How it works
The total number of days between the two dates is divided by 7 to get full weeks. Any leftover days are shown separately.
Weeks Between Dates — Calculate Weeks Between Two Dates Online
Pick a start date and an end date and the page reports how many full weeks separate them, plus the leftover days that don't fit into a complete week. Also shows the total day count, an approximate month count (using a 30.44-day average month), and a years-with-fractional-remainder display. Useful for tracking pregnancy weeks (40 weeks gestation = 280 days from last menstrual period), counting weeks until or since a project milestone, planning a multi-week trip, computing an apartment lease's remaining weeks, or scheduling weekly recurring events.
About this tool
The calculation parses both dates as UTC midnight to avoid daylight-saving offsets adding or subtracting an hour mid-month and bumping the week count. Total days is `Math.floor((end - start) / 86_400_000)`. Full weeks is `Math.floor(days / 7)`, remaining days is `days % 7`. Approximate months use 30.44 days per month (the Gregorian average accounting for the 365.2425-day mean year), and years use 365.2425 — useful for ballpark thinking but not precise enough for legal age computation (use the dedicated age calculator for that). Past dates are handled with `Math.abs`, so flipping the inputs gives the same magnitude. Common reference points: pregnancy is conventionally 40 weeks = 280 days from LMP (rule of Naegele), a 13-week quarter is one common business reporting period, a 12-week sprint is the now-classic Brian Moran scheduling unit, an EU work week is 5 weekdays = 35-40 hours, a typical mortgage is 1565-2087 weeks (30-40 years).
- Full weeks + remaining days, plus total days count
- Approximate months (30.44-day average) and years (365.2425 day mean)
- UTC midnight parsing — ignores DST/timezone hour shifts
- Math.abs on the difference — accepts dates in either order
- Live recalc as you change either date
- Useful for pregnancy week tracking (280-day Naegele rule)
- Useful for project sprint planning, lease tracking, age in weeks
- Distinguishes "approximate months" from precise calendar-month count
- No upload — both dates stay in your browser
- Mobile-friendly date pickers via HTML5 input type=date
Free. No signup. Your inputs stay in your browser. Ads via Google AdSense (consent required).
Frequently asked questions
How does the 40-week pregnancy convention work (Naegele's rule)?
Naegele's rule, formulated by Franz Karl Naegele (Heidelberg professor of obstetrics, 1812), estimates pregnancy due date as LMP (last menstrual period) + 280 days = LMP + 40 weeks = LMP + 9 calendar months + 7 days. It assumes a 28-day menstrual cycle with day-14 ovulation, so the actual gestational age from conception is ~38 weeks. Modern obstetric practice (ACOG Committee Opinion 700, May 2017, "Methods for Estimating the Due Date") confirms Naegele's rule as the starting point but recommends first-trimester ultrasound crown-rump length (CRL) for redating if discordant by > 5 days in trimester 1 or > 7 days in trimester 2. Beyond 22 weeks, ultrasound becomes too imprecise to override LMP-based EDD. Practical use: enter LMP date as start, today as end, see week count + remaining days.
What's the "12 Week Year" sprint methodology?
The 12 Week Year (Brian P. Moran & Michael Lennington, Wiley, 2013, ISBN 978-1-118-50923-4) is an alternative to annual goal-setting: replace 12-month plans with 12-week sprints (≈ one quarter), with weekly accountability + lead-and-lag indicators. The premise: a 12-week horizon is short enough to maintain urgency but long enough for substantive progress. Common variants: 4 sprints per year with a 1-week buffer between each (52 / (12+1) ≈ 4); some teams use 13-week (the standard fiscal quarter); some use 6-week (Basecamp-style Shape Up). Paired with the weeks-calculator: enter sprint start + today, see week count to gauge sprint progress.
How does EU vs US work-week length differ?
EU Working Time Directive 2003/88/EC (Article 6, read with Article 16(b)) caps average weekly working time including overtime at 48 hours over a 4-month reference period. UK + Ireland have individual opt-out clauses; some sectors (transport, healthcare) have sector-specific derogations. Typical EU national implementation is 35-40 hour standard week (France 35h since the Aubry Law in force February 2000, Germany ~40h, Spain 40h, Netherlands ~38h). US Fair Labor Standards Act 1938 sets 40-hour standard (fully phased in by 1940) with overtime above (1.5× pay), no maximum cap on hours worked. So a US software engineer pulling 60-hour weeks indefinitely is legal; a 60-hour week sustained over a 4-month reference period in the EU would exceed the 48h cap, with the UK individual opt-out or sector-specific derogations being the main paths to legal 60-hour weeks. Useful when comparing employment offers across jurisdictions or computing project timelines with EU vs US contractors.
