Mbps vs MB/s - Why Internet Speed Units Are So Confusing

Why "100 Mbps" Only Downloads at 12.5 MB/s

Your ISP advertises "up to 1 Gbps," yet actual file downloads max out at 125 MB/s. A speed test shows "500 Mbps," but Steam displays "62.5 MB/s." This isn't a connection problem - it's a difference in units.

The world of internet speeds is rife with confusing units. This article untangles the confusion around speed units, from the difference between Mbps and MB/s, to the tricks behind ISP advertising, and how to read your "real" speed.

Bits vs. Bytes - The 8x Trap

The root of the confusion lies in the coexistence of two units: "bits" and "bytes."

• Bit (b): The smallest unit of data. A single digit of 0 or 1
• Byte (B): 8 bits = 1 byte. A unit that can represent a single alphanumeric character (many characters, such as Japanese or Chinese, take 3 bytes in UTF-8)

Network speeds are traditionally expressed in bits per second (bps), while file sizes are expressed in bytes (B). This difference in convention creates an 8x discrepancy.

So on a "100 Mbps connection," the theoretical maximum download speed is 12.5 MB/s. The connection isn't slow - the units are just different.

Why Network Speeds Use Bits and File Sizes Use Bytes

The Networking World: Bits Are Natural

Communication lines physically transmit signals of 0s and 1s. Electrical signal on/off, optical pulse presence/absence, radio wave modulation - all are bit-level operations. For telecom engineers, expressing a line's capacity as "how many bits per second it can send" is perfectly natural.

The Computing World: Bytes Are Natural

Computers process data in byte units. Memory addressing, filesystem block sizes, CPU register widths - all use bytes (or multiples thereof) as the base unit. Expressing a file size as "1 MB" rather than "8,000,000 bits" is far more intuitive for humans.

The ISP's Advantage

There's also a marketing reason why ISPs express speeds in bits. "100 Mbps" is a number 8 times larger than "12.5 MB/s," giving consumers the impression of "fast." This isn't fraud - it's the industry-standard notation - but it does tend to mislead consumers. The same unit confusion applies when evaluating VPN protocol speeds, where encryption overhead further reduces effective throughput.

SI Prefixes vs. Binary Prefixes - Another Layer of Confusion

On top of the bit/byte confusion, there are two schools of thought on prefixes.

• SI prefixes (decimal): 1 KB = 1,000 bytes, 1 MB = 1,000,000 bytes, 1 GB = 1,000,000,000 bytes
• Binary prefixes: 1 KiB = 1,024 bytes, 1 MiB = 1,048,576 bytes, 1 GiB = 1,073,741,824 bytes

Hard drive manufacturers use SI prefixes (decimal), while Windows converts sizes using binary prefixes but keeps the "GB" label. This is why "a 1 TB hard drive shows up as 931 GB" (some operating systems, such as macOS, use decimal display instead). 1 TB (decimal) = 1,000,000,000,000 bytes ≈ 931 GiB (binary).

In the context of network speeds, the "M" in Mbps always refers to the SI prefix (decimal, 1,000,000). The binary prefix confusion doesn't apply here.

What "Best Effort" Means

Nearly all internet connections offered by ISPs in Japan are "best effort." "Up to 1 Gbps" means "the theoretical maximum under ideal conditions" and does not guarantee that speed at all times.

Many factors affect actual speeds:

• Number of users sharing the same line (especially apartment-type fiber connections)
• Time-of-day congestion (speeds tend to drop during evening peak hours)
• Router and LAN cable performance (older equipment can become a bottleneck)
• Wi-Fi signal conditions (attenuation from walls and distance)
• Processing capacity and bandwidth of the destination server

Shared bandwidth is particularly noticeable on public Wi-Fi networks, where many users compete for the same connection.

Check your connection info on IP Check-san and measure your actual speed with a speed test site. Understanding the gap between your ISP's advertised speed and your real-world speed is the first step toward improving your network environment.

Speed Unit Quick Reference

The formula: MB/s = Mbps ÷ 8. Remember just this, and you'll never be confused by internet speed units again.

Summary

The confusing nature of internet speed units stems from the historical conventions of the telecom industry (bits) and the computing industry (bytes). If you know that "Mbps ÷ 8 = MB/s," you can correctly interpret both ISP advertisements and download speed displays. To learn more about how your device is identified on the network independently of connection speed, check out our article on what an IP address is.

Frequently Asked Questions

Is 12.5 MB/s normal on a 100 Mbps connection?

Yes, that is the theoretical maximum. 100 Megabits per second divided by 8 equals 12.5 Megabytes per second. In practice, actual speeds are slightly lower due to protocol overhead, network congestion, and other factors.

Why are internet speeds measured in bits?

The telecommunications industry has historically measured data transmission in bits because serial communication sends data one bit at a time. The computer industry uses bytes because processors and storage work in byte-sized chunks. This historical divide persists today.

Why does a 1 TB hard drive show as 931 GB in the OS?

Drive manufacturers use SI units where 1 TB = 1,000,000,000,000 bytes, while Windows displays sizes converted using binary units where 1 TiB = 1,099,511,627,776 bytes. The same number of bytes appears as a smaller number when measured in binary units.