Weeks Between Dates — ISO 8601 Week Counter
Pick two dates — the calculator returns ISO 8601 weeks between, the fractional value, the start- and end-week labels (W01-W53) and the remainder days. The widget below stacks one strip per full week, plus a short amber strip for the remainder.
Quick Conversion
Formula: days = weeks × 7
ISO Week Strips
20w + 2d20 full ISO weeks + 2 days (fractional: 20.2857w).
Inputs
- Total days
- 142
- Full weeks
- 20
- Remainder days
- 2
- Fractional
- 20.2857
- ISO start
- W2
- ISO end
- W22
Common span presets
What do these weeks really mean?
20 full weeks plus 2 days maps directly onto European finance vocabulary — invoices and project plans use "week N" with ISO labels W01-W53. From W2 to W22, the count is exactly the number of Monday-Sunday spans crossed.
For agile teams, this maps to sprint count: a 2-week sprint cadence at 20 weeks = 10 sprints. For pregnancy tracking, this is the gestational-week count (~40 weeks full-term). For school terms (US standard 16-week semester, UK 12-week trimester), the count is the academic week tally.
The fractional value (20.2857) is what spreadsheet formulas use when computing weekly hourly rates or weekly compound interest. The 7-day cycle is robust against DST because date-only inputs work in floating midnight.
Standards and bodies governing week numbering
| Standard / Body | Year | Definition |
|---|---|---|
| Babylonian week | c.1000 BCE | Seven planets → seven days. |
| ISO 8601 / TC 154 | 1988 (rev 2019) | Week 1 = week with first Thursday. Mon-first ordering. |
| US convention | 19th c. → | Sun-first; Excel default WEEKNUM type 1. |
| Long ISO years | 28-year cycle | 53 weeks when Jan 1 = Thu (or Wed in a leap year). 2026 is long. |
| Postgres / MySQL | 2000s | Implement ISO weeks natively. |
| Bevan-Cooper algo | 1989 | Computing ISO weeks from Gregorian dates. |
Weeks ↔ days ↔ months
| Weeks | Days | Months (avg) |
|---|---|---|
| 1 | 7 | 0.23 |
| 2 | 14 | 0.46 |
| 4 | 28 | 0.92 |
| 8 | 56 | 1.84 |
| 13 | 91 | 2.99 |
| 26 | 182 | 5.98 |
| 52 | 364 | 11.96 |
| 100 | 700 | 23.00 |
| 200 | 1400 | 46.00 |
| 500 | 3500 | 114.99 |
For year-level math, see Years between dates.
Formula
fullWeeks = ⌊(end - start) / (7 × 86,400,000)⌋Worked: start = 2026-01-05 (Mon, W02-2026), end = 2026-05-27 (Wed, W22-2026). Δ days = 142. fullWeeks = ⌊142/7⌋ = 20; remainder = 2 d. Fractional = 142/7 = 20.2857 w. ISO span W02 → W22 = 20 ISO weeks elapsed.
Recent calculations
Save up to ten week-counts locally.
Walkthrough — stacking weeks
- Type the start date in the first yellow input.
- Type the end date — the calculator swaps if you reverse them.
- Watch one violet strip per full week tile into a grid, capped at 24 for visibility.
- An amber strip on the end represents remainder days (its width is proportional).
- Read fractional weeks, ISO start/end and remainder days from the side panel.
The history of the ISO week
In 2026, a Frankfurt project manager opens a fresh slide deck and writes 'W14 → W32, 2026' as the sprint plan. ISO weeks are the lingua franca of European finance and project management — quietly canonical.
The seven-day week itself reaches back to Babylonian astronomy (c.1000 BCE), where one day was assigned to each of the seven classical wandering bodies (Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn). The names survive in Romance and Germanic languages. The Christian week placed Sunday first, the Jewish week placed Saturday as Sabbath, and modern ISO 8601 placed Monday first.
ISO 8601 was first published in 1988 by the International Organization for Standardization and revised in 2000, 2004 and 2019. It defines week 1 as the first week of the year containing a Thursday — equivalently, the week containing 4 January. Week numbers run W01-W52 in 'short' years and W01-W53 in 'long' years (2026 is long; 2027 is short).
Why Thursday? Because to be 'in' a year, a week must have most of its days inside that year. Four days (the majority of 7) lands on Thursday or later — so the first week with Thursday in the new year wins. This rule was authored by ISO/TC 154 and is implemented natively by every JVM, .NET, PostgreSQL, MySQL and modern JavaScript runtime.
Long ISO years occur when Jan 1 is a Thursday, or Jan 1 is a Wednesday in a leap year. They follow a 28-year cycle, broken every 400 years by the Gregorian century rule. The next long years after 2026 are 2032, 2037 and 2043.
Practical adoption is uneven. European industry, Swiss watch and German automotive manufacturing live in ISO weeks. American calendars typically use the 'US week' which starts on Sunday. This calculator reports ISO weeks because they sort and arithmetic-add cleanly; the US convention is reachable by adding 1 to the start weekday if needed.
See also days between, date range and years between dates.
Three worked ISO-week examples
Example 1 — 2-week sprint
Start 2026-05-04 (Mon, W19), end 2026-05-17 (Sun, W20). Total = 13 d. Full weeks = 1, remainder = 6 d. Fractional = 1.857 w. Sprints often quote 'W19-W20 2026'.
