📏How many weeks between two dates?

✓ Tested & verified Updated: Aug 14, 2026 How we calculate this

Weeks between two dates

From

— weeks

— to — — — weeks in decimal

In days: —
In fortnights: —
Working weeks (Mon–Fri): —
Same weekday at both ends?: —

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How many weeks are there between two dates?

The DaysUntilNow weeks calculator divides the span into whole weeks and the days left over, so 100 days between two dates is 14 weeks and 2 days rather than 14.29 weeks. A remainder is the normal case: a week divides evenly into a span only when both ends fall on the same weekday, and the result says outright whether yours do.

The decimal is there as well, on its own line, because spreadsheets and progress trackers want it — but it is the second answer rather than the first. Underneath it the same span appears in days, in fortnights, and in working weeks at five days each, which is the figure a project plan runs on.

Weeks are the natural unit for anything with a weekly rhythm: pregnancy, school terms, sprints, notice periods, pay cycles. All of those are counted in whole weeks from a fixed start, and all of them leave a remainder that has to go somewhere.

The week is the only unit of time in common use that has no relationship to anything astronomical. Days come from the Earth turning, months from the Moon, years from the orbit — the week is a human invention seven days long, and it does not divide into any of them. Everything awkward about counting in weeks follows from that one fact.

Days into weeks, at a glance

Days	Whole weeks and remainder
30	4 weeks and 2 days
45	6 weeks and 3 days
60	8 weeks and 4 days
90	12 weeks and 6 days
100	14 weeks and 2 days
180	25 weeks and 5 days
280	40 weeks exactly
365	52 weeks and 1 day
366	52 weeks and 2 days

Two rows in that table are worth keeping. 280 days is the only round figure in it, and it is round on purpose — it is the length of a pregnancy under the standard dating rule, which is written in days precisely so that it lands on a whole number of weeks. And 365 days is 52 weeks and one day, which is the source of nearly every calendar oddity people notice.

Why a year is never a whole number of weeks

That single leftover day is why a fixed date does not keep its weekday. Every common year pushes it forward by one, and a leap year pushes it forward by two, so a birthday that falls on a Friday this year is on a Saturday next year and skips to a Monday when a 29 February intervenes.

It is also why the week-numbering year does not match the calendar year. Under ISO 8601, weeks run Monday to Sunday and week 1 is the week containing 4 January, which leaves the first days of January belonging to the last week of the previous year — and once every five or six years, the year needs a 53rd week to absorb the accumulated remainder. The calendar tools group has the week-number side of this; here the concern is only the span between two dates.

The places that count in weeks rather than months

Pregnancy. Dated from the first day of the last period, in weeks, because a month is too coarse and the milestones are weekly.

School and university terms. Timetables, teaching weeks and reading weeks are all counted from a fixed start, and the remainder at the end of term is exactly what everyone argues about.

Notice and probation periods. Two weeks’ notice and half a month are different lengths, and which one the contract says decides the leaving date.

Sprints and iterations. Two-week and three-week cycles are the default in software planning, so “how many sprints fit before the deadline” is a weeks question with a remainder that has to be planned around rather than ignored.

Pay cycles. Weekly, fortnightly and semi-monthly are three different schedules, and only the first two are actually measured in weeks.

Fortnights, biweekly and the 27-payday year

A fortnight is exactly fourteen nights and always has been. Semi-monthly pay is not a fortnight: it is half a month, so the interval swings between 13 and 17 days and there are 24 of them in any year. Fortnightly pay has 26 periods covering 364 days, which is why the calendar eventually catches up and squeezes in a 27th pay date.

Checking your own year takes two entries here: put the first pay date of the year in as the start and 31 December as the end, and read the fortnights line. Anything of 26 or more means a 27th date lands inside the year. For the calendar-day version of the same question, use days between dates; for the working-day version with holidays removed, the business days calculator; and to find the date a given number of weeks out rather than the count, days from today takes weeks as a unit.

Frequently asked questions

Is a year exactly 52 weeks?

No. 365 days is 52 weeks and 1 day, and a leap year is 52 weeks and 2 days. That leftover is why a fixed date walks forward one weekday a year and two across a leap day: 4 July is a Saturday in 2026, a Sunday in 2027 and a Tuesday in 2028. It is also why a span measured in weeks almost always comes out as whole weeks plus a remainder rather than a round number.

Why does 2026 have 27 biweekly pay periods instead of 26?

Because 26 fortnightly periods are only 26 × 14 = 364 days — one day short of a common year and two short of a leap year. The leftover accumulates until, roughly every 11 years, a 27th pay date falls inside the calendar year. 2026 is such a year: an employer whose biweekly pay dates fall on Thursdays pays on 1 January and again on 31 December, both Thursdays, for 27 in total. Employers either divide the annual salary by 27 or leave the per-period amount alone and absorb the extra.

Is pregnancy 40 weeks or 9 months?

Both, loosely. Naegele’s rule puts the due date 280 days — exactly 40 weeks — after the first day of the last menstrual period, and counts that first day as day one even though conception follows about two weeks later. Nine calendar months from the same date is only 273 to 276 days depending on which months you cross, which is why the rule is written as “nine months plus seven days”. Weeks are used precisely because calendar months are 28 to 31 days long.

Does "biweekly" mean twice a week or every two weeks, and is a fortnight always 14 days?

A fortnight is always exactly 14 nights — the word is Old English feowertiene niht, “fourteen nights” — so it is unambiguous. “Biweekly” is not: dictionaries record both “every two weeks” and “twice a week” as current usage, which is why payroll and publishing say semi-weekly or semi-monthly when it matters. The two payroll schedules are genuinely different: semi-monthly pay gives 24 periods a year, biweekly gives 26.

How we calculate this

The span is counted as whole days between two UTC midnights, then divided by seven. The quotient is the whole weeks and the remainder is reported as days, which is the form the answer is wanted in; the decimal equivalent is shown separately rather than instead, because the two are useful to different people and rounding one to produce the other loses the remainder.

Fortnights are the same span divided by fourteen and shown to one decimal, since a part-fortnight has no natural name. Working weeks count the Monday-to-Friday days in the range and divide by five, so a span that starts mid-week reports a fraction rather than pretending to start on a Monday; public holidays are not deducted here, which is what the business days calculator is for. The last line compares the two weekdays at the ends of the span, because that is the quickest check that a remainder is real: if both ends fall on the same weekday, the span is a whole number of weeks and nothing else needs explaining.

Worked example

With From 2026-08-14, To 2026-11-22, this page works out 14 weeks and 2 days. Friday, August 14, 2026 to Sunday, November 22, 2026 — 14.29 weeks in decimal

In days100
In fortnights7.1
Working weeks (Mon–Fri)14.2
Same weekday at both ends?No — Friday to Sunday

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