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Collatz Calculator

Follow a number through the Collatz sequence to 1

A whole number from 1 up.

Steps to reach 1

111

Highest value reached
9,232
Numbers in the sequence
112
27 → 82 → 41 → 124 → 62 → 31 → 94 → 47 → 142 → 71 → 214 → 107 → 322 → 161 → 484 → 242 → 121 → 364 → 182 → 91 → 274 → 137 → 412 → 206 → 103 → 310 → 155 → 466 → 233 → 700 → 350 → 175 → 526 → 263 → 790 → 395 → 1186 → 593 → 1780 → 890 → 445 → 1336 → 668 → 334 → 167 → 502 → 251 → 754 → 377 → 1132 → 566 → 283 → 850 → 425 → 1276 → 638 → 319 → 958 → 479 → 1438 → 719 → 2158 → 1079 → 3238 → 1619 → 4858 → 2429 → 7288 → 3644 → 1822 → 911 → 2734 → 1367 → 4102 → 2051 → 6154 → 3077 → 9232 → 4616 → 2308 → 1154 → 577 → 1732 → 866 → 433 → 1300 → 650 → 325 → 976 → 488 → 244 → 122 → 61 → 184 → 92 → 46 → 23 → 70 → 35 → 106 → 53 → 160 → 80 → 40 → 20 → 10 → 5 → 16 → 8 → 4 → 2 → 1

Rule: if the number is even, halve it; if it is odd, triple it and add 1. The conjecture says every start reaches 1.

Runs in your browser. Nothing is uploaded.

About the Collatz Calculator

The Collatz conjecture is a famous unsolved puzzle. Pick any whole number: if it is even, halve it; if it is odd, triple it and add one. Repeat. The conjecture says you always end up at 1. This calculator follows any starting number step by step, lists the whole sequence, and tells you how many steps it took and the highest value it reached on the way. Type a whole number to start from and the Collatz sequence appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.

Updated October 2026

How to use the Collatz Calculator

  1. 1Enter a whole number to start from.
  2. 2Pick the units or options from the menus if the Collatz sequence tool offers them.
  3. 3Read the Collatz sequence in the large result box.
  4. 4Look at the extra rows for related figures.
  5. 5Press Copy result to keep the answer.

When you need the Collatz sequence

The sequence is a favourite for classroom exploration, programming practice and recreational maths. Looking at which small numbers take surprisingly long, such as 27, 97 and 871, is a fun way to see how unpredictable simple rules can be.

The numbers rise and fall like hailstones in a cloud, which is why the sequence is also called the hailstone sequence. No one has found a number that does not reach 1, and checks by computer go up to enormous values, but there is no proof.

    How the Collatz sequence is worked out

    If n is even, the next number is n ÷ 2. If n is odd, the next number is 3 × n + 1. The sequence stops at 1. Starting from 27 takes 111 steps and climbs as high as 9,232 before falling to 1. Starting from 7 gives 7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1, which is 16 steps.

    Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.

    • Handles very large starts
    • Shows every step
    • Step count and peak value

    Mistakes to avoid with Collatz sequence

    The calculator uses exact big-number arithmetic, so large starts are fine, but it stops after 100,000 steps to protect your browser. A start of 0 or a negative number is not allowed, and decimals do not apply.

      On phone and computer

      On a phone, type into the boxes one after another and read the Collatz sequence underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the Collatz sequence move. The collatz calculator works the same on both. If you like number puzzles, try Fibonacci Calculator and Number Sequence Calculator for other patterns. Prime Number Calculator and Factorial Calculator explore two more classics.

        Features

        • The conjecture is unproved
        • 27 is a classic example
        • Stops at 100,000 steps
        • Clear messages when the Collatz sequence inputs make no sense
        • Copy the result with one tap
        • Private: nothing is uploaded

        Frequently asked questions

        How do I calculate Collatz sequence?

        If n is even, the next number is n ÷ 2. If n is odd, the next number is 3 × n + 1. The sequence stops at 1. Starting from 27 takes 111 steps and climbs as high as 9,232 before falling to 1. Starting from 7 gives 7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1, which is 16 steps.

        What is the formula for Collatz sequence?

        If n is even, the next number is n ÷ 2. If n is odd, the next number is 3 × n + 1. The sequence stops at 1. The collatz calculator applies it exactly and rounds only the displayed answer to six significant figures.

        Has the Collatz conjecture been proved?

        No. It has been checked for every starting number up to about 2 to the power of 68, but no proof exists.

        Which small number takes the most steps?

        Below 100, the number 97 takes 118 steps, the most of any start under 100.

        Why 3n + 1?

        It is the simplest rule that mixes growth and shrinkage. Using 3n − 1 or 5n + 1 gives sequences that behave differently.

        Is this collatz calculator free and private?

        Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.

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