How to verify a credit card number in your head (why? because it is interesting you boring farts!)

[Hatter The Mad]

Edit: Sorry for the typo, corrected.

Edit 2: I didn't realise that so many people were not generally curious about everyday things, like myself. If you are not, don't waste your time reading


Ok, perhaps not in your head, but here is how to do it:

Take the long strip, and double every other number starting from the right (i.e. skip the first).

So, if your credit card number is:

4417 1234 5678 9113

4 * 2 = 8
1 * 2 = 2

1 * 2 = 2
3 * 2 = 6

5 * 2 = 10
7 * 2 = 14

9 * 2 = 18
1 * 2 = 2


Then take each digit and starting again from the right on that strip, add them to the ones we did not double (double digits become the sum of their parts - like 12 = 1+2). In other words, slot them back in like so:

8+4+2+7
+
2+2+6+4
+
1+0+6+1+4+8
+
1+8+1+2+3
=
70

All of these numbers add up to a number divisible by 10, and all valid credit card numbers do.

Try it and see!

 

[Russinating]

When would this ever be useful?

 

[Glaucus]

Why would any normal person need to know, odd thread is odd. On top of that how many people could do that in their head, that's a lot of numbers to remember.

 

[Castiel]

When would this ever be useful?

or be able to do it in your head on the fly?

 

[yhack]

8 * 2 = 14

Nope.

 

[PardonTheWait]

a) Who the **** can do that in their head?

b) Is that one of those sums that would work with any 16 digit number? I bet it is.

 

[Hatter The Mad]

Why would any normal person need to know, odd thread is odd. On top of that how many people could do that in their head, that's a lot of numbers to remember.

Why does any 'normal' person need to know anything? What's wrong with being curious about stuff?

Blissful ignorance is for knuckle draggers

 

[Hatter The Mad]

Nope.

YOU DID NOT SEE THIS

 

[Noxia]

I don't get why this is ever going to be needed.
Or is it more of a "fun" thingy.

 

[-BA-]

Hmm my post vanished. Weird. Pretty much said same thing as everyone else. Weird post with incorrect maths.

 

[Glaucus]

There's a lot of stuff normal people need to know. Randomly checking and card number is not one of them.

 

[Judgeneo]

Then take each digit and starting again from the right add them to the ones we did not double (double digits become the sum of their parts - like 12 = 1+2). In other words, slot them back in like so::

(8+4+2+7)
+
(2+2+6+4)
+
(1+0+6+1+6+8)
+
(1+8+1+2+3)

All of these numbers add up to a number divisible by 10, and call valid credit card numbers do.

Try it and see!

You worked from the left, Confused :s

 

[doiks14]

Suppose it could be useful for card verification for a software/website. I can't see doing this using my head though.

 

[Castiel]

Why does any 'normal' person need to know anything? What's wrong with being curious about stuff?

Blissful ignorance is for knuckle draggers

You cant even multiply 8 by 2!

Knuckledraggers.....

 

[GJ02]

snip

lolz my comment is your sig, a good thing or a bad thing ? lol

 

[Hatter The Mad]

You worked from the left, Confused :s

You start from the right on the original digit strip.

 

[Strife212]

b) Is that one of those sums that would work with any 16 digit number? I bet it is.

No it's a part of the CC system.

 

[Hatter The Mad]

You cant even multiply 8 by 2!

Knuckledraggers.....

I made a typo. The horror of it.

 

[Cosimo]

When would this ever be useful?

To impress folk on a forum.

 

[Judgeneo]

You start from the right on the original digit strip.

Yeah I realised that, but the numbers I quoted can be read from left to right from the original credit card number.


The question is, now we have a formulae for a valid credit card number, can someone give us the formulae for the correct security codes to go with them?