Mbps vs MB/s: Why Your Download Is 8× Slower Than Your Internet Plan

Your internet plan says 100 Mbps, but your download manager shows about 12 MB/s, and nothing is wrong. Internet speeds are quoted in megabits per second (Mbps), while file sizes and download rates are shown in megabytes (MB). There are 8 bits in a byte, so you divide the megabit figure by 8: 100 Mbps ÷ 8 = 12.5 MB/s. That factor of 8 is the single most common source of confusion about internet speed.

Bits, bytes, and the factor of 8

A bit is the smallest unit of digital data, a single 0 or 1. A byte is a group of 8 bits, and it’s the unit we measure files in: a photo is a few megabytes, a movie a few gigabytes.

Data transmission, though, has always been measured in bits per second, a convention inherited from telecoms. So two different units describe the same pipe:

• Bit-rate, bits per second: bps, Kbps, Mbps, Gbps. This is what your ISP sells.
• Byte-rate, bytes per second: B/s, KB/s, MB/s, GB/s. This is what your browser or download manager reports.

Because a byte is 8 bits, you convert between them by a factor of 8:

• Mbps → MB/s: divide by 8
• MB/s → Mbps: multiply by 8

The capital-versus-lowercase “B” is the giveaway: lowercase b is bits, uppercase B is bytes. “Mbps” and “MB/s” look almost identical but differ eightfold.

Conversion table

Internet plan (Mbps)	Download speed (MB/s)	1 GB file takes about
25 Mbps	3.1 MB/s	5 min 20 s
50 Mbps	6.25 MB/s	2 min 40 s
100 Mbps	12.5 MB/s	1 min 20 s
300 Mbps	37.5 MB/s	27 s
500 Mbps	62.5 MB/s	16 s
1,000 Mbps (1 Gbps)	125 MB/s	8 s

These are theoretical maximums. In practice, expect roughly 10-20% less because of protocol overhead, Wi-Fi, and the limits of the server you’re downloading from.

Why ISPs use the bigger-sounding number

Marketing plays a part, “100 Mbps” sounds eight times faster than “12.5 MB/s”, but the deeper reason is genuine: the bit is the fundamental unit of data transmission, and network engineering has always counted in bits. Your download manager then counts in bytes because that’s how files are sized. Neither is wrong; they’re just two scales for the same thing, and the factor of 8 sits between them.

How long will a download take?

Transfer time is simply the file size divided by the speed, once both are in the same unit. The trick is converting first:

1. Convert the file size to bits: bytes × 8. A 1 GB file is 8,000,000,000 bits.
2. Divide by the speed in bits per second. On 100 Mbps (100,000,000 bps): 8,000,000,000 ÷ 100,000,000 = 80 seconds.

That’s why a 1 GB game update on a “100 Mbps” connection takes over a minute, not ten seconds. Our bandwidth converter turns any speed between Mbps and MB/s instantly, and the download time calculator estimates transfer times (with an optional real-world overhead adjustment) from a file size and your connection speed.

A note on decimal vs binary

There’s a second, smaller subtlety. Networking uses decimal units, 1 Mbps is exactly 1,000,000 bits per second. Storage sometimes uses binary units, where 1 MiB (mebibyte) is 1,048,576 bytes. That distinction can shift a calculation by a few percent, but it’s separate from, and much smaller than, the eightfold bits-versus-bytes gap. For everyday internet-speed questions, the factor of 8 is the one that matters.

Frequently asked questions

Is 100 Mbps the same as 100 MB/s?

No, and this is the most common mistake. 100 Mbps (megabits) equals only 12.5 MB/s (megabytes), because a byte is 8 bits. A 100 Mbps connection downloads a 1 GB file in about 80 seconds, not 10.

How do I convert Mbps to MB/s?

Divide by 8. Megabits per second ÷ 8 = megabytes per second. So 100 Mbps = 12.5 MB/s, 300 Mbps = 37.5 MB/s, and 1 Gbps = 125 MB/s. To go the other way, multiply MB/s by 8.

Why is my download slower than my internet speed?

Partly the bits-versus-bytes confusion (your download shows bytes, your plan quotes bits), and partly real-world losses: Wi-Fi interference, a slow or busy server, distance, and protocol overhead typically cut 10-20% off the theoretical maximum. If a download is far slower than 1/8 of your plan speed, the bottleneck is usually the other end or your Wi-Fi.

How fast is a 1 Gbps connection?

1 Gbps is 1,000 Mbps, which is 125 MB/s. It downloads a 1 GB file in about 8 seconds in theory, though at those speeds your hard drive, Wi-Fi, or the source server often becomes the limiting factor rather than the connection itself.

What’s the difference between Mb and MB?

Lowercase “Mb” (megabit) is 1,000,000 bits; uppercase “MB” (megabyte) is 1,000,000 bytes, which is 8,000,000 bits. So 1 MB = 8 Mb. Internet speed uses Mb (Mbps); file sizes use MB. The single letter’s case makes an eightfold difference.

Why do ISPs advertise in megabits instead of megabytes?

The bit is the fundamental unit of data transmission and the historical standard in telecoms and networking, so speeds have always been quoted in bits per second. The larger-sounding number is a marketing bonus, but the underlying reason is a genuine engineering convention.