Mbps vs MB/s: Network Speed Units Explained

Your ISP advertises a "100 Mbps" connection. You start a large file download and your download manager reports 11.5 MB/s. A moment of confusion: are you getting what you paid for? Yes — you are. The units are different, and understanding why is the key to making sense of every speed test, bandwidth spec, and transfer estimate you'll ever read.

The short answer: 1 MB/s = 8 Mbps. Divide a speed in Mbps by 8 for MB/s, so a 100 Mbps plan moves at most 12.5 MB/s.

Bits vs Bytes: The Core Confusion

Network speeds are measured in bits per second. File sizes are measured in bytes. One byte = 8 bits. This single fact explains most of the apparent discrepancy between advertised speeds and observed download rates.

100 Mbps ÷ 8 = 12.5 MB/s (maximum theoretical download rate)

Even a perfect line delivers a little less file data than that, because headers share it. Over Ethernet, a full-size packet takes 1,538 bytes of line time to carry 1,460 bytes of your data: the frame takes 38 bytes, counting its preamble and the gap between frames, and the IP and TCP headers 40 more. That is 94.9%, so a 100 Mbps Ethernet link carries at most about 11.87 MB/s of a download, or 11.77