2026-09-14 9 min

Week Number Calculator

Find the ISO week number of any date — plus week start/end, US week, quarter, day-of-year, and days left in the year.

Calculator

Date

Results update automatically

Results update live as you type

Status:W44 · 20262026-10-26 → 2026-11-01Monday · Q4

Week Summary

ISO weekW44 of 2026

Week range2026-10-26 → 2026-11-01

US weekWeek 43

DateMonday, October 26, 2026

Year positionDay 299 · 67 left

Year Progress

Day of yearDay 299 / 365

ISO week elapsedW44 / 53

Days left in year67 days

Breakdown

Measure	Value
Week-year	2026
Week range	2026-10-26 → 2026-11-01
US week	Week 43
Quarter	Q4
Day of year	299
Weeks in year	53

Results

Optimal

ISO Week

W44

ISO 8601 week of week-year 2026

Week-Year

2026

Year the ISO week belongs to

Week Range

2026-10-26 → 2026-11-01

Monday to Sunday (ISO)

Weekday

Monday

Day of week of the date

US Week

Week 43

Sunday-start week of the calendar year

Day of Year

299

Ordinal day within the year

Quarter

Calendar quarter of the date

Days Left in Year

67days

Midnights remaining after this date

ISO weeks follow the Thursday rule with Monday starts; the US week starts Sunday. All boundaries are UTC midnights.

Visualizations

Week Position

44ISO week

43US week

43Day of year ÷7

Year Progress

Day of year299

Week vs Weeks in Year

ISO week44

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Monday, October 26, 2026 sits in W44 of 2026 (53-week year)

Formula

ISO 8601 defines week 1 as containing the first Thursday; shifting any date to its week's Thursday reveals both the week-year and the count base.

ISO week via Thursday rule; range = Monday→Sunday; US week Sunday-start

Alternative Form

Weeks-in-year = ISO week of Dec 28 (52 or 53)

Variables

Symbol	Name	Unit	Description
D	Input date	UTC date	Anchor midnight
W	ISO week	1–53	Thursday-rule count
Yw	Week-year	year	Year of the week's Thursday
U	US week	1–53	Sunday-start count from Jan 1

Explanation

The Thursday trick works because every ISO week contains exactly one Thursday, and week 1 is defined as the week with the year's first Thursday (equivalently, containing Jan 4).

Derivation

Thursday = D − ((ISOweekday−4)); W = 1 + round((Thursday − firstThursday)/7d); range Monday→Sunday; U from Sunday anchor; Dec-28 week gives 52/53.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Parse the date as a UTC midnight.

D = Date(YYYY-MM-DD + T00:00:00Z)

Date:2026-10-26

Intermediate: Time-zone-proof anchor

Result: Monday, October 26, 2026

Step 2

Apply the ISO Thursday rule to find the week-year.

Shift to Thursday of the same week; its year is the week-year

Weekday:Monday

Intermediate: Jan 1 can belong to week 52/53 of the prior week-year

Result: Week-year 2026

Step 3

Count Thursdays from the week-year start for the week number.

W = 1 + round((Thursday − first Thursday) ÷ 7 days)

ISO Week:W44

Intermediate: Always 1–53, exact by construction

Result: W44 of 2026

Step 4

Step back to Monday and forward to Sunday for the range.

Start = D − ISO weekday index; End = Start + 6

Start:2026-10-26

End:2026-11-01

Intermediate: Monday-start, UTC midnights

Result: 2026-10-26 → 2026-11-01

Step 5

Read the parallel US Sunday-start week for American contexts.

US week = floor((Sunday − Jan 1) ÷ 7) + 1

US Week:43

Intermediate: Week 1 contains Jan 1

Result: US week 43

Step 6

Place the date in the year: ordinal day, quarter, remainder.

Day-of-year = floor((D − Jan 1) ÷ day) + 1

Day of Year:299

Quarter:Q4

Days Left:67

Intermediate: 53-week ISO year

Result: Day 299, Q4, 67 days left

Final Answer

W44

Worked Examples

See how the calculator works with different scenarios.

Scenario

September 14, 2026 (a Monday).

Inputs

Date:2026-09-14

Calculation

Thursday of week = Sep 17 → week-year 2026 Count → W38

Result

W38 of 2026, Sep 14–20

Interpretation

A Monday input opens its own week — ideal for sprint labels like 2026-W38.

Understanding Your Result

Your Result: ISO Week: W44

Typical range: Years hold 52 ISO weeks, occasionally 53. Week 1 contains Jan 4; mid-September is around W37.

