Mbps vs MB/s: How to Calculate Real Download Times

Your internet provider promises 100 Mbps. You start downloading a game and the launcher shows 11.8 MB/s. It looks like you are getting about a tenth of what you pay for – but in most cases nothing is wrong. The two numbers use different units: one counts bits, the other counts bytes.

Bits vs bytes

A bit (lowercase b) is a single 0 or 1. A byte (uppercase B) is 8 bits. Network speeds are traditionally measured in bits per second because data travels over a connection one bit at a time. File sizes are measured in bytes because computers store data in bytes.

• Mbps or Mb/s = megabits per second – used by internet providers and speed tests
• MB/s or MBps = megabytes per second – used by browsers, Steam, cloud apps and file managers

The capitalization of the b is the whole difference, and it is a factor of eight.

The ÷8 rule

MB/s = Mbps ÷ 8 and Mbps = MB/s × 8

A 100 Mbps connection can move at most 100 ÷ 8 = 12.5 MB/s. The 11.8 MB/s in the launcher is about 94% of the theoretical maximum – a normal result once network overhead is included.

How long will a download take?

Time (seconds) = file size (MB) ÷ speed (MB/s)

Example 1: a 5 GB game on a 100 Mbps plan. 5 GB = 5,000 MB, and 100 Mbps = 12.5 MB/s, so 5,000 ÷ 12.5 = 400 seconds, or 6 minutes 40 seconds in ideal conditions. Real connections lose some capacity to protocol overhead and congestion; assuming 90% efficiency, plan for about 7 minutes 24 seconds.

Example 2: uploading a 1 GB video on a plan with 10 Mbps upload. 10 Mbps = 1.25 MB/s, so 1,000 ÷ 1.25 = 800 seconds, or 13 minutes 20 seconds. Many home plans have much lower upload than download speeds, which is why uploads feel slow.

Download time for a 10 GB file

Add roughly 10% for real-world conditions. On fast plans the limit is often not your connection but the server you download from, your Wi-Fi or the speed of the disk you are writing to.

The second trap: GB vs GiB

Storage adds another source of confusion. Internet speeds and most file sizes online use decimal units, where 1 GB = 1,000,000,000 bytes. Windows, however, reports sizes in binary units – 1 GiB = 1,073,741,824 bytes – while still labeling them "GB". A file advertised as 5 GB therefore appears in Windows as about 4.66 GB. Nothing is missing; it is the same number of bytes counted with a different unit. Our guide to digital storage units explains the IEC binary prefixes in detail.

Why your speed test and download disagree

• Wi-Fi vs cable. Wi-Fi signal, distance and interference often cap real speeds well below the plan. Test with an Ethernet cable to see what the line delivers.
• The remote server. A download can only be as fast as the server sending it.
• Other devices. Streams and updates on other devices share the same connection.
• Disk speed. On gigabit connections, an older hard drive can be the bottleneck.

To convert between bits, bytes and their decimal or binary multiples, use the digital storage converter; for the bit-level background, see the binary converter.