The decimal number $1$ in binary is also $1$. This is because binary, like the decimal system, starts counting from zero, and the next number after $0$ is $1$.

Here's how you can calculate it:

1. The binary system is base 2, so each position represents powers of 2 (i.e., $2^0$, $2^1$, $2^2$, etc.).
2. For $1$ in decimal, you need to find the largest power of 2 that fits into $1$.
   • $1 = 2^0 = 1$
3. So, $1$ in decimal is represented as $1$ in binary.

Therefore, the binary equivalent of the decimal number $1$ is:

$1_{10} = 1_2$

Would you like any further details or calculations?

Here are some related questions:

1. What is the binary representation of the decimal number 2?
2. How do you convert the decimal number 5 to binary?
3. What is the binary representation of the number 15?
4. How can binary addition be performed?
5. How do binary and decimal systems differ in counting?

Tip: When converting from decimal to binary, always break down the number into sums of powers of 2.