Week Number API

A free JSON endpoint for week numbers.

Send a GET request to /api/week and you get back the ISO week, US week, date range, holidays and more as clean JSON. No key, no sign-up. It works from a script, a spreadsheet, a bot or directly in browser JavaScript.

Prefer the visual tools? Use the week calculator or browse the full 2026 calendar.

Endpoints

Current week

GET /api/week

By date

GET /api/week?date=2026-05-22

By week and year

GET /api/week?week=21&year=2026

Pretty-printed

GET /api/week?pretty=1

Try it live

Date (optional)

GET /api/week

Press "Run request" to call the API.

Example response

{
  "ok": true,
  "iso_week": 22,
  "iso_year": 2026,
  "us_week": 22,
  "start": "2026-05-25",
  "end": "2026-05-31",
  "total_weeks_in_year": 53,
  "is_53_week_year": true,
  "quarter": 2,
  "region": "uk",
  "holidays": [ ... ],
  "adjacent": { "previous": { ... }, "next": { ... } },
  "links": {
    "self": "https://weekisit.com/week/22/2026",
    "calendar": "https://weekisit.com/week-numbers-2026",
    "ics": "https://weekisit.com/week/22/2026?format=ics"
  },
  "generated_at": "2026-05-27T00:00:00Z"
}

Response fields

Field	Type	Description
`iso_week`	integer	ISO 8601 week number, 1 to 53
`iso_year`	integer	ISO week-numbering year (can differ from calendar year)
`us_week`	integer	US week number, Sunday-start
`start`	string	Monday of the week, YYYY-MM-DD
`end`	string	Sunday of the week, YYYY-MM-DD
`start_label`	string	Human-readable start date
`end_label`	string	Human-readable end date
`total_weeks_in_year`	integer	52 or 53
`is_53_week_year`	boolean	True if the year has 53 ISO weeks
`quarter`	integer	Calendar quarter, 1 to 4
`day_of_year`	integer	Day number within the year
`days_in_year`	integer	365 or 366
`region`	string	Holiday region: us, uk, ca or au
`holidays`	array	Public holidays falling in this week
`adjacent`	object	Previous and next week, with dates and URLs
`links`	object	Canonical, calendar and .ics links
`generated_at`	string	UTC timestamp of the response

Code examples

JavaScript (fetch)

const res = await fetch('https://weekisit.com/api/week');
const data = await res.json();
console.log(`It is week ${data.iso_week} of ${data.iso_year}`);

Python (requests)

import requests
r = requests.get("https://weekisit.com/api/week", params={"date": "2026-05-22"})
print(r.json()["iso_week"])

cURL

curl "https://weekisit.com/api/week?week=21&year=2026&pretty=1"

API questions and answers

Do I need an API key?

No. The API is free and open. There is no key, no sign-up and no authentication. Just make a GET request and you get JSON back.

Can I call it from the browser?

Yes. The API sends an Access-Control-Allow-Origin header, so you can call it with fetch or XMLHttpRequest directly from any website's front-end code.

Is there a rate limit?

Please keep usage reasonable. Responses are cached for 60 seconds, so caching results on your side is both polite and faster. Heavy automated use should cache aggressively.

Which week numbering does it use?

Both. The response includes the ISO 8601 week (Monday start) and the US week (Sunday start), so you can use whichever your application needs.

Can I use it in a commercial product?

Yes, you are welcome to build on it. A link back to weekisit.com is appreciated but not required. Please cache responses to keep load reasonable.

Week calculator ISO week number Week calendar 2026 Current week number