Binary, Hex and Decimal: How to Convert Between Number Bases

Oct 21, 2026 · 2 min read · Developer

We count in tens because we have ten fingers. Computers count in twos because a switch is either on or off. Programmers also use hexadecimal, which counts in sixteens. The same number can be written in all of them.

The four common bases

• Decimal (base 10) uses digits 0 to 9. This is everyday counting.
• Binary (base 2) uses only 0 and 1.
• Octal (base 8) uses 0 to 7. You still see it in file permissions.
• Hexadecimal (base 16) uses 0 to 9 and A to F.

One number, many ways

The number 255 is 11111111 in binary and FF in hex. Notice that hex is much shorter. One hex digit stands for exactly four binary digits, which is why developers like it.

Where you meet hex

• Colours in web design, such as #2563EB. Each pair is red, green and blue.
• Memory addresses and error codes.
• Hashes and IDs.

Converting decimal to binary by hand

Keep dividing the number by 2 and write down the remainders. For 13: 13 divided by 2 is 6 remainder 1, then 3 remainder 0, then 1 remainder 1, then 0 remainder 1. Read the remainders from bottom to top: 1101.

In code

(255).toString(2)    // "11111111"
(255).toString(16)   // "ff"
parseInt("ff", 16)   // 255

Prefixes

Code often marks the base with a prefix: 0b for binary, 0o for octal, 0x for hex. Our converter accepts them.

Big numbers

Normal JavaScript numbers lose accuracy when they get huge. The Number Base tool on our Developer Converters page uses BigInt, so long values stay exact.

Type a number, choose its base, and you see all four forms at once. If you enter a digit that is not allowed, such as 9 in binary, it tells you.

Frequently asked questions