Kilobytes. Megabytes. Gigabytes. Terabytes. You’ve seen these words on every computer ad, every phone spec sheet, every plan from your internet provider. And unless someone explained them to you at some point, there’s a good chance they’ve stayed vague ever since — numbers you compare without quite knowing what you’re comparing.

Here’s what they actually are, in terms worth remembering.

It all starts with a single bit

A computer stores everything — every letter, every photo, every song, every email — as a sequence of on/off signals. On or off. One or zero. Each individual on-or-off signal is called a bit. Eight bits grouped together form a byte. A byte is roughly enough storage to hold one letter of the alphabet. The letter A. The letter Q. One character of text.

From there, the units scale up by roughly a thousand at each step:

What things actually weigh

The sizes above are approximate — but here’s a more practical sense of scale:

• A single text email: a few kilobytes. A thousand of them wouldn’t fill a megabyte.
• A high-quality photo from a modern phone: 3–8 MB. A thousand photos: 3–8 gigabytes.
• A three-minute song in MP3 format: 3–5 MB.
• A full music album: 50–100 MB.
• A standard-definition movie: 700 MB to 1.5 GB.
• A high-definition movie: 4–15 GB.
• A 4K movie: 50–100 GB.

The lesson here is that text is almost weightless and video is almost the opposite. Email takes up almost no room. Family videos take up a significant amount. A folder full of 4K footage from a camera is a completely different category of storage problem than a folder full of Word documents.

Storage vs. memory — two different things

This is where most people get confused, because computers use both measurements for two completely different things. Both are measured in gigabytes. Neither is the same as the other.

Storage (the hard drive or SSD) is the filing cabinet. It’s where your files live permanently — your documents, photos, music, installed programs. When you turn the computer off, the files stay. A typical home computer today has 256 GB to 1 TB of storage.

Memory (RAM) is the desk space. It’s working room — where the computer holds whatever it’s actively using right now. When you close a program, what was in RAM is cleared. When you turn off the computer, RAM empties completely. A typical home computer today has 8–16 GB of RAM.

When a computer slows down because “it’s running out of memory,” that’s the desk getting crowded — too many programs open at once. When a computer warns you it’s low on disk space, that’s the filing cabinet getting full. The fix for one is not the fix for the other.

Internet speed: a different unit entirely

Internet providers advertise speeds in megabits per second — written as Mbps. This is not the same as megabytes. A bit is one-eighth of a byte. So a 100 Mbps connection transfers roughly 12.5 MB of data per second. The distinction matters when you’re trying to figure out how long a download will take.

If you have a 100 Mbps internet connection and you’re downloading a 1 GB file:

• 1 GB = 1,000 MB
• 1,000 MB ÷ 12.5 MB/second = about 80 seconds

Most people don’t need to do this math — but knowing that your internet provider is talking about bits while your computer is talking about bytes explains why the two numbers never seem to match.