What is Mbps vs MB/s?
Understanding the difference between ISP bandwidth numbers (bits) and file download speeds (bytes), and how to convert between them.
Your ISP sold you a "100 Mbps" plan. You run a speed test and it confirms roughly 100 Mbps. Then you download a file and your browser reports the transfer running at about 12 MB/s. Same connection, two numbers that look almost nothing alike — and it's one of the most common sources of "is my internet actually slower than advertised" confusion. The short answer: it isn't slower. Mbps and MB/s are measuring the same thing in two different units, and the entire difference comes down to a single letter's capitalization.
Bits and Bytes Are Different Units
A bit is the smallest unit of digital data — a single 0 or 1. A byte is a group of 8 bits. That factor of 8 is the entire source of the confusion:
- Mbps — megabits per second (lowercase "b"). This is how internet speed is marketed and sold.
- MB/s or MBps — megabytes per second (capital "B"). This is how file sizes, downloads, and storage are measured.
Since 1 byte = 8 bits, converting is a straight division: MB/s = Mbps ÷ 8.
Why Internet Speed Is Sold in Bits
Networking equipment moves data as a continuous serial stream of individual bits, so bits-per-second was the natural unit going back to early telecom and dial-up modems, and the convention stuck. It also happens to produce a bigger, more impressive-looking number for marketing — "100 Mbps" sounds much faster than "12.5 MB/s," even though they describe the identical connection.
Why Files Are Measured in Bytes
Storage has always been byte-addressed — a 4 GB file occupies 4 billion bytes on disk, not bits. So operating systems, download managers, and browsers naturally report transfer progress in the same unit as the file size itself: bytes (KB, MB, GB). That's why a download bar shows "MB/s" while your router's label and your ISP's bill both say "Mbps."
The Conversion Table
| Advertised Speed | Theoretical Max Transfer Rate |
|---|---|
| 25 Mbps | ≈ 3.1 MB/s |
| 50 Mbps | ≈ 6.3 MB/s |
| 100 Mbps | ≈ 12.5 MB/s |
| 250 Mbps | ≈ 31.3 MB/s |
| 500 Mbps | ≈ 62.5 MB/s |
| 1000 Mbps (1 Gbps) | ≈ 125 MB/s |
These are theoretical ceilings, not guarantees — real-world throughput is usually somewhat lower once protocol overhead, Wi-Fi conditions, and the server on the other end are accounted for (see our guide on why Wi-Fi runs slower than Ethernet).
A Practical Example
Say you're downloading a 10 GB game update on a genuine 100 Mbps connection (≈12.5 MB/s). At that rate:
10,000 MB ÷ 12.5 MB/s = 800 seconds = 13 minutes 20 seconds
Knowing the MB/s figure — not just the Mbps one — is what actually lets you estimate how long a real download will take. Our Network Tools page includes a calculator that does this conversion for any file size and speed automatically.
How SpeedFox Reports Your Result
Because this mix-up is so common, every speed reading on SpeedFox — the live gauge during a test and every figure on your results page — shows both units side by side: Mbps as the primary, industry-standard figure, and a smaller MBps reading right next to it, always derived from the exact same measurement. No mental division required.
Key Takeaways
- Mbps (megabits) is how internet plans are sold; MB/s (megabytes) is how files and downloads are measured.
- 1 byte = 8 bits, so MB/s is always Mbps divided by 8.
- A "slow download" showing MB/s isn't necessarily slower than your Mbps plan — check the math first.
- Real-world throughput is usually somewhat below the theoretical maximum from either unit.