The Real Reason Might Surprise You: Why Are PINs Only 4 Digits Long?

This is a perfect example of how human limitations sometimes shape technology.

As digital technology becomes more widespread, we all face the need to protect our data through various authentication methods. One of the most common solutions is the PIN.

We use four-digit PINs to access bank cards, mobile devices, and other electronic services. But why exactly four digits? Wouldn’t more digits be safer? Let’s dive into the history behind it!

Don’t Worry—A Four-Digit PIN Is Secure Enough​

The history of PINs dates back to the mid-1960s, when the first ATMs were invented. On June 27, 1967, the first-ever ATM, developed by John Shepherd-Barron, was installed at a Barclays Bank branch in London. At the time, plastic bank cards didn’t exist yet, so customers used special paper vouchers with a weak radioactive coating to withdraw cash—these were issued by the bank upon request.

Even then, Shepherd-Barron realized there needed to be a better way to protect users' money.

Why Are PINs Only 4 Digits Long?​

The only option was for bank customers to enter a unique PIN to access their accounts and confirm transactions at the ATM.

Shepherd-Barron originally suggested a six-digit PIN (PIN stands for Personal Identification Number). He tested the system on people close to him, including his wife, Caroline. However, Caroline had difficulty remembering six digits and said the longest code she could reliably recall was four digits.

This made the inventor realize that PINs needed to be both secure and convenient for users. Before long, the four-digit PIN became the global standard that we still use today.

Are Four-Digit PINs Really Secure?​

Despite advancements in technology and the rise of more complex authentication methods, the four-digit PIN remains a crucial security feature. It's true that a four-digit PIN is relatively easy to guess using brute-force methods, as there are only 10,000 possible combinations (from 0000 to 9999).

To protect cardholders, banks limit the number of incorrect PIN attempts.​

For example, if you enter the wrong PIN three times in a row while making a purchase or withdrawing cash from an ATM, your card may be locked—either temporarily or permanently, depending on your bank’s policy. To regain access, you'll typically need to visit a branch in person with proper identification.