Mbps vs MB/s: Why 100 Mbps Downloads at 12 MB/s
Your internet plan is sold in bits. Your download manager counts bytes. There are eight bits in a byte, and that factor explains everything.
16 February 2026 · 5 min read · Technology
A 100 Mbps connection downloading at "only" 12 MB/s is not being throttled. It is doing exactly what it promised.
Bits and bytes
- bit (lowercase b) — a single 1 or 0
- byte (uppercase B) — eight bits, historically one character of text
Network speeds are measured in bits per second. File sizes are measured in bytes. So:
MB/s = Mbps ÷ 8
100 Mbps ÷ 8 = 12.5 MB/s. Real-world overhead from TCP/IP headers takes another 3–7% off, which lands you around 11.8–12.1 MB/s.
Why ISPs use bits
Partly history — telecoms measured line capacity in bits per second long before consumer file transfers existed. Partly marketing: "1000 Mbps" is a bigger number than "125 MB/s", and bigger numbers sell.
Quick conversion table
| Plan speed | Theoretical max | Realistic |
|---|---|---|
| 25 Mbps | 3.13 MB/s | ~2.9 MB/s |
| 100 Mbps | 12.5 MB/s | ~11.8 MB/s |
| 300 Mbps | 37.5 MB/s | ~35 MB/s |
| 1 Gbps | 125 MB/s | ~112 MB/s |
Try it on the Mbps to MB/s converter.
Estimating a download
Time in seconds = file size in MB ÷ (plan Mbps ÷ 8).
A 4.7 GB file (4,700 MB) on a 100 Mbps line: 4,700 ÷ 12.5 = 376 seconds, about 6¼ minutes.
What actually slows you down
The line speed is rarely the bottleneck:
- Wi-Fi. A weak 2.4 GHz signal can cap you at 20 Mbps regardless of the plan.
- The server. A single source may limit per-connection speed.
- Disk write speed. An old spinning drive tops out around 100 MB/s.
- Shared contention. Cable segments slow down at peak hours.
Before upgrading a plan, run a speed test on Ethernet. If wired speed already matches your plan, more bandwidth will not help.
Related conversions
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy