420 (four hundred [and] twenty) is the natural number following 419 and preceding 421.

In mathematics

420 is:

• the sum of four consecutive primes (${\displaystyle 101+103+107+109}$).
• the sum of the first twenty positive even numbers.
• a zero of the Mertens function^[1] and is sparsely totient.^[2]
• a pronic number.^[3]
• the smallest number divisible by the numbers 1 to 7; as a consequence of that, it is a Harshad number in bases 2 to 10, except in base 5.
• a 141-gonal number.
• a balanced number.^[4]

In other fields

• 420 is a slang term that refers to the consumption of cannabis. April 20th is commonly celebrated as a holiday dedicated to the drug, due to the numerical form of the day being 4/20.
• 420 is the country calling code for Czech Republic.