Guide · updated May 2026 · 4 min read

Mbps vs MB/s: the difference in 2 minutes

Got a "Gigabit" fiber contract and your downloads peak at 125 MB/s? Nothing's broken: you're looking at two different units measuring the same thing. Here's why, with the bare-bones math.

The rule: 8 bits = 1 byte

It all starts with a 60-year-old computing convention: a byte is 8 bits. Always has been, always will be, no debate.

Every confusion stems from this:

• Mbps = Megabits per second. Lowercase "b".
• MB/s = MegaBytes per second. Uppercase "B".

Conversion is trivial: 1 MB/s = 8 Mbps. To convert Mbps to MB/s, divide by 8.

Quick example: "1 Gigabit" fiber (1000 Mbps). Divided by 8 = 125 MB/s theoretical download speed. That's exactly the number you see in Chrome or Steam's progress bar.

Why ISPs and downloads use different units

The reason is historical-commercial, not technical:

• ISPs talk in bits because the entire network infrastructure (routers, switches, modems) is designed and specified in bits. It's the network engineer's native tongue.
• Operating systems show bytes because a file is measured in bytes, not bits. When Chrome downloads a 5 MB PDF, having the speed in MB/s instantly tells you "1 second".

Telco marketing also prefers Mbps for a more pedestrian reason: 1000 Mbps sounds way better than 125 MB/s in advertising, even though it's the same number.

Practical math: how long to download?

All calculations start by converting line speed (Mbps) to download speed (MB/s) by dividing by 8. Then: time = file size / speed.

Example 1: Netflix movie download (4 GB)

• 1 Gbps FTTH = 125 MB/s theoretical → 4 GB / 125 MB/s = ~32 seconds (in practice 40-60 seconds).
• 100 Mbps FTTC = 12.5 MB/s theoretical → 4 GB / 12.5 MB/s = ~5 and a half minutes.
• 20 Mbps ADSL = 2.5 MB/s theoretical → 4 GB / 2.5 MB/s = ~27 minutes.

Example 2: Steam game (80 GB)

• 1 Gbps FTTH → ~10-15 minutes (Steam-limited, not line-limited).
• 200 Mbps FTTC → ~50 minutes.
• 100 Mbps FTTC → ~1 hour 50 minutes.

Example 3: iPhone update (8 GB)

• 1 Gbps FTTH → ~1-2 minutes.
• 100 Mbps 5G FWA → ~12 minutes.

Why does it always take longer in practice? Three reasons: TCP/HTTP protocol overhead (~5-10%), destination server limits, possible throttling. In real-world calculations, count on 80-90% of nominal speed.

Conversion table

Common speeds, converted to effective MB/s:

• 10 Mbps → 1.25 MB/s
• 20 Mbps → 2.5 MB/s (max ADSL)
• 30 Mbps → 3.75 MB/s
• 50 Mbps → 6.25 MB/s
• 100 Mbps → 12.5 MB/s
• 200 Mbps → 25 MB/s
• 300 Mbps → 37.5 MB/s
• 500 Mbps → 62.5 MB/s
• 1000 Mbps (1 Gbps) → 125 MB/s
• 2500 Mbps (2.5 Gbps) → 312.5 MB/s (XGS-PON next-gen)
• 10000 Mbps (10 Gbps) → 1250 MB/s (future home)

MB vs MiB: the trap few know

A subtlety that drives purists crazy: there are two definitions of "Mega" in computing:

• MB decimal (SI): 1 MB = 1,000,000 bytes. Used by hard drive vendors, SSDs, telcos.
• MiB binary: 1 MiB = 1,048,576 bytes (= 1024 KiB = 2²⁰ bytes). Used by operating systems and RAM.

The difference is 4.86%. That's why a "1 TB" disk shows as 931 GB in your OS: the manufacturer sells 1,000,000,000,000 bytes (decimal); the OS counts in 1024 (binary).

For consumer downloads this distinction is almost always negligible: in the math above we used the informal mixed convention (1 GB = 1000 MB) which is the most common.

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