Online Tool Store
· 5 min read
Top 3 Week Number Lookup Alternatives Worth Using
Manesh Jayawardhana
CIO & Co-founder
You need to know which week number a date falls in — for a project deadline, a manufacturing schedule, or filling in a form that asks for “Week 34” instead of a calendar date.
There are two competing standards worth knowing about: ISO 8601, which starts weeks on Monday and is used internationally in business and manufacturing, and the US convention, which starts on Sunday. Getting the wrong one can genuinely throw off a date by a week.
How to judge a week number lookup tool
Uses ISO 8601 correctly. Week 1 is defined as the week containing the year’s first Thursday — a tool that gets this edge case right around New Year’s is doing it correctly.
Shows the full week date range. Knowing “Week 34” is less useful without seeing the Monday-to-Sunday dates it actually covers.
No signup for a quick lookup. This is an instant, one-off reference check.
Offers both ISO and regional standards if relevant. If you work across US and international contexts, having both available in one tool avoids confusion.
The comparison
| Tool | Best for | Free tier | Watch out |
|---|---|---|---|
| GeneralBlue | Choice between ISO 8601 and US standard | Free, no signup mentioned | Ads shown on the page |
| Devryo | Extras like fiscal quarter and leap year status | Free, no registration | Fewer standard options — ISO 8601 only |
| TodaysDateAndTime | Full-year calendar view with week numbers | Free | Specific standard support not independently confirmed |
| Week Number Lookup | ISO 8601 week number and week year with Monday-Sunday range | Free, no signup | ISO 8601 only, no US-standard toggle |
Facts checked August 2026; tools change their plans.
GeneralBlue
GeneralBlue lets you choose between ISO 8601 (Monday start, the international business standard) and the US standard (Sunday start, common for retail and school calendars), returning the complete week range, quarter, and year progress.
It isn’t for someone who wants an ad-free page — the site displays ads, which is worth knowing if a clean, minimal interface matters to you.
Devryo
Devryo adds genuinely useful extras beyond the week number itself: fiscal quarter (Q1-Q4), automatic leap year detection, day of year, and week start/end dates, all with no registration required.
It isn’t for someone who needs the US-standard (Sunday-start) week numbering — it’s built around ISO 8601 specifically.
TodaysDateAndTime
TodaysDateAndTime provides a full-year calendar view showing week numbers, day-of-year counts, and remaining days in the year at a glance, useful for planning across the whole year rather than checking one date at a time.
It isn’t for someone who wants every standard-support detail confirmed — the specific breadth of standard options wasn’t independently verified.
Week Number Lookup
Our tool finds the ISO 8601 week number and week year for any date, along with that week’s Monday-to-Sunday date range — entirely in your browser.
A real limitation: it covers ISO 8601 only — for the US Sunday-start convention specifically, you’d need a tool like GeneralBlue that offers both.
Which one to pick
If you need the standard ISO 8601 week number with its exact date range, use our Week Number Lookup.
If you specifically need the US Sunday-start week numbering, use GeneralBlue.
If you want extras like fiscal quarter or leap year status alongside the week number, use Devryo.
If you’re planning across a full year and want a calendar view, use TodaysDateAndTime.
How to do it with Week Number Lookup
- Open the Week Number Lookup.
- Enter any date.
- Read the ISO 8601 week number, week year, and that week’s Monday-to-Sunday range.
Browse the full tools directory for more free, browser-based date and time tools.
You might also need
If the deadline or event you’re tracking is in a different time zone, our Time Zone Converter converts a date and time into ten major world cities at once, using accurate daylight saving handling.
Frequently asked questions
Is there a free week number lookup that doesn’t need an account?
Yes. Our Week Number Lookup and all three alternatives here find week numbers without requiring signup.
What’s the difference between ISO week numbering and the US standard?
ISO 8601 weeks start on Monday, with Week 1 defined as the week containing the year’s first Thursday — this means the first few days of January can sometimes belong to the last week of the previous year. The US convention simply starts weeks on Sunday and typically treats the week containing January 1st as Week 1, which can produce a different week number for the same date.
Why does Week 1 sometimes seem to start in the previous year?
Under ISO 8601, if January 1st falls on a Friday, Saturday, or Sunday, those early January days are actually part of the last ISO week of the previous year, not Week 1 of the new year — this trips people up regularly and is exactly the kind of edge case a correctly built ISO week calculator handles automatically, per the ISO 8601 date and time standard.
Final thought
For business or manufacturing contexts that reference “week numbers,” confirm whether ISO 8601 or a regional standard is expected before you rely on the result — the two can disagree by a week at certain points in the year, especially right around January 1st.