By the end of this lesson you will be able to
- Put KB, MB, GB and TB in order and give a real example of each.
- Estimate how many photos, songs or films fit in a given amount of storage.
- Check total and free space on Ubuntu and on Windows.
- Explain the difference between RAM and storage with an everyday comparison.
Shops quote laptop sizes as '8 GB memory, 256 GB storage', and error messages say 'not enough space'. Without a feel for these numbers you cannot judge what to buy or why a computer has filled up. This lesson gives you that feel, and then clears up a mix-up that catches almost every beginner: memory and storage are not the same thing.
Counting information
You met the Bit and the Byte earlier. A byte is enough to hold one letter. Because bytes are tiny, bigger units are used: a kilobyte (KB) is about a thousand bytes, a megabyte (MB) about a thousand kilobytes, a gigabyte (GB) about a thousand megabytes and a terabyte (TB) about a thousand gigabytes. Each step is a thousand times bigger than the one before.
| Unit | Size | Everyday example |
|---|---|---|
| Byte (B) | 1 letter | The letter A |
| Kilobyte (KB) | about 1,000 bytes | A short email or a page of plain text (a few KB) |
| Megabyte (MB) | about 1,000 KB | A phone photo (3 to 5 MB) or a song (3 to 6 MB) |
| Gigabyte (GB) | about 1,000 MB | A film in good quality (1 to 5 GB) or about 250 songs |
| Terabyte (TB) | about 1,000 GB | Hundreds of films, or a large collection of photos |
Read this figure as text
A bar chart on a logarithmic scale comparing a document, a photo, a song, an app and a film.
Bars in megabytes on a logarithmic scale, so each bar is far longer than the one above. A text document is about 0.05 MB (50 KB). A phone photo is about 4 MB. A song is about 5 MB. A typical app installer is about 150 MB. A film in good quality is about 3,000 MB, which is 3 GB. The film is almost a thousand times bigger than a photo.
Quick sums help. A 128 GB phone has about 128,000 MB. At 4 MB a photo, that is around 32,000 photos in theory, but the operating system and apps take a share. A 1 TB external drive holds roughly 250,000 photos, or several hundred standard films.
Checking free space
Your storage drive, either an SSD or a hard disk, has a fixed capacity (total size). What matters day to day is the free space: how much of that is still empty. When free space runs very low the computer can become slow and refuse to save files. Check it every month or so.
Look at capacity and free space
On Ubuntu 24.04
Step 1: Open Settings
Click the system menu at the top right of the screen, then the Settings icon (a gear). In Settings choose System, then About.
You should see: You see device information including Disk Capacity near the bottom of the list.
Step 2: See free space for a folder
Open Files, right-click empty space in your Home folder, or a folder in the sidebar, and choose Properties. Look at the Basic tab.
You should see: A line called Free space shows how much room is left on the drive that holds the folder.
On Windows 11
Step 1: Open Settings
Press the Windows key and I together, then choose System, then Storage.
You should see: A bar shows how much of the drive is used, split into categories such as Apps, Temporary files and Documents.
Step 2: See free space in File Explorer
Open File Explorer and click This PC in the left pane.
You should see: Each drive has a bar and a line such as '120 GB free of 237 GB'. The bar usually turns red when space is nearly full.
- Marker 1: Main drive, where your files live
- Marker 2: Removable USB drive
- Marker 3: Free space: the number to watch
Read this figure as text
A simulated window listing two drives with their free space, and numbered markers on the name, the bar and the free space text.
A window titled This PC lists two drives. Local Disk (C:) is marked 1: it shows 120 GB free of 237 GB, so a bit under half is used. USB Drive (D:) is marked 2: it shows 7.2 GB free of 14.5 GB. Marker 3 points to the free-space text, which is the number to watch. The bar under each name fills with the used space.
RAM is not storage
Imagine you are working at a desk with a filing cabinet beside it. The cabinet holds everything you own, in drawers, and it keeps it for years. The desk holds only what you are working on right now, and anything on it has to be cleared away at the end of the day. Reaching for something on the desk is instant. Fetching from the cabinet takes a moment.
RAM (random access memory) is the desk: fast, small, and cleared when the power goes off. Storage (the SSD or hard disk) is the cabinet: bigger, slower, and keeps its contents with the power off. When you open a document, the computer copies it from storage into RAM so that the processor can work on it quickly. When you press Save, it copies the changes back to storage.
