The Numbers That Never Quite Add Up
You bought a 1 TB hard drive. Your computer says it has 931 GB. You download a file listed as 4.7 GB. Your internet plan offers 50 Mbps. These numbers are all measuring data, but they are not all using the same definitions. Understanding how storage units work explains every confusing number you have ever seen on a spec sheet, a download bar, or a file manager.
The Base Units
Everything starts with bits and bytes.
A bit is the smallest unit of digital data — a single binary value, either 0 or 1. Bits are used to measure data transfer speeds (your internet connection, for example).
A byte is 8 bits. A byte can represent 256 different values (2⁸ = 256), which is enough to encode a single character of text. File sizes are measured in bytes or multiples of bytes.
The Confusion: 1,000 vs 1,024
Here is where it gets complicated. There are two competing definitions for storage prefixes:
Decimal (SI) prefixes — used by hard drive manufacturers and internet speeds:
- 1 KB = 1,000 bytes
- 1 MB = 1,000,000 bytes
- 1 GB = 1,000,000,000 bytes
Binary prefixes — used by operating systems and RAM:
- 1 KiB (kibibyte) = 1,024 bytes
- 1 MiB (mebibyte) = 1,048,576 bytes
- 1 GiB (gibibyte) = 1,073,741,824 bytes
Hard drive manufacturers use the decimal definition because it makes their drives sound bigger. Windows and older macOS versions use binary definitions for file sizes. This is exactly why a "1 TB" drive shows as 931 GB in Windows — the drive manufacturer counts in powers of 1,000, and Windows counts in powers of 1,024. Neither is wrong; they are just different conventions.
Modern macOS switched to decimal definitions in 2009, so a 1 TB drive does show as approximately 1 TB on a Mac.
Storage Unit Reference Table
| Unit | Decimal Definition | Binary Equivalent |
|---|---|---|
| 1 KB | 1,000 bytes | 0.976 KiB |
| 1 MB | 1,000,000 bytes | 0.954 MiB |
| 1 GB | 1,000,000,000 bytes | 0.931 GiB |
| 1 TB | 1,000,000,000,000 bytes | 0.909 TiB |
| 1 PB | 1,000,000,000,000,000 bytes | very large |
Real-World File Size Reference
Knowing roughly how large different types of files are helps you estimate storage needs:
- Plain text file: a few KB
- High-resolution photo (JPEG): 3–8 MB
- MP3 song (3 minutes): 3–5 MB
- 1-hour podcast (MP3): 50–100 MB
- HD movie (1080p): 4–8 GB
- 4K movie: 20–80 GB
- iOS or Android app: 50 MB to 2 GB
- Modern AAA video game: 50–150 GB
Internet Speed vs Storage Size
Internet speeds are measured in megabits per second (Mbps), not megabytes. This catches people out constantly.
To convert Mbps to MB/s, divide by 8. A 100 Mbps connection downloads at 12.5 MB/s. A 1 Gbps fibre connection downloads at 125 MB/s. So downloading a 4 GB file on a 100 Mbps connection takes about 320 seconds — roughly 5 minutes, assuming a perfect connection with no overhead.
How to Use the Toobits Data Storage Converter
Enter any value, select the unit, and see instant conversions across bits, bytes, KB, MB, GB, TB, and PB. The converter uses decimal definitions (powers of 1,000) which match hard drive marketing, internet speeds, and most modern operating system displays. It is the fastest way to cross-check file sizes, storage capacities, or bandwidth calculations without doing the arithmetic yourself.