Guide · Written by M.C. · updated September 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.

Mbps to MB/s: divide by 8. So 1000 Mbps = 125 MB/s, 500 Mbps = 62.5 MB/s, 100 Mbps = 12.5 MB/s. To go the other way, multiply MB/s by 8 (e.g. 20 MB/s = 160 Mbps). Use the converter below for any value.

Conversion table: Mbps to MB/s

The values people look up most, already worked out. The rule never changes: divide by 8.

How long a download takes

Enter your speed in the converter above and these times update on their own.

These are theoretical times: in practice add 10-20% for protocol overhead, and the speed of the server you download from matters too.

What your provider is actually promising

The gap between the advert and your download bar has two separate causes, and it helps to keep them apart.

• Bits versus bytes. Connections are sold in megabits, files are measured in megabytes, and there are eight bits in a byte. A 100 Mbps line downloads at roughly 12 MB/s. Nothing is wrong.
• Average versus guaranteed. In the UK, advertised "average" speeds are the ones at least half of customers get at peak time — so being below average is normal for half the country by definition. The figure that actually protects you is the minimum guaranteed speed on your Key Facts document. In the US, the equivalent reference is the typical speed on the broadband consumer label.

So before you conclude you are being short-changed, work out which of the two you are comparing against. Dividing the advertised megabits by eight tells you what to expect in the download bar; the guaranteed figure tells you whether you have a complaint.

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.

"Fibre" doesn't mean the same thing on both sides of the Atlantic

Part of the confusion about speed starts with the word on the advert. In the UK, a package sold as "fibre" has for years been allowed to describe a line where the fibre stops at a green cabinet in the street and the last stretch to your house is still copper. That last stretch is what caps your speed, and it depends on how far you live from the cabinet — which is why two neighbours on the same package can measure very different numbers.

• Full fibre (sometimes sold as FTTP or "ultrafast") runs glass all the way to the property. Distance stops mattering.
• Part-fibre (FTTC) is fibre to the cabinet, copper to you. Speeds fall with distance and can vary through the day.
• Cable is a third thing again: coaxial from a shared local segment, which is why evening congestion shows up on cable more than on full fibre.

In the US the labels differ but the physics don't: DSL and cable behave like the copper and coax cases above, while fibre-to-the-home behaves like full fibre. Before blaming your provider for a low number, work out which of the three you actually have — it changes what is reasonable to expect and what is worth complaining about.

Common conversions people search

The most asked conversions on Google, answered directly. These are the numbers you actually need:

• 1000 Mbps to MB/s = 125 MB/s (1 Gbps fiber)
• 500 Mbps to MB/s = 62.5 MB/s
• 300 Mbps to MB/s = 37.5 MB/s
• 200 Mbps to MB/s = 25 MB/s (typical FTTC max)
• 100 Mbps to MB/s = 12.5 MB/s (typical FTTC / entry FTTH)
• 50 Mbps to MB/s = 6.25 MB/s
• 30 Mbps to MB/s = 3.75 MB/s (Netflix 4K minimum)
• 25 Mbps to MB/s = 3.125 MB/s
• 10 Mbps to MB/s = 1.25 MB/s (ADSL entry)
• 2.5 Gbps to MB/s = 312.5 MB/s (XGS-PON fiber)
• 10 Gbps to MB/s = 1250 MB/s (10G residential future)

And the other direction — MB/s to Mbps, multiply by 8:

• 5 MB/s to Mbps = 40 Mbps
• 10 MB/s to Mbps = 80 Mbps
• 20 MB/s to Mbps = 160 Mbps
• 50 MB/s to Mbps = 400 Mbps
• 100 MB/s to Mbps = 800 Mbps
• 125 MB/s to Mbps = 1000 Mbps (1 Gbps)

Trick to remember: divide Mbps by 8 for MB/s. Multiply MB/s by 8 for Mbps. That's it.

FAQ