Collatz Sequence Calculator

About this calculator

The Collatz conjecture (also called the 3n+1 problem) proposes a deceptively simple rule: starting from any positive whole number, if it's even, halve it; if it's odd, triple it and add one; repeat. The conjecture — unproven despite being checked for every number up to enormous limits — claims this process always eventually reaches 1. This calculator runs the rule for any starting number and shows the full sequence, the total number of steps, and the highest value reached along the way.

The problem is a favorite in recreational and number theory mathematics because it's trivial to state but has resisted proof for nearly a century — mathematician Paul Erdős reportedly said mathematics wasn't ready for it. Students and hobbyists use a Collatz calculator to explore how wildly the sequence length and peak value can vary between nearby starting numbers, and educators use it as an approachable entry point into open problems in mathematics.