Never lose a lock combination again



I have personally gone through what seems like hundreds of combination locks over my lifetime. They take the form of gym lockers, bike locks, shed locks, etc.

Combination locks are great because it means that you don't have to worry about losing the key, but if you don't remember the lock combo, you are in trouble.



The solution to this problem is to write an encrypted version of the combo directly on the lock itself using a sharpie, and then all you have to do is work back from the encrypted version if you forget the lock combination. This can be as simple as adding your birthday to the number, and when you need to recover the number, you just subtract your birthday from it.

So if your birthday is:

10 / 13 / 77

and your lock is:

19 / 22 / 41

You write on the back of the lock

 29/ 35 / 118  (which is from 10 19 / 13 22 / 77 41).

An alternative encryption might be to concatenate the combination and add meaningless numbers. Unless you knew how to extract the correct digits, any attempt to use the numbers would be a waste of time.

For example, if your combination is 21 / 36 / 08, you might write the numbers 21421636108 which would require you to ignore the first three digits and the digits in between - 214(21)6(36)1(08) Any number of other methods would work as long as you don't forget how they work!


When you need to recover your lock combination, you would just subtract your birthday from the encrypted code on the back of the lock to recover the original number

(29-10=19 / 35-13=22 / 118-77=41)

This means that all you need is one three number combo to unlock all of the locks you own.


  • To simplify the math, you can reduce your birth year to just the last digit.
  • To increase security against people who might know your birthday, you can use another 3 number set such as your mother's birthday, or the last 6 digits of your phone number broken into 2 digit numbers.

Regardless of what you choose, choose something you can remember and use it consistently for all your locks.

If All Else Fails

Ultimately you can recover the combination even if you forget your birthday:

Another Process

Use a substitution cipher. Think of a 10 letter phrase with no repeating letters. For example, MUSTY FLAVOR. Each letter represents a number 1-9,0 according to its position.

So if your lock is:

19 / 22 / 41

You write on the back of the lock

MO / UU / TM  

Its extremely difficult to guess this combination unless you know the substitution cipher (and more difficult to figure out than a birthday.)

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