Whole weeks and the remainder
A span rarely divides into whole weeks, so the answer is given two ways: as whole weeks plus leftover days, and as a decimal. From 3 February 2027 to 19 May 2027 is 105 days, which is exactly 15 weeks. From 3 February to 20 May is 106 days, which is 15 weeks and 1 day, or 15.14 weeks.
Weeks and week numbers are different things
The number of weeks between two dates is a duration. The week number of a date is a position in the calendar year. Two dates 15 weeks apart can have week numbers that differ by 15, or by 16 if the span crosses a year boundary under ISO numbering, or by a negative number if it crosses into a new year. The result shows the week number at each end so the difference is visible rather than surprising.
Worked example — a term
A school term running from Monday 7 September 2026 to Friday 18 December 2026 is 102 days from start to end, or 14 weeks and 4 days — 103 days if you count both the first and the last day. It runs from ISO week 37 to ISO week 51. Removing twenty-eight weekend days and the Columbus Day, Veterans Day and Thanksgiving observances leaves 71 teaching days. Labor Day, Monday 7 September, is the start date itself and so is outside the count; if the term is described as beginning the day after the holiday, that is already what this figure assumes.
Why counting by weeks is often better
Weekly cycles survive month-length variation. A plan expressed as "14 weeks" is unambiguous; a plan expressed as "three and a half months" is not, because it depends which months. For anything with a weekly rhythm — pay periods, sprints, terms, treatment schedules — the week count is the number that actually means something.
The convention, stated plainly
The week count is the day count divided by seven. The day count follows the same rule as everywhere on this site: the start date is not counted, so a span from a Wednesday to the Wednesday fifteen weeks later is 105 days and exactly 15 weeks. Add one more day and it is 106 days, which is 15 weeks and 1 day, or 15.14 weeks as a decimal. Both forms are shown because they are used for different things: whole weeks plus a remainder is what a schedule wants, and the decimal is what a spreadsheet wants.
A whole-week span always starts and ends on the same weekday. That is the quickest sanity check there is — if your two dates fall on different weekdays, the answer cannot be a whole number of weeks, and any tool that gives you one has rounded.
Worked example — a fifteen-week block
From Wednesday 3 February 2027 to Wednesday 19 May 2027 is 105 days, exactly 15 weeks. It runs from ISO week 5 to ISO week 20 — a difference of 15, which matches, because the span stays inside one calendar year. Removing thirty weekend days and Presidents' Day on Monday 15 February leaves 74 working days. Extend the end by one day to Thursday 20 May and the day count becomes 106, the week count becomes 15 weeks and 1 day, and the working-day count becomes 75 — the arithmetic that catches people is that adding a single day to a span can change three different figures by three different amounts.
Where the week count and the week number stop agreeing
Subtracting one ISO week number from another gives the right answer only when both dates sit in the same ISO year. Across a year boundary it does not: 2026 has 53 ISO weeks, so a span running from week 50 of 2026 to week 3 of 2027 is six weeks, not minus forty-seven. And the ISO year does not line up with the calendar year — 1 January 2027 falls in ISO week 53 of 2026, while 31 December 2029 falls in ISO week 1 of 2030. The result panel shows the ISO year alongside the week number at each end for exactly this reason. The full rule, with the years where the two conventions disagree, is on the week number page.
Where other tools disagree
- Rounding to whole weeks. A tool that reports "15 weeks" for a 106-day span has silently discarded a day. We report the remainder.
- Weeks of 7.0 versus calendar weeks. Some tools count the number of week boundaries crossed rather than dividing by seven, which gives a different answer for any span not starting on the first day of a week.
- Inclusive counting. Counting both endpoints adds a day and therefore about 0.14 of a week. We show the exclusive figure as the primary answer and state the inclusive day count alongside it.
Related counts
For the same span in raw days, with the inclusive and exclusive figures side by side, use days between dates. For the count with weekends and federal holidays removed, use working days between two dates. To find the date a given number of weeks ahead rather than counting a span you already have, use weeks from today or the date calculator.