Day of Year & Week Number Calculator (ISO, US & Fiscal)
Get DOY, week number, and the exact week date range (ISO/US/fiscal) plus business days left with weekends and holidays—ready to copy into reports.
Overview
Trying to label a sprint as W52, reconcile a payroll week, or confirm the day-of-year (DOY) for a KPI cutoff? The tricky part is that “week number” can mean ISO, US/locale, or a fiscal rule—each can disagree near New Year.
This page gives you the week number and the start/end dates of that week, plus progress and business-day counts you can paste into tickets, spreadsheets, and status reports.
Last Verified: December 2025
Quick reference: ISO vs US vs Fiscal
| System | Week starts | What defines “Week 1” | What “year” the week belongs to | Typical use | Common gotcha |
|---|---|---|---|---|---|
| ISO-8601 | Mon–Sun | Week containing Jan 4 (first Thursday rule) | ISO week-year (can differ from calendar year) | Sprints, EU reporting, many software teams | Jan 1–3 can be in the previous ISO year |
| US/Locale | Sun–Sat | Week 1 begins on Jan 1 (partial week allowed) | Calendar year | Some payroll, US scheduling, local calendars | Week numbers shift vs ISO at year boundaries |
| Fiscal | Mon–Sun | Configurable: first day / first full week / first 4-day week | Fiscal year label = year the FY ends in | Corporate reporting, finance ops, internal calendars | Different companies use different “week 1” policies |
How the numbers are computed
Day-of-year (DOY)
- Jan 1 = DOY 1.
- Non-leap years have 365 days; leap years have 366.
- Practical implication: from March 1 onward, DOY is +1 in leap years compared with non-leap years.
ISO week number and ISO week-year
- Weeks run Monday to Sunday.
- Week 1 is the week that contains January 4.
- The label YYYY in YYYY-Www is the ISO week-year, not always the calendar year.
US/Locale week number
- Weeks run Sunday to Saturday.
- Week 1 starts on January 1, even if that week is not a full 7 days.
- This choice is why ISO and US week numbers often disagree in early January.
Fiscal week number
- You pick a fiscal year start month (common: April).
- You pick the week 1 rule: first day, first full week, or first 4-day week.
- Fiscal weeks are counted as Monday–Sunday, and the fiscal year label is the year the FY ends in.
Business days left (weekends + holidays)
- The count excludes your chosen weekend pattern (Sat/Sun, Fri/Sat, etc.).
- The count also excludes any exact holiday dates you paste (YYYY-MM-DD, one per line).
- Tip: if your workplace observes a holiday on a different day (e.g., “observed Monday”), paste the observed date for accurate counts.
Examples with real dates
Example A: 2025-12-25 (Christmas Day)
- DOY: 359 / 365 (2025 is not a leap year).
- ISO: 2025-W52 (Thu). ISO week range: 2025-12-22 to 2025-12-28.
- US/Locale: Week 52. US week range: 2025-12-21 to 2025-12-27.
- Fiscal (Apr start, first 4-day week): FY2026 week 39. Fiscal week range: 2025-12-22 to 2025-12-28.
Example B: ISO year crossover
- 2016-01-01 is 2015-W53-5 in ISO terms.
- That happens because the ISO “week-year” follows the Jan 4 rule, not the calendar year boundary.
Example C: Leap day and DOY
- 2024-02-29 is DOY 60.
- From 2024-03-01 onward, DOY is +1 compared with a non-leap year date with the same month/day.
Common mistakes and how to avoid them
- Using calendar year with ISO weeks: “2026-W01” can start in late December 2025. When you share ISO weeks, share the full label YYYY-Www, not just “week 1”.
- Assuming Week 53 is an error: ISO week 53 is valid in specific year alignments. If you see W53, check whether your system is ISO and whether the year starts on Thu (or leap year Wed).
- Mixing week-start days: If one report uses Mon–Sun and another uses Sun–Sat, your “weekly totals” can shift by up to 1 day at both ends. Align the week system before comparing KPIs.
- Time-zone boundary drift: “Today” depends on the chosen IANA time zone. Near midnight UTC, two teams can legitimately be on different dates, changing DOY and sometimes the week-year.
- Business-day inclusivity confusion: Decide whether you mean “days left after today” or “including today.” If you need a strict policy, state it in the report notes and keep it consistent.
- Holiday lists missing observed dates: The business-day logic only excludes the dates you paste. If your organization observes a holiday on a different day, paste that observed date.
Practical uses
- Sprint planning: “Ship in ISO 2026-W05” plus the exact week range for stakeholder emails.
- Payroll and scheduling: Confirm whether your “Week 1” follows ISO or a locale rule before exporting timesheets.
- Quarter/year close: Track DOY and business days left to plan cutoffs and approvals.
- Analytics bucketing: Avoid off-by-one-week errors when grouping events by week in BI tools.
Assumptions and limits
- Week systems here are ISO (Mon–Sun), US/Locale (Sun–Sat), and a continuous fiscal Mon–Sun rule set.
- Holiday handling is manual: only the pasted dates are excluded (no automatic country calendars).
- Some organizations use retail calendars like 4-4-5. Those are not modeled; treat results as an approximation if your policy differs.
Disclaimer: Educational and planning use only. For payroll, legal, or audited reporting, verify the week definition used by your organization and systems of record.
FAQ
Why can the ISO week belong to the previous year?
ISO week 1 is the week that contains January 4 (equivalently, the first Thursday). So dates near New Year can be in the prior ISO week-year; 2016-01-01 is 2015-W53-5.
Why do some years have Week 53?
An ISO year has week 53 when the year starts on a Thursday, or when a leap year starts on a Wednesday. Those alignments fit an extra full Monday–Sunday week.
What's the difference between ISO and US week numbers?
ISO weeks start Monday and week 1 contains January 4. US/locale weeks often start Sunday and count week 1 from January 1, even if it is a partial week.
Does changing the time zone change the week number?
It can for "today" and for dates near midnight UTC. A moment that is Dec 31 in one zone may be Jan 1 in another, which changes DOY and sometimes the week-year.
How is day-of-year (DOY) counted?
January 1 is DOY 1. Non-leap years have 365 days; leap years have 366, so DOY after February increases by 1 in leap years.
How are fiscal weeks computed here?
Choose the fiscal start month and the week 1 rule (first day, first full week, or first 4-day week). Weeks are Monday–Sunday and the fiscal year label is the year the fiscal year ends in.
How are business days left calculated?
From the selected date to the end of the current week/month/quarter/year, the tool excludes your chosen weekend pattern and any pasted holiday dates (YYYY-MM-DD).
Do you support 4-4-5 retail calendars?
Not yet. This tool uses continuous Monday–Sunday weeks; if your finance calendar uses 4-4-5 or 5-4-4 periods, treat results as an approximation.