420 became “the weed number” because a small group of California high-school friends in the early 1970s started using “4:20” as their private code for meeting up to smoke, and that in-joke eventually went global through cannabis and music culture.

The core origin story

Most historians of cannabis culture point to a group of five students at San Rafael High School in Marin County, California, in 1971, later nicknamed the “Waldos.”

  • They heard a rumor about an abandoned marijuana patch near Point Reyes.
  • They agreed to meet after school at 4:20 p.m. by a campus statue to search for it.
  • “4:20” became their code word for smoking or doing anything weed-related.
  • Over time, they just shortened it to “420,” and it spread through their wider social circle.

The legendary hidden weed field never really mattered; the meeting time and the code word did.

How 420 spread from an inside joke

The jump from a local high-school code to worldwide slang happened gradually but had a few key boosters.

  • Some of the Waldos had loose connections to the Grateful Dead scene in the Bay Area, where the slang “420” started circulating among fans.
  • In 1990, Deadheads in California printed flyers inviting people to “smoke 420” at 4:20 p.m. on April 20.
  • A copy of that flyer reached the magazine High Times; they printed it and began using and explaining “420” in their coverage, amplifying it to a national and then global audience.

Once it appeared in a high-profile cannabis magazine and in a big touring fan community, “420” became a recognizable wink for weed almost everywhere.

Myths people think are the reason

Because 420 feels oddly specific, a lot of urban legends grew around it. Most of these are fun, but wrong.

Common myths include:

  • Police code myth: That “420” is a police radio code for “marijuana in progress.”
  • Chemistry myth: That there are 420 active chemicals in cannabis.
  • Famous birthdays myth: That it’s tied to Bob Marley’s birthday or other famous figures’ birthdays.
  • Bob Dylan math myth: That it comes from his song “Rainy Day Women #12 & 35,” since 12 × 35 = 420.

None of these has solid evidence; they’re backfilled explanations people made up after 420 was already in use. The Waldos’ 4:20 p.m. meet-up story is the one with names, dates, and documentation behind it.

From 4:20 p.m. to the date 4/20

Once “420” was firmly tied to weed, turning it into a date was an easy next step.

  • “420” as a time → 4:20 p.m., the classic “spark up” moment.
  • In U.S. date format, April 20 is 4/20, which made the day itself a natural “holiday.”
  • By the 1990s and 2000s, April 20 became a de facto cannabis day with public gatherings, protests for legalization, and brand promos.
  • Today, many cities host 4/20 events, and dispensaries and brands use the date for sales and campaigns.

So “420” is both:

  • A time: 4:20 p.m., associated with smoking.
  • A date: April 20 (4/20), the unofficial weed holiday.

Why 420 stuck as a cultural symbol

Several things helped 420 turn from a local code into a global cultural shorthand.

  • It’s short, numeric, and easy to hide in plain sight (clocks, prices, usernames).
  • It’s vague enough that outsiders might not notice, which made it a useful bit of code when cannabis was heavily stigmatized or illegal.
  • Media and pop culture references (magazines, movies, music, online forums) repeatedly used “420,” reinforcing it as the weed number.
  • Legalization waves in places like parts of the U.S., Canada, and Europe turned 4/20 events into visible public festivals, making the association even stronger.

Think of 420 as a small inside joke that escaped its original friend group and, over a few decades, became a global cultural tag for cannabis.

TL;DR: 420 is “the weed number” because a group of California teens in the early ’70s used 4:20 p.m. as their daily meet-up time to smoke, turned “420” into code for weed, and that code spread through the Grateful Dead scene, High Times magazine, and wider cannabis culture until it became a worldwide symbol and the basis for the 4/20 cannabis “holiday.”

Information gathered from public forums or data available on the internet and portrayed here.