Week Number Calculator
ISO week number, day of year, and days left for any date.
What it does
ISO week numbers run Monday to Sunday with week one holding the first Thursday of January. That Thursday rule keeps every week at seven days and every year at 52 or 53 whole weeks, which plain division cannot do.
The tool above takes any date. It returns the ISO week, the day of year, and days left, all computed in UTC so zones never matter.
Edge days surprise once a year. January first through third can belong to week 52 or 53 of the prior year, while December 29 onward can open week one of the next.
Project plans love week numbers because they dodge month lengths entirely. Sprint names, release trains, and payroll periods all read cleaner as W41 than as mid October.
How to use
- Pick any calendar date.
- Read the ISO week for planning labels.
- Read day of year for ordinal math.
- Use days left for year end countdowns.
Formula
- dayOfYear
- Ordinal day from 1 to 365 or 366.
- weekday
- Monday start numbering for the ISO rule.
- +10 / 7
- Thursday anchoring folded into week count.
The Thursday of each week decides its year: a week belongs to whichever year holds its Thursday. That single rule creates 53 week years.
Worked example
Example: 10 Oct 2026
Date 10 October 2026, a Saturday.
- Day of year: Jan 31 + Feb 28 + Mar 31 + Apr 30 + May 31 + Jun 30 + Jul 31 + Aug 31 + Sep 30 + 10 = 283.
- Weekday: Saturday is 6 in Monday start.
- Week: floor((283 - 6 + 10) / 7) = floor(287/7) = 41.
- Answer: W41, day 283, with 82 days left in 2026.
Reference table
| Date | Week | Day |
|---|---|---|
| 1 Jan 2026 (Thursday) | W01 | Day 1 |
| 1 Apr 2026 | W14 | Day 91 |
| 1 Jul 2026 | W27 | Day 182 |
| 1 Oct 2026 | W40 | Day 274 |
Limits
- ISO system only. US week numbering differs.
- No holiday or workday logic included.
- Leap seconds ignored in day counts.
- Fiscal calendars follow their own rules.
Common mistakes
- Dividing day of year by seven for the week.
- Starting weeks on Sunday in ISO contexts.
- Assuming January 1 opens week 1.
- Forgetting 53 week years exist.
- Mixing US and ISO numbers in one plan.
Try it yourself
Quick checks. Answers sit tucked under each question.
Week of 1 Jan 2026 (Thursday)?
Thursday anchors week 1 of 2026. So W01, day 1.
Day of year for 4 Jul 2026?
181 days before July plus 4 = day 185.
Questions
What week is it now?
Enter today above. The tool reads the live date rules, not a fixed table.
Why does January 1 read W52?
Its Thursday falls in the prior year, so the week belongs there.
When do 53 week years happen?
When Thursday anchors 53 Thursdays: leap years starting Thursday and common years starting Thursday.
ISO vs US week numbers?
ISO starts Monday with Thursday rule. US weeks often start Sunday with January 1 in week 1.
How many weeks a year?
Fifty two usually, fifty three sometimes. The tool counts exactly.
What is day of year for?
Ordinal math: spreadsheets, astronomy, and Julian style day counts.