Example 2 — pregnancy term
Start 2026-01-05 (Mon, W02), end 2026-10-12 (Mon, W42). Total = 280 d ≈ 40 weeks (full-term gestation). Fractional = 40.000 w.
Example 3 — Q4 fiscal block
Start 2026-10-01 (Thu, W40), end 2026-12-31 (Thu, W53). Total = 91 d. Full weeks = 13, remainder = 0 d. Fractional = 13.000 w. 2026 is a long ISO year (W01-W53).
Pro tips & common mistakes
- Mistake — ISO vs. US week confusion
- ISO weeks start Monday and define W1 by first Thursday. US weeks start Sunday and define W1 as the week containing Jan 1. They can differ by ±1 week label.
- Pro tip — long ISO years
- Long years (53 weeks) follow a 28-year cycle. 2026, 2032, 2037 are long. If you see a W53, the year is long.
- Pro tip — agile cadence
- Scrum standard 2-week sprint = 14 days = exactly 2 ISO weeks. Plan kickoffs on Monday for the cleanest W-label alignment.
What planners say
“European finance schedules everything in ISO week numbers. Punching two dates and seeing W14 → W32 saves me a colored-calendar lookup every Monday.”
“Argentina's collective bargaining cycles run on calendar weeks. The fractional-week output is what makes the 1.5-week ramp negotiable.”
“I cross-check parish week-of-baptism entries. ISO weeks plus the exact remainder days is the citation form I now insist on.”
“Japanese gakkou schedules in weekly granularity. Knowing the start week and end week pre-empts a half-hour spreadsheet build.”
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Glossary
- ISO week (W01-W53)
- Per ISO 8601, W01 is the week containing the first Thursday of the year. Weeks always run Monday-Sunday.
- Long ISO year
- A 53-week year. Occurs when Jan 1 is Thursday, or Jan 1 is Wednesday in a leap year. 2026 is long; 2027 is short.
- Week year
- The year a week officially belongs to under ISO 8601. Can differ from the calendar year for dates Dec 29-31 and Jan 1-3.
- US WEEKNUM type 1
- Excel default: Jan 1 = W01; Sun = day 1. Differs from ISO weeks by up to ±1 label.
- Gestational week
- Pregnancy convention: week 1 begins at the first day of the last menstrual period. Full-term gestation = 40 weeks.
Methodology & review
ISO week computation uses the Bevan-Cooper algorithm (1989): shift each date to its Thursday, then count weeks from Jan 1 of that Thursday's year. Reviewed against ISO 8601:2019 (Annex A) and PostgreSQL EXTRACT(week FROM ...) semantics.
Last reviewed: 2026-05. Author: Toolokit time-date team.
ISO 8601 vs US week conventions
| Property | ISO 8601 | US convention |
|---|---|---|
| First day of week | Monday | Sunday |
| Week 1 definition | Week with first Thursday | Week containing Jan 1 |
| Max week number | 52 or 53 | 52 or 53 (Excel mode 1) |
| Adopted by | EU, ISO members, finance, project mgmt | US calendar apps, retail planning |
| Excel function | ISOWEEKNUM | WEEKNUM(date,1) |
| PostgreSQL | EXTRACT(week FROM ...) | EXTRACT(doy FROM ...) / 7 |
| Examples this calculator uses | ✓ | — |
Where weeks-between math matters
Agile sprint planning
1-week, 2-week, 3-week cadences map cleanly to W-labels. Mondays-first ISO alignment keeps labels stable.
European finance
Invoice cycles, fiscal-quarter boundaries, regulatory filings use W-labels almost exclusively.
Pregnancy tracking
Gestational weeks (week 1 = first day of LMP); 40-week full-term anchor.
Academic terms
16-week US semester, 12-week UK trimester, 10-week quarter system at Stanford and Caltech.
Pharma trials
Phase-II dosing windows often quoted in weeks (W1-W12 etc.).
Retail planning
Holiday-quarter buildouts; back-to-school assortment windows; supply-chain lead-time tracking.
Long vs short ISO years (next two decades)
| Year | Jan 1 weekday | Leap year? | Length |
|---|---|---|---|
| 2024 | Monday | Yes | W01-W52 (short) |
| 2025 | Wednesday | No | W01-W52 (short) |
| 2026 | Thursday | No | W01-W53 (long) |
| 2027 | Friday | No | W01-W52 (short) |
| 2028 | Saturday | Yes | W01-W52 (short) |
| 2029 | Monday | No | W01-W52 (short) |
| 2030 | Tuesday | No | W01-W52 (short) |
| 2031 | Wednesday | No | W01-W52 (short) |
| 2032 | Thursday | Yes | W01-W53 (long) |
| 2037 | Thursday | No | W01-W53 (long) |
| 2043 | Thursday | No | W01-W53 (long) |
Long ISO year rule: Jan 1 is a Thursday, OR Jan 1 is a Wednesday in a leap year.
How this tool compares
| Feature | This tool | Excel WEEKNUM | Excel ISOWEEKNUM |
|---|---|---|---|
| ISO week labels | ✓ | Mode 21 | ✓ |
| Fractional week count | ✓ | — | — |
| Remainder days | ✓ | MOD() | MOD() |
| Visual week strips | ✓ | — | — |
| Long-year detection | ✓ | — | Returns 53 |
Pure day count between two dates.
Weekday/weekend split with shaded calendar.
Years plus decimal year-count.
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