Hours Calculator

Two calculators sit above this. The first gives the gap between two clock times on the same day, and the second works between two full dates when a shift runs overnight or a job spans weeks. Both report the answer as hours and minutes, as decimal hours, and as plain minutes, because payroll, invoicing and timesheets each want a different one.

What an hour is

An hour is 60 minutes, a minute is 60 seconds, and there are 24 hours in a day. The second is the part with a rigorous definition, fixed since 1967 as a specific count of caesium oscillations, and everything above it is built from that by multiplication.

Almost everyone reads time on one of two systems, and the difference between them causes more scheduling errors than the arithmetic ever does.

The 12-hour clock

The 12-hour clock runs 1 to 12 twice a day. AM is ante meridiem, before midday, and PM is post meridiem, after noon. Analogue clock faces carry no marker for which half you are in, so the distinction lives entirely in context or in the letters beside a digital display.

The convention is that 12 AM is midnight and 12 PM is noon, which is the single most misread pair of labels in scheduling. The logic is not obvious, since 12 PM comes before 1 PM yet noon is the boundary rather than something after it. If a deadline matters, writing "12 midnight" or "12 noon" removes the doubt entirely, and 11:59 PM is a common way of avoiding the question altogether.

The 24-hour clock

The 24-hour clock runs 00:00 to 23:59, with the day starting at midnight. Hours 00 to 11 are the AM half and 12 to 23 the PM half, so 17:30 is 5:30 PM. Nothing is ambiguous, which is why it is the international standard, the format in ISO 8601, and the one used in aviation, medicine, transport timetables and the military.

Converting is simple in both directions. For a PM time, add 12 to the hour unless it is already 12. For a 24-hour value above 12, subtract 12 and add PM. Midnight is the awkward one: 12 AM is 00:00, and 24:00 is occasionally used to mean the end of a day rather than the start of the next.

Hours in different time periods

Description	Hours
Hours in a day	24
Hours in a week	168
Hours in a month	672 for a 28-day month 696 for a 29-day month 720 for a 30-day month 744 for a 31-day month 730.5 on average
Hours in a year	8,760 for a 365-day year 8,784 for a 366-day year 8,766 on average
Hours in a decade	87,648 for a decade with 2 leap years 87,672 for a decade with 3 leap years 87,660 on average
Hours in a century	876,600

The month and year figures are ranges rather than single numbers because months and years vary in length. The averages come from the Gregorian year of 365.25 days as used for long spans: 8,766 hours in an average year, and 730.5 in an average month.

Decimal hours against hours and minutes

This is where timesheets go wrong. Three hours and 42 minutes is 3.7 hours, not 3.42. Minutes run to 60 rather than 100, so converting means dividing the minutes by 60 rather than sliding them behind a decimal point.

The error is quiet and expensive. Recording 7 hours 30 minutes as 7.30 instead of 7.5 loses twelve minutes of pay per shift, which is around an hour a week. Payroll systems almost always want decimal hours, while rosters and clocks show hours and minutes, so a conversion happens somewhere in nearly every workflow. The calculators above print both so no one has to do it in their head.

A few conversions worth memorising: 15 minutes is 0.25, 20 minutes is 0.333, 30 minutes is 0.5, 40 minutes is 0.667, and 45 minutes is 0.75.

Shifts that cross midnight

A shift from 10 PM to 6 AM is eight hours, but subtracting the clock numbers gives a negative. The first calculator handles this by reading an end time that is not after the start as belonging to the next day, and it says so when it does. Where a span runs over more than one night, or over a date boundary you care about, the second calculator takes full dates and removes the guesswork.

Daylight saving is the other trap. On the day clocks go forward, a shift from 9 AM to 5 PM is seven hours of actual time rather than eight; on the day they go back it is nine. Both calculators work in local time, so a span crossing a changeover reflects the real elapsed hours rather than the difference between the numbers on the clock face.

Rounding and paid time

Employers in the United States are permitted to round recorded time to the nearest quarter hour under the Fair Labor Standards Act regulations, provided the rounding is neutral over time and does not systematically favour the employer. In practice this produces the widely used seven-minute rule: anything from 1 to 7 minutes past the quarter rounds down, and 8 to 14 rounds up.

Rounding that only ever goes one way is not permitted, and rounding cannot be used to shave time an employee actually worked. Unpaid breaks are a separate question again: a 30-minute lunch inside a nine-hour span leaves eight and a half paid hours, so subtract breaks before the total goes anywhere near a payslip.

Reading your result

Take the line that matches where the number is going. Hours and minutes suit a roster or a conversation, decimal hours suit payroll and invoicing, and plain minutes suit anything being summed across many short intervals, since minutes never need a carry.

Check AM and PM before anything else, because an eight-hour error usually means one of them is wrong rather than the arithmetic. If the answer looks about twelve hours out, that is almost always a noon or midnight label rather than a mistake in the calculation.

For a running week of shifts rather than a single span, the Time Card Calculator totals several days at once. To add or subtract a fixed amount of time from a date instead of measuring between two, use the Time Calculator.

What the law counts as paid time

Under the Fair Labor Standards Act, overtime is owed after 40 hours in a workweek, paid at one and a half times the regular rate. The workweek is a fixed, recurring 168-hour period that an employer sets and does not shift around to avoid overtime. There is no federal daily overtime, so a 12-hour day inside a 38-hour week carries no premium under federal law. Several states run their own rules on top of that, California among them, and the state rule applies when it is more generous.

Breaks split into two kinds. A short rest break, usually 5 to 20 minutes, counts as paid work time when it is offered. A meal period of 30 minutes or more, taken free of duties, does not. Neither is required by federal law at all, so an unpaid half-hour lunch is a matter of company policy or state law rather than a federal entitlement. Eating at your desk while covering the phones is not a bona fide meal period, and that time is payable.

Rounding, and when it stops being legal

Rounding a punch to the nearest five minutes, tenth of an hour or quarter hour has been accepted practice for decades. The Department of Labor's regulation at 29 CFR 785.48 permits it "provided that it is used in such a manner that it will not result, over a period of time, in failure to compensate the employees properly for all the time they have actually worked."

The condition is the whole rule. Rounding has to be neutral, cutting both ways over time. A system that rounds every clock-in up to the quarter hour and every clock-out down is not rounding, and the shortfall is unpaid wages. Small numbers accumulate: seven minutes shaved from each of five shifts a week is about 30 hours a year.

Decimals, and the error hiding in them

Payroll runs on decimal hours, so minutes have to be divided by 60. Fifteen minutes is 0.25 and 45 minutes is 0.75, both exact. Twenty minutes is 0.333333 recurring, and this is where hand-built timesheets lose money. Entering 0.33 instead for a 20-minute stretch loses 0.000033 of an hour each time, which is invisible on one line and real across a payroll of hundreds. Keep four decimal places in intermediate steps and round once, at the end, on the total.