Binary and Data: Why Computers Use 0 and 1

Why only two symbols?

Inside a computer are billions of tiny switches called transistors. A switch can be OFF or ON — nothing in between. We write OFF as 0 and ON as 1. That is binary: a counting system with only two digits. Simple for the machine, and — with the right codes — enough for everything.

Counting in binary

You count 0, 1, 2, 3… Binary counts 0, 1, then runs out of digits and moves left, just like you do after 9:

Decimal:  0   1   2   3   4   5
Binary:   0   1   10  11  100 101

Each binary digit is called a bit. Eight bits together make a byte — enough combinations (256) for every letter, number and symbol on your keyboard.

How text becomes binary

A code called ASCII gives every character a number: the letter A is 65, which in binary is 01000001. Your messages, documents and code files are all stored as long lines of these numbers.

How images and sound become binary

• Images are grids of tiny dots (pixels); each pixel’s colour is stored as numbers
• Sound is measured thousands of times per second; each measurement is a number
• Video is simply many images plus sound, all as numbers

The beautiful idea

Text, photos, music, apps, this webpage — at the deepest level, all of it is 0s and 1s flowing through switches. Once you truly see this, computers stop being mysterious: they are just very, very fast at very simple things.

Go deeper — measuring data

Data sizes build up in steps of roughly a thousand: 1 kilobyte (KB) ≈ 1,000 bytes (a short text), 1 megabyte (MB) ≈ 1,000 KB (a photo), 1 gigabyte (GB) ≈ 1,000 MB (a short HD video), 1 terabyte (TB) ≈ 1,000 GB (a big hard drive). When an app says “128 GB storage”, it is counting bytes — billions of them, each one 8 bits of 0s and 1s.

Practice task (20 minutes)

Take the first letter of your name. Find its ASCII number from any ASCII table, then convert that number to binary by repeatedly dividing by 2 and writing the remainders backwards. Check your answer: it should be exactly 8 bits. Do the same for one more letter.

Common mistakes to avoid

• Mixing up bits and bytes — internet speeds are sold in bits, file sizes shown in bytes
• Thinking 1 KB is exactly 1,000 bytes in every context — formally it is 1,024 in binary counting
• Skipping binary as “theory” — it returns in networking, security and exams