The 8:1 rule

A bit is one binary digit — the atom of data. A byte is 8 bits — the atom of storage and files. Every “my internet is slower than advertised” complaint starts by confusing the two, because the industry uses both in the worst possible way:

• Your plan is sold in bits. Mbps = megabits per second. “Gigabit fiber” = 1,000 megabits per second.
• Your downloads are measured in bytes. Browsers, Steam, torrent clients and OS progress bars all report MB/s — megabytes per second.

So a 100 Mbps connection tops out at 100 ÷ 8 = 12.5 MB/s. Not broken, not throttled — the same number in different units. This single conversion is the answer to the most-asked download question on the internet.

The conversion table worth memorizing

Advertised speed	Theoretical MB/s	Real-world MB/s (~90%)
10 Mbps	1.25 MB/s	~1.1 MB/s
25 Mbps	3.1 MB/s	~2.8 MB/s
50 Mbps	6.25 MB/s	~5.6 MB/s
100 Mbps	12.5 MB/s	~11.3 MB/s
300 Mbps	37.5 MB/s	~34 MB/s
500 Mbps	62.5 MB/s	~56 MB/s
1 Gbps	125 MB/s	~113 MB/s
2.5 Gbps	312.5 MB/s	~280 MB/s

The third column is the honest one: TCP/IP headers, TLS records and the occasional retransmission shave 5–15% off the top. That’s not your ISP cheating — it’s the cost of moving data reliably.

Why bits won for networks (and bytes won for files)

Networking inherited bits from telecommunications, where line rates were always described in bits per second — a 56k modem was 56 kilobits. Storage inherited bytes because files, memory and disks are byte-addressed — nobody sizes a photo in bits. Marketing then cemented the split: “1,000 Mbps” simply looks more impressive than “125 MB/s”, so no ISP has an incentive to switch.

The result is a permanent translation step between what you buy and what you see. Speed tests report in Mbps to match your plan; download managers report in MB/s to match your files. Neither is wrong — they’re just speaking different units about the same pipe.

A second split: MB vs MiB

Bits vs bytes explains the 8× gap; there’s also a quieter 7% gap inside file sizes. Decimal units (used by drive makers and the SI system) go 1 KB = 1,000 B. Binary units go 1 KiB = 1,024 B, 1 MiB = 1,024 KiB. Operating systems frequently compute in binary but label it decimal — Windows “GB” is usually GiB underneath.

That means the “100 GB” game in your library might be 100 GiB = 107.4 GB of actual transfer — a real difference on a capped connection or a slow line. The calculator carries a decimal/binary switch so you can match whichever unit the source is really using; the download-time answer shifts accordingly.

Sanity-checking a speed test

Run a test and read it in this order:

1. Mbps result — compare to your plan. Within ~90% on a wired test is normal.
2. ÷ 8 — that’s the best MB/s you can ever see on this connection.
3. × 0.85–0.9 — that’s the MB/s a healthy download should actually show.

If step 3 is far below what your apps report, the bottleneck isn’t the plan — it’s Wi-Fi airtime, a rate-limited server, a VPN hop, or an overloaded hour. Why your real download speed is lower than advertised goes through each tax in order, and the download-time math page applies all of this to real file sizes.

Frequently asked questions

Is Mbps the same as MB/s?

No — they're an 8:1 ratio apart. Mbps is megabits per second; MB/s is megabytes per second. Since 1 byte = 8 bits, divide Mbps by 8 to get MB/s: 100 Mbps = 12.5 MB/s, 1,000 Mbps (gigabit) = 125 MB/s.

Why do ISPs advertise in megabits instead of megabytes?

Partly convention — networking hardware has always been rated in bits — and partly because 500 Mbps looks bigger than 62.5 MB/s. It's the same speed either way; the bits figure is just eight times larger numerically.

My speed test says 300 Mbps but downloads show 37 MB/s — is that right?

Yes, that's a healthy line: 300 ÷ 8 = 37.5 MB/s theoretical. If you see ~33–35 MB/s, the missing slice is protocol overhead — normal. If you see 3–4 MB/s, something else is wrong: Wi-Fi limits, a capped server, or an old cable.

What's the difference between Mb and MB, and MiB?

Lowercase b is bits, uppercase B is bytes — a factor of 8. MB vs MiB is a different split: MB is decimal (1,000 KB) and MiB is binary (1,024 KiB), a ~7% gap at the giga level. Our calculator has a unit-system switch for exactly this.

How do I convert my download speed to real MB/s quickly?

Mbps ÷ 8 = theoretical MB/s; then multiply by ~0.9 for what a download manager will actually report on a good wired connection. So 200 Mbps → 25 MB/s on paper → ~22–23 MB/s real.