- Small: about 8 to 32 GB
- Very fast
- Holds what is open now
- Emptied at switch-off
Temporary working space.
- Large: 256 GB to several TB
- Slower
- Keeps files with power off
- Holds documents, photos, apps
Long-term keeping.
Read this figure as text
Two columns comparing RAM as a small fast desk and storage as a large slower cabinet.
Left column, RAM (the desk): small, typically 8 to 32 GB; very fast; holds what you are working on right now; emptied when you switch off or restart. Right column, Storage (the cabinet): large, typically 256 GB to several TB; slower than RAM; keeps files when the power is off; holds your documents, photos, apps and the operating system. Opening a file copies it from the cabinet to the desk. Saving copies it back.
- Step 1: In storageSafe with power off
- Step 2: OpenCopy loaded into RAM
- Step 3: EditChanges live in RAM
- Step 4: SaveWritten back to storage
Read this figure as text
A flow showing a file being copied from storage into RAM, edited, and then saved back to storage.
Step 1: the document is kept in storage. Step 2: you open it, and a working copy is loaded into RAM. Step 3: you type, and the changes happen in RAM only. Step 4: you press Save, and the changed document is written back to storage so it survives switching off.
A common confusion: buying more storage does not make a slow computer faster at running many programs. That is a job for RAM. A full drive, on the other hand, causes problems because the computer needs spare room to save files and work. Both matter, for different reasons.
Common mistakes
✖ Instead of: Thinking '8 GB memory' means 8 GB of space for files.
✔ Do this: Memory is RAM, the temporary workspace. File space is called storage. Check both numbers when you read a specification.
✖ Instead of: Expecting a 256 GB drive to show exactly 256 GB.
✔ Do this: Different counting methods and the operating system itself mean the usable space is a little less.
✖ Instead of: Never looking at free space until errors appear.
✔ Do this: Check monthly. Try to keep at least 10 to 20 per cent of the drive free.
If something goes wrong
- The computer says 'not enough space' when I try to save or copy a file.
- Check free space. Move large files you do not need often to an external drive, or empty the Trash or Recycle Bin after checking it. Never delete system folders.
- I cannot find where the space went.
- On Windows use Settings, System, Storage and open each category. On Ubuntu use the Disk Usage Analyzer application if it is installed, or sort folders by Size in Files. Downloads and Videos are the usual culprits.
- A new drive shows less capacity than the box says.
- That is the difference between counting in thousands and in 1,024s, plus space the system reserves. It is normal.
Key terms
- Gigabyte
- A unit of data size equal to about one billion bytes.
- RAM
- The computer's short-term working space, which holds what you are using right now and is emptied when it switches off.
- Storage
- The part of a computer that keeps your files and programs even when it is switched off.
- SSD (solid-state drive)
- A fast storage device with no moving parts that keeps your files when the power is off.
- Free space
- The part of a drive's capacity that is still empty and ready for new files.
Practise it for real
Reading is not the same as being able to do it. Complete these on your own computer, then tick only what you have actually done. These ticks are your own honest record: nothing checks them for you.
Find your own numbers
On your laptop, follow the steps in this lesson to find the total capacity and free space of your main drive. Then find your RAM: on Ubuntu open Settings, System, About and read Memory; on Windows open Settings, System, About and read Installed RAM. Write down capacity, free space and RAM, and work out roughly what percentage of the drive is free.
Challenge: will it fit?
A friend has a 64 GB phone with 20 GB free. She wants to keep 2,000 photos of about 4 MB each and 10 films of about 2 GB each. Calculate whether both fit, and suggest two things she could do if they do not. Show your sums in MB or GB, with units.
Knowledge check
6 questions. You need 80% to pass. Mistakes here are part of learning, and you can retry as often as you like.
Summary
- KB, MB, GB and TB each step up by about a thousand times.
- Photos and songs are a few MB; a good-quality film is a few GB.
- Free space is what is left on the drive; check it regularly.
- RAM is the temporary desk; storage is the long-term cabinet.
- Unsaved work lives only in RAM and is lost if the power fails.
Checklist: can you do these?
Marking a lesson as read does not prove you can do it. Skills count as demonstrated when you pass the quiz (not yet) and complete the practical tasks (not yet).
Related lessons
References and further reading
Official sources for details that can change between versions.
- Check your storage space: Windows (Microsoft)
- Ubuntu Desktop guide (Ubuntu)