Bits, bytes, GB and TB: storage sizes explained
In short: Computers store everything as tiny on-off switches called bits. Eight bits make a byte, and bigger units build up from there: kilobytes, megabytes, gigabytes and terabytes. Knowing the rough sizes helps you choose a phone, a cloud plan or a memory card, and work out what is filling your storage.
Everything is ones and zeros
Inside every computer, information is stored as millions of tiny switches that are either on or off. Each one is called a bit (short for "binary digit"), and it holds a 1 or a 0. On its own a bit says very little, but in groups they can represent anything: letters, numbers, colours, sounds and video.
Eight bits make one byte. A byte is roughly enough to store one letter of text. Everything else is just a bigger pile of bytes.
The units from small to huge
| Unit | Roughly | Everyday example |
|---|---|---|
| Kilobyte (KB) | 1,000 bytes | A short email with no attachments |
| Megabyte (MB) | 1,000 KB | A phone photo is usually a few MB; a song is around 3 to 5 MB |
| Gigabyte (GB) | 1,000 MB | About an hour of HD video streaming can use 1 to 3 GB |
| Terabyte (TB) | 1,000 GB | A large laptop drive or external backup drive |
You may see slightly different numbers, such as 1,024 instead of 1,000. Computers count in powers of two, so a "1 TB" drive often shows up as about 931 GB in your settings. Nothing is missing; it is just two ways of counting.
Storage is not the same as speed
Here is a common mix-up. Storage is measured in bytes (MB, GB). Internet speed is measured in bits per second (Mbps). Because a byte is eight bits, a 100 Mbps connection downloads roughly 12 megabytes per second at best. Our guide Internet speed explained covers this in detail.
Why your phone fills up
A 128 GB phone sounds huge, but the operating system and built-in apps take a chunk before you start. After that, the biggest users are usually:
- Videos, especially 4K video, which can use several hundred MB per minute.
- Photos, which add up quickly when you take thousands.
- Messaging apps that save every photo and video friends send you.
- Downloaded films, music and podcasts for offline use.
- Games, some of which are several GB each.
Tip: On most phones, Settings then Storage shows exactly what is using the space. Start with the biggest item rather than deleting lots of small things.
Choosing the right size
- For a phone used mainly for messages, browsing and some photos, 128 GB is usually comfortable.
- If you film lots of video or keep many games, consider 256 GB or more.
- For backups, choose an external drive or cloud plan at least as big as the data you want to protect, with room to grow.
Whatever size you choose, keep a backup. Storage can fail, and phones can be lost. See How to back up your files and photos.
Cloud storage uses the same units
Online storage plans for photos and files are also sold in GB and TB. A free plan often gives a few GB, which fills quickly if your phone backs up every photo and video. Check how much your backups use before choosing a paid plan, and remember that the space is usually shared between email, photos and documents on the same account.
Deleting a file on your phone may not free up cloud space, and deleting it from the cloud may delete it from your phone too, depending on how syncing is set up. Read the message on screen carefully before you confirm. If you are tidying up, move precious photos to a second place first, such as a computer or an external drive.
Key takeaways
- A bit is a single on-off switch; eight bits make a byte.
- KB, MB, GB and TB each go up by about a thousand times.
- Storage is measured in bytes, internet speed in bits per second.
- Videos, photos and messaging apps are usually what fills a phone; check the Storage settings page.
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