How does ISO 8601 number calendar weeks?
ISO 8601:2019 specifies that weeks start Monday (not Sunday like the US convention) and Week 1 of any year contains the first Thursday of that year — equivalently, the first 4-day week. This means Week 1 may start in the previous year's December (if 1 January is a Friday/Saturday/Sunday) or in early January. A year contains either 52 ISO weeks (most years) or 53 ISO weeks (when 1 January falls on a Thursday, or when 1 January is Wednesday and the year is a leap year — equivalently 31 December is Thursday, or Friday in a leap year). 53-week years: 2020, 2026, 2032, 2037, 2043. The ISO week format is YYYY-Www-D (e.g., 2026-W18-3 = Wednesday of ISO week 18 of 2026). Used in fiscal-week reporting, ERP systems, and project management tools like Jira and Linear configurable to ISO weeks; the weeks-calculator's day+week breakdown matches the ISO format for fiscal-week reconciliation.
Why use 30.44 days/month for approximate-month conversion?
30.44 days is the Gregorian-exact average month: 365.2425 (Gregorian mean year) ÷ 12 = 30.4368 ≈ 30.44 days/month. The Gregorian year averages 365.2425 days because the leap year rule (divisible by 4, except divisible by 100, except divisible by 400) gives 97 leap years per 400-year cycle: (97 × 366 + 303 × 365) / 400 = 365.2425. The simpler Julian approximation 365.25/12 = 30.4375 days/month is also widely used (e.g., the days-between-dates calculator in this portfolio uses Julian for simpler mental math); the difference is ~0.0006 days/month ≈ 0.9 minutes per month, which compounds to ~10.8 minutes per year — negligible for ballpark estimation. For exact calendar-month arithmetic (Jan 31 + 1 month = Feb 28, NOT Feb 31), use the dedicated date calculator.
Sources (5)
- International Organization for Standardization (2019). ISO 8601-1:2019 — Date and time format including ISO 8601 week numbering (weeks start Monday; Week 1 contains the first Thursday of the year, equivalently the first 4-day week); year may contain 52 or 53 ISO weeks (53-week years 2020, 2026, 2032, 2037, 2043). Distinct from US convention where weeks start Sunday and Week 1 contains 1 January. ISO Technical Committee 154 (TC 154); supersedes ISO 8601:2004.
- Naegele, F. K. (1812). Naegele's rule for estimated due date — LMP (last menstrual period) + 280 days = LMP + 40 weeks = LMP + 9 calendar months + 7 days; assumes 28-day cycle with day-14 ovulation. Modern obstetric practice (ACOG Committee Opinion 700, May 2017) confirms Naegele's rule as starting point but recommends first-trimester ultrasound CRL (crown-rump length) for redating if discordant by > 5 days T1 / > 7 days T2. Franz Karl Naegele, Heidelberg professor of obstetrics, 1812; modern guidance from American College of Obstetricians and Gynecologists (ACOG) Committee Opinion 700 "Methods for Estimating the Due Date", May 2017.
- Moran, B., & Lennington, M. (2013). The 12 Week Year: Get More Done in 12 Weeks than Others Do in 12 Months — alternative to annual goal-setting; 12-week sprint cycle (≈ one quarter) with weekly accountability + lead-and-lag indicators. Adopted in OKR-adjacent contexts and by sales/sports/personal-development teams. Brian P. Moran & Michael Lennington, The 12 Week Year (Wiley, 2013); ISBN 978-1-118-50923-4.
- European Parliament & Council (2003). EU Working Time Directive 2003/88/EC — average weekly working time including overtime not to exceed 48 hours over a 4-month reference period (Article 6 read with Article 16(b)); minimum 11 consecutive hours daily rest + 24 consecutive hours weekly rest (Articles 3, 5); typical EU implementation 35-40 hour standard week. UK + Ireland have individual opt-out clauses; some sectors (transport, healthcare) have sector-specific derogations. European Parliament and Council Directive 2003/88/EC of 4 November 2003; consolidated Working Time Directive replacing 93/104/EC; transposed into national labour law (Spain transposes via the Estatuto de los Trabajadores, with Real Decreto-Ley 8/2019 adding the time-recording obligation). US comparison: Fair Labor Standards Act 1938 (40-hour standard fully phased in by 1940) with overtime above (no maximum cap).
- World Wide Web Consortium (W3C) (2018). Web Content Accessibility Guidelines (WCAG) 2.1 — Success Criterion 4.1.3 Status Messages. W3C Recommendation 5 June 2018; carried unchanged into WCAG 2.2 (Recommendation 5 October 2023).
These are the original publications the formulas in this tool are based on. Locate them by journal name and year on Google Scholar or PubMed.