Kaprekar's Routine Calculator

How it is calculated

Kaprekar's routine works on a four-digit number. Pad it with leading zeros to four digits, sort the digits from largest to smallest to get D, from smallest to largest to get A, and compute D - A. The difference is padded to four digits again and the step is repeated until the number 6174 appears. The tool reports how many subtractions were needed.

Numbers from 0 to 9999 are accepted, so 12 is treated as 0012. Numbers whose four digits are all identical (0000, 1111 ... 9999) have no result, since they collapse to 0 and never reach 6174. Starting from 6174 itself needs 0 steps.

Common mistakes

• Dropping the leading zero: 0378 must stay four digits, and leaving it as 378 changes how the digits are sorted in later steps.
• Trying numbers with four identical digits such as 1111; they give 0 after one step and the tool returns nothing.
• Entering a number with more than four digits or a negative number, which is outside the supported range.
• Expecting the same step count for numbers that look similar: 3524 takes 3 steps but 2111 takes 5.
• Applying the routine to 3-digit or 5-digit numbers and expecting 6174; those cases lead to other constants or cycles, and this tool only handles four digits.