the clock produces a figure that matches nothing on the actual time sheet.
Doing this without the arithmetic
All of the above is exactly what the time card calculator on this site does for a full week at once: enter each day’s clock in and clock out, enter the unpaid break in minutes, and it handles overnight shifts, decimal and hours-and-minutes display, optional rounding, and daily-then-weekly overtime, with nothing uploaded and nothing saved anywhere but your own browser. For a single day or a single stretch of hours rather than a full week, the hours calculator does the same subtraction without the weekly layout.