What This Means

The ISO week plus its Monday–Sunday range is the unambiguous weekly address of your date; the US week, quarter, and day-of-year translate it for other conventions.

Why It Matters

Sprints, payroll weeks, and epidemiological reports all run on week numbers. Mixing ISO and US weeks silently shifts deadlines by a day.

Professional Interpretation

Use ISO labels (2026-W37) for anything that sorts or crosses borders; use US weeks only for explicitly American office calendars.

Recommendations

•Label sprints as YYYY-Www
•Confirm ISO vs US before publishing
•Budget a 53rd payroll week in long years
•Use the range for meeting invites
•Copy ISO format into tickets

Educational Content

Introduction

The Week Number Calculator maps any date to its ISO 8601 week, the Monday–Sunday range, the parallel US week, and its position in quarter and year.

Concept Explanation

ISO week 1 contains the first Thursday; every week's Thursday shares its week-year. Counting Thursdays from the year's first gives a boundary-proof week number.

Applications

▸Naming sprints and releases

▸Aligning payroll weeks

▸Scheduling recurring Monday meetings

▸Comparing year-over-year weekly metrics

▸Planning around 53-week years

Benefits

•Boundary-proof ISO math
•Monday range for invites
•US week for American calendars
•53-week-year flag for payroll

Limitations

•ISO only as canonical; fiscal week calendars differ
•Whole days; times ignored
•Proleptic rules before 1582

Best Practices

•Prefer ISO labels for sorting
•State the convention in shared docs
•Check Dec 28 for 53-week years
•Pair with Quarter for reporting

Common Misconceptions

Myth: Week 1 always starts January 1.

Truth: No — ISO week 1 contains the first Thursday, so it can start Dec 29–Jan 4.

Myth: ISO and US weeks are interchangeable.

Truth: They differ by start day and year-boundary rules; off-by-one weeks are common at New Year.

Myth: Every year has 52 weeks.

Truth: Most do, but years starting Thursday (or leap years starting Wednesday) hold 53.

Practical Tips

•Label sprints YYYY-Www
•Check New Year boundaries
•Confirm convention with partners
•Use ranges in invites
•Watch 53-week payroll years
•Copy ISO into tickets
•Pair with Quarter for reports

Common Mistakes

Assuming Jan 1 is week 1

Apply the Thursday rule — Jan 1 often closes the prior week-year.

Mixing Monday and Sunday starts

ISO starts Monday; US starts Sunday. Pick one per document.

Forgetting 53-week years

Weekly budgets and payroll need the extra week in long years.

Sorting US week labels as text

Only ISO YYYY-Www labels sort correctly.

Assumptions

This calculator makes the following assumptions to provide estimates:

Thursday Rule: ISO 8601 week definition throughout.

UTC Midnights: Dates are whole UTC days.

Monday Start (ISO): Ranges run Monday to Sunday.

Sunday Start (US): US weeks run Sunday to Saturday.

Limitations

Be aware of these limitations when using this calculator:

No Fiscal Weeks: 4-4-5 retail calendars are not modelled.

No Times: Weekday times ignored.

Gregorian Scope: Proleptic before 1582.

Frequently Asked Questions

Answers to common questions about this calculator.

What week number is today?

Enter today's date — the calculator returns the ISO week, the Monday–Sunday range, and the US week side by side.

Why does January 1 sometimes belong to last year?

Under ISO rules the week belongs to its Thursday's year, so early-January days can sit in week 52/53 of the prior week-year.

ISO vs US week — which should I use?

ISO (Monday start, Thursday rule) for international, sortable, payroll-safe labels; US (Sunday start) for American office calendars that expect it.

Which years have 53 weeks?

Years starting on Thursday, plus leap years starting on Wednesday — detected here via the week of Dec 28.

Can I get the date range of a week?

Yes — the Monday→Sunday range is computed for the ISO week of any date you enter.

Do weeks affect quarters?

Only loosely; use the Quarter Calculator when reporting cutoffs must be exact.

How accurate are these date calculations?

All math uses UTC midnight-to-midnight day arithmetic, so daylight-saving shifts never add or remove an hour from a day count. Results are exact for the Gregorian calendar for any date from 1900 through 2100.

Do leap years affect the result?

Yes, automatically. February 29 exists in leap years (divisible by 4, except centuries unless divisible by 400), and every total-days figure in this calculator counts it exactly.

Why do months give uneven results?

Months are 28 to 31 days long, so converting days to months is approximate. This calculator reports exact years, months, and days using calendar arithmetic, plus exact totals in days, weeks, and hours so nothing is hidden by rounding.

Which time zone is used?

