100 Minute Clock Calculator

One hour, one hundred slots

Decimal hours cut the hour into 100 equal pieces called hundredths. One hundredth is 60 / 100 = 0.6 minutes, which is 36 seconds. That is the whole system: a clock that reads 7:45 holds 465 minutes, and 465 / 60 = 7.75 hours, which prints on a time card as 7.75 or as 775 hundredths.

The reverse trip is just as short. A decimal reading of 7.37 hours means 7 hours plus 0.37 of an hour, and 0.37 x 60 = 22.2 minutes, so the clock reading is 7:22 and 12 seconds. Payroll software never shows those seconds; it stops at the hundredth and lets the 0.2 of a minute fall where it may.

Nothing in the arithmetic is new. The minute hand has always divided the hour into 60, and payroll divides it into 100 for one reason: multiplying money by 7.75 is a single step, while multiplying money by 7:45 is not a calculation at all until somebody converts the 45.

Why payroll counts in hundredths

A paycheck is hourly rate times hours. With decimal hours that is one multiplication: $24.00 x 7.75 = 186.00. With clock notation the same shift would need a division inside the multiplication, and every extra step is a place for a typo that costs somebody money.

Hundredths also give every minute the same weight on paper. In a tenth of an hour system, one tenth is 6 minutes, so 22 minutes does not land on a mark and gets rounded. In hundredths, 22 minutes is 22 / 60 = 0.3667 hours, which rounds to 0.37 and sits 12 seconds from the truth. The rounding is smaller than the tenth system would allow, and it is the same size in every hour of the day.

There is a real cost to the rounding, and it goes both ways. A shift of 8:20 is 8.3333 hours, but a hundredths time card records 8.33, and at $24.00 an hour that pays $199.92 instead of $200.00, a gap of 8 cents against the worker. The 22 minute shift rounds the other way and adds about 5 cents. Over a year of time cards the gaps mostly cancel, which is the argument regulators accept, and the next sections show where the law draws the line.

Minutes to decimal hours, minute by minute

The table below covers the first half hour in single minutes, rounded to the nearest hundredth the way time cards round. Read any row as: that many minutes is that fraction of an hour.

Every sixth minute lands on a clean tenth, which is why many payroll systems round punches to the nearest 6 minutes. The ladder runs 6 / 12 / 18 / 24 / 30 / 36 / 42 / 48 / 54 minutes against 0.10 / 0.20 / 0.30 / 0.40 / 0.50 / 0.60 / 0.70 / 0.80 / 0.90 hours, and 60 minutes closes it at 1.00. Check one row by hand: 36 / 60 = 0.6, so 36 minutes is 0.60 hours or 60 hundredths.

Anything between the marks rounds to the closer one. 34 minutes is 0.5667 and shows as 0.57, while 35 minutes is 0.5833 and shows as 0.58. The maximum error from rounding to a hundredth is half a hundredth, or 18 seconds, which is why hundredths beat tenths (a maximum error of 3 minutes) wherever the punches are not forced onto 6 minute marks.

The other direction: decimal hours back to h:mm

Going from decimal to a clock reading means multiplying the part after the decimal point by 60. Take 0.37: that is 0.37 x 60 = 22.2 minutes, so 7.37 hours reads as 7:22 plus 12 seconds. The whole number part never changes; only the decimal tail converts.

Two decimals convert cleanly and two do not. 0.25, 0.50 and 0.75 land on 15, 30 and 45 minutes because quarters divide 60 evenly. Hundredths like 0.37 or 0.83 leave a remainder of seconds, and a time card throws those away. If a schedule says 8.10 hours and you want to know when to stop, the answer is 8:06 on the dot, because 0.10 of an hour is a whole number of minutes.

A week of time cards at $24.00 an hour

Here is a five day week converted the way payroll does it, with each punch pair turned to hundredths and then multiplied by the rate.

Check the total two ways and they agree here: 39.50 x 24.00 = 948.00, and the five daily amounts already sum to $948.00. That agreement is luck of this particular week. Wednesday is the row to watch, because 7:58 is 7.9667 hours and the time card rounds it up to 7.97, moving $0.08 into the paycheck. Sum the rounded daily amounts every week and small drifts can collect in either direction; sum the exact minutes first, 2,370 of them, then convert once, and the drift disappears.

The same week at a different rate scales by multiplication alone. At $31.00 an hour it is 39.50 x 31.00 = 1,224.50, and at $18.50 it is 730.75. Hours never change with the rate, which is the argument for keeping hours in decimal form everywhere a rate can be attached to them.

The 7/8 minute rounding rule in 29 CFR 785.48(b)

Federal regulation allows employers to record start and stop times to the nearest 5 minutes, one tenth of an hour, or quarter hour, as long as the practice does not end up shorting workers over time. That is 29 CFR 785.48(b), the Department of Labor's rounding rule, and the current eCFR text (2026) keeps the original wording about rounding practices averaging out.

The Department of Labor explains the quarter hour version in Fact Sheet #53: time from 1 to 7 minutes may be rounded down, while 8 to 14 minutes must be rounded up to the next quarter hour. A clock in at 8:07 records as 8:00, but a clock in at 8:08 records as 8:15. The seven minutes on the boundary are worth 7 / 60 x 24 = 2.80 at $24.00 an hour, which is why the direction of the round matters to both sides.

The rule has teeth in one direction only. Rounding has to be neutral on its face and has to even out in practice, so a system that always rounds late arrivals up to the quarter hour and early arrivals back to the shift start would fail the test. Hundredths rounding, at a maximum error of 18 seconds per punch, sits well inside the tolerated tenth of an hour, which is one reason payroll vendors default to it.

The 100 minute clock is a different animal

A true 100 minute clock divides the hour into 100 minutes of 36 seconds each, the way payroll hundredths do, and that is the sense this page uses. History tried a bigger version. France's decimal time of 1793 divided the day into 10 hours of 100 minutes, each decimal minute running 86.4 standard seconds, and the decimal hour stretched to 2.4 ordinary hours. The scheme lasted a few years before the 24 hour day won back the clocks.

The difference matters when a number crosses systems. A reading of 7.75 on a decimal clock face (of a 10 hour day) would be most of the day gone, while 7.75 payroll hours is a normal shift. If a form or a device says hundredths, assume payroll convention; if it says decimal hours of the day, check whether the day holds 10 or 24 of them before you translate.

Where decimal hour conversions go wrong

The classic error is reading a decimal as clock minutes. 7.37 hours is 7 hours and 22.2 minutes, not 7:37. The decimal tail is a fraction of an hour, so 0.37 means 37% of 60 minutes. Anyone who writes 7:37 for 7.37 has handed the employee 15 minutes of phantom time.

The second error is adding clock notation as if it were decimal. 1:45 plus 2:30 is 4:15, which is 4.25 hours, while adding the decimal forms gives 1.75 + 2.50 = 4.25. Same answer, but only if you convert before adding. Add 45 + 30 and divide by 100, as a 100 minute clock would, and the sum comes out wrong by more than half an hour.

The third error is trusting the rounded figure for money. 8:20 rounds to 8.33 and pays $199.92 at $24.00 an hour rather than $200.00. On a single shift that is noise; on 2,080 hours a year, a systematic 0.0033 hour shortfall is 6.86 hours, or $164.64 at the same rate. When the stakes are a year of paychecks, keep the exact minutes until the last multiplication, and let the hundredths rounding land once, not fifty times.