Binary Coded Decimal, usually shortened to BCD, is a different way of storing numbers in binary — instead of converting a whole number into one long binary value, BCD converts each decimal digit separately, which turns out to be useful in specific, practical situations.
How BCD Actually Works
In standard binary, the number 47 is converted as a whole into 101111. In BCD, each digit is converted on its own using exactly 4 bits: 4 becomes 0100, and 7 becomes 0111, so 47 in BCD is written as 0100 0111 — two separate 4-bit groups rather than one combined binary number.
A Worked BCD Example
Converting 92 to BCD: the digit 9 becomes 1001, and the digit 2 becomes 0010, giving 1001 0010 as the full BCD representation — compare that to standard binary, where 92 converts as a whole number to 1011100, a shorter but less directly readable result.
Why This Uses More Bits
BCD is noticeably less space-efficient than standard binary, since each decimal digit always takes a full 4 bits even though 4 bits can represent values up to 15 — meaning combinations from 10 through 15 are simply never used in BCD. Standard binary packs the same number into fewer total bits by treating it as one value instead of digit by digit.
So Why Use BCD at All
BCD’s advantage is that converting between BCD and the decimal digits humans read is trivial — each 4-bit group maps to exactly one digit, with no place-value math required. This makes BCD useful in devices that need to display numbers directly, like digital clocks, calculators, and some older electronic displays, where converting from standard binary to decimal digits for display would otherwise require extra circuitry.
Where BCD Still Shows Up
Even though general-purpose computing moved almost entirely to standard binary for efficiency, BCD survives in financial software and some embedded systems, where avoiding the small rounding errors that can occur when converting binary fractions to decimal matters more than saving a few bits of storage.
Signed Numbers Work Differently Too
A related concept, the signed binary converter, deals with a separate problem: how to represent negative numbers in binary at all, since a plain binary number has no built-in minus sign. Common approaches reserve one bit specifically to indicate positive or negative, which changes how the remaining bits are interpreted compared to an unsigned binary number.
See Standard Binary for Comparison
To see how a number converts under standard (non-BCD) binary instead, our binary translator and our guide on how to read binary both use the ordinary whole-number method, which makes a useful side-by-side comparison against the digit-by-digit approach BCD uses.