Dates are parsed and compared in UTC to keep results identical in every time zone. Enter the calendar date as written; the exact hour of day is not modelled unless a time field is shown.

What date format should I enter?

Use the date picker or YYYY-MM-DD format, for example 2026-09-14. The calculator validates February 30 and similar impossible dates and falls back to today when a field is empty.

Can the end date be before the start date?

If the dates arrive reversed, the calculator swaps them internally and still reports a positive span, so the breakdown always describes the true distance between the two dates.

What is the difference between total days and calendar Y/M/D in the Week Number Calculator?

Total days is the exact midnight count, while Y/M/D is the borrowed calendar read like 1 y 2 m 5 d. Use total days for prorating and money math, and Y/M/D for birthdays, tenure and anniversary wording in the Week Number Calculator.

How does the Week Number Calculator handle February 29 birthdays and leap days?

February 29 is counted exactly whenever it falls inside the span, and Feb 29 birthdays roll to Feb 28 in common years for anniversary math. The leap rule is divisible by 4, except centuries unless divisible by 400.

How does month-end clamping work, for example Jan 31 plus one month?

Adding months keeps the same day number when possible and clamps to the last valid day otherwise, so Jan 31 plus one month becomes Feb 28 or Feb 29 in a leap year. The result keeps the correct weekday and month length context.

What is the difference between ISO week numbers and US week numbers?

ISO weeks start Monday and week 1 contains the first Thursday, so Jan 1 can belong to the prior year week 52 or 53. US week counting often starts Sunday and labels Jan 1 as week 1. The Week Number Calculator uses ISO Monday-start logic for consistency.

How is day-of-year computed and does it include Feb 29?

Day-of-year counts Jan 1 as 1 through Dec 31 as 365 or 366, with Feb 29 included as day 60 in leap years. Days remaining and percent elapsed are derived from 365 or 366 accordingly.

Can I use historic dates before 1900 or far-future dates after 2100?

Yes for planning math, with proleptic Gregorian rules extended backward and forward. Results before the 1582 Gregorian reform are arithmetic rather than historical, so cite them as calculated rather than observed for legal history.

What is the difference between Julian dates and Gregorian calendar dates?

Julian dates count continuous days from noon Jan 1, 4713 BC for astronomy, while Gregorian dates are civil month/day/year. The Week Number Calculator uses Gregorian civil dates; convert separately if you need Julian day numbers for astronomical work.

How do results compare with Excel DAYS, DATEDIF or Google Sheets?

Excel DAYS matches the exclusive midnight count, and DATEDIF Y/M/D matches borrowed subtraction when both use the same inclusive setting. If you see a one-day gap, check inclusive versus exclusive first, then 1900 versus 1904 date systems in Excel.

What does the Week Number Calculator show when start and end are the same date?

An exclusive same-date span reads 0 days, while inclusive reads 1 day, because inclusive counts the endpoint itself. This is correct for empty versus single-day ranges like same-day deadlines.

How can I print, save or share my calculation?

Use the copy and print controls to archive the headline plus breakdown with inputs and the inclusive setting noted. Paste into payroll, HR or client records so the one-day boundary choice is auditable later.

How are weekdays and weekend flags determined?

Weekdays come from the UTC day-of-week for each date, with Monday to Friday treated as workdays in business modes. The Week Number Calculator never shifts a weekday for time zones because dates are fixed at UTC midnight.

Why does 1582 or very old dates sometimes look odd?

The Gregorian calendar skipped 10 days in October 1582 when adopted, and Britain and others adopted later. The calculator applies uniform proleptic rules, which is exact for modern planning but not a historical record for those transition years.

Can I chain add and subtract steps, like plus 90 days then plus 6 months?

Yes. Take the result ISO date from step one as the anchor for step two, keeping the same month-end clamping rule each time. Chaining preserves weekday accuracy better than adding a single averaged day total.

Is the Week Number Calculator free on mobile and without sign-up?

Yes. Every Atlascalcs date tool runs instantly in the browser on phone, tablet or desktop with no account and no fee. Results update live as you change dates.

How do I choose inclusive versus exclusive for contracts and payroll?

Use inclusive when both endpoints are worked or billed, such as a rental from Mar 1 to Mar 3 counting 3 days. Use exclusive for elapsed time like SLAs and delivery windows, where Mar 1 to Mar 3 is 2 midnights.

What should I check if my hand count differs by one day?

Recheck three things in order: inclusive versus exclusive, whether you counted the start day by hand, and Feb 29 inside the span. Flipping the include toggle once proves the one-day delta in the step trace.