---
title: "Leap year and week number — free, runs in your browser"
description: "Whether a year is a leap year, the ISO week number, and the day of the week for any date. Free, no sign-up, and nothing you enter leaves your browser."
url: https://atoneplace.net/tools/leap-year-checker/
site: "At One Place"
---

# Leap year and week number

Whether a year is a leap year, the ISO week number, and the day of the week for any date.

## How it works

Tells you whether a year is a leap year, gives the ISO 8601 week number for a date, and names the weekday.

The leap year rule has three parts, and the third catches almost everyone: divisible by 4 is a leap year, unless divisible by 100, unless also divisible by 400. So 1900 was not a leap year and 2000 was. The correction exists because the tropical year is about 365.2422 days, not 365.25 — the quarter-day approximation drifts by three days every four centuries, and the century rule removes exactly that.

ISO week numbering is not the same as "which week of the year". Week 1 is the week containing the first Thursday, so the first days of January often belong to the last week of the previous year. A year has 52 or 53 ISO weeks, and weeks always start on Monday.

Both are why date arithmetic should always use a library or the platform date object rather than being derived by hand.

> This tool runs entirely in your browser. Nothing you enter is sent to a server, logged or stored, and the page keeps working with the network disconnected.

## Common questions

### Was 1900 a leap year?

No. It is divisible by 100 but not by 400. 2000 was, because it satisfies the 400 rule.

### Why do leap years skip some centuries?

Because the tropical year is 365.2422 days, not 365.25. Adding a day every four years overshoots by about three days per four centuries, and the century rule removes them.

### Can 1 January be in week 52?

Yes. ISO week 1 is the week containing the first Thursday, so early January dates often belong to the previous year’s final week.

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## Sources

- Calculated on this page — At One Place (/about/)