I Built a Free Week Number API (No Key Required) — Here's How

Shanwen Zhao — Sun, 21 Jun 2026 09:21:28 +0000

I built a free REST API for week number calculations. No API key required, CORS enabled, runs on Cloudflare's edge network.

🚀 Quick Examples

GET /api/week → Current week
GET /api/week?date=2026-12-25 → Week for Christmas
GET /api/week?year=2026&week=53 → Date range for Week 53

📊 What I Built

The entire site has 18,000+ static HTML pages generated programmatically across 15 languages. All hosted on Cloudflare Pages free tier.

Features:

🗓️ Instant week number — Open the page, see today's week immediately
🌍 15 languages — Chinese, English, Japanese, Korean, Spanish, French, German, Portuguese, Russian, Arabic, Hindi, Thai, Vietnamese, Indonesian, Turkish
📡 Free JSON API — No API key needed, CORS enabled
🔧 Embeddable widget — Add week numbers to your website
🧬 Viral tools — Life in Weeks visualization, Birth Week calculator, Countdown timer, Week Comparison tool
📅 Printable calendars — Full-year week calendars
📘 ISO 8601 guide — Complete reference
🧩 Chrome extension — One-click week number

🛠️ Tech Stack

Pure HTML/CSS/JS (no framework)
Cloudflare Pages + Functions for the API
Programmatic SEO with structured data (hreflang, BreadcrumbList, FAQ schema)
10+ interactive tools

🔗 Links

Website: weeknumber.cc
API Explorer: weeknumber.cc/api-explorer.html

Would love feedback from the dev community! 🙏

I Built a Week Number Calculator with 18000+ Static Pages and a Free API —— Here's How

Shanwen Zhao — Sun, 21 Jun 2026 08:50:04 +0000

A few months ago I realized something every time I needed to know what week it was I had to Google it. The results were cluttered with ads and outdated information. So I decided to build weeknumber.cc —— a clean fast free week number calculator.

What does it do

It tells you the current week number. But it does a LOT more

Date Week conversion —— Any date to week number any week number to dates
Full year calendars —— 2015 through 2035 every year visualized
15 languages —— Chinese English Japanese Korean Spanish French German Portuguese Russian Arabic Hindi Thai Vietnamese Indonesian Turkish
Free JSON API —— No API key CORS enabled
Embeddable widget —— One line of HTML to add week numbers to any website
Chrome Extension —— Quick popup in your browser
Viral tools —— Birth week calculator countdown timer life-in-weeks visualizer comparison tool quiz

The Numbers

18000+ static HTML pages —— Every week of every year every date every month every language plus history pages
15 languages with proper hreflang tags
18623 URLs in the sitemap
Zero frameworks —— Pure HTML/CSS/JS
0 hosting cost —— All on Cloudflare Pages free tier

The API my favorite part

Developers love this

bash

Get current week

curl https//www.weeknumber.cc/api/week

Get week for a date

curl https//www.weeknumber.cc/api/weekdate=2026-12-25

Get dates for a specific week

curl https//www.weeknumber.cc/api/weekyear=2026&week=1

Returns clean JSON
json

"week" 25
"year" 2026
"startdate" "2026-06-15"
"enddate" "2026-06-21"
"system" "ISO 8601"

No API key. No rate limits. Just works.

The Widget

Add this to any website

html
div class="wn-widget" data-lang="zh"/div
script src="https//www.weeknumber.cc/widget.js" async/script

That's it. It renders a beautiful week number badge in any of 15 languages.

How I built it

The entire site is static HTML. I use a build script that generates every page programmatically —— every week of every year every date conversion every language variant. The result 18000+ pages that load instantly from Cloudflare's global edge network.

The API runs on Cloudflare Pages Functions their edge function platform. The OG image generation and widget are also serverless edge functions.

Why it's working

Programmatic SEO —— 18000+ pages each targeting specific long-tail keywords like "week 25 2026" "what week is June 15" "25 2026"
Hreflang tags —— Every page properly signals language variants to search engines
Structured data —— FAQ and BreadcrumbList schema on every page
100% Core Web Vitals —— Pure HTML zero JavaScript on main pages loads in milliseconds
Free backlinks —— The API and widget naturally attract developer links

Try it

Visit weeknumber.cchttps//www.weeknumber.cc

The API widget and all tools are completely free. I'd love to hear your feedback

DEV Community: Shanwen Zhao