Three different questions
MB and GB commonly describe data quantity or storage capacity, while Mbps describes transfer rate: megabits per second. A 500 MB file is not a 500 Mbps connection, and a 50 Mbps connection does not transfer 50 megabytes each second.
Lowercase b means bit, while uppercase B means byte. In this context, one byte equals eight bits.
Decimal and binary units
In decimal SI prefixes, mega means one million and giga means one billion. Storage and communications therefore often use:
- 1 MB = 1,000,000 bytes.
- 1 GB = 1,000 MB = 1,000,000,000 bytes.
- 1 Mbps = 1,000,000 bits per second.
Some computing contexts use binary units, whose precise names include an i: 1 MiB = 1,048,576 bytes and 1 GiB = 1,024 MiB. NIST recommends dedicated binary prefixes to avoid ambiguity. An operating system may therefore show a different value from a drive label because of units and formatting, not because all the capacity “disappeared.”
A quick reference table
| Unit | Name | Value | Common use |
|---|---|---|---|
| bit | bit | Smallest binary unit | Network rates |
| B | byte | 8 bits | File size and storage |
| MB | decimal megabyte | 1,000,000 bytes | File size and capacity |
| GB | decimal gigabyte | 1,000 MB | Storage and data allowances |
| MiB | binary mebibyte | 1,048,576 bytes | Some computing systems |
| Mbps | megabits/second | 1,000,000 bits/second | Network transfer rate |
Download-time example: 500 MB at 50 Mbps
Convert the file from megabytes to megabits: 500 × 8 = 4,000 megabits. Then divide by the rate:
The theoretical time is 80 seconds, or 1 minute 20 seconds, assuming decimal units, a sustained 50 Mbps actual rate, and no overhead. Real transfers can take longer because of congestion, server limits, Wi‑Fi, protocols, and device activity.
Plan speed is not necessarily guaranteed speed
An advertised rate may be “up to” a value, and upload speed may differ from download speed. One speed test is not a permanent verdict; repeat it at suitable times and conditions, and compare over Ethernet if you want to reduce Wi‑Fi variability. For monthly allowances, distinguish total data in gigabytes from instantaneous transfer rate in megabits per second.
Common mistakes
- Dividing MB by Mbps without multiplying file size by eight.
- Reading MB/s as Mbps.
- Assuming GB and GiB are identical.
- Using 1,024 everywhere even though SI prefixes are decimal.
- Treating theoretical time as a guaranteed actual download time.
Frequently asked questions
Why doesn't a 500 MB file download in 10 seconds at 50 Mbps?
Because 50 Mbps means a theoretical 6.25 MB/s before overhead. Then 500 ÷ 6.25 = 80 seconds.
Does 1 GB always equal 1,024 MB?
No. In decimal units, 1 GB = 1,000 MB. In binary units, 1 GiB = 1,024 MiB.
Are network speed and data usage the same?
No. Speed is an instantaneous rate; usage is an accumulated quantity. A 100 GB allowance and 100 Mbps speed describe different things.
Sources and references
- NIST — Metric (SI) prefixes: decimal prefix values and dedicated binary prefixes. Accessed 2026-11-06.
- The time calculation uses 1 byte = 8 bits and decimal units, and does not claim to measure a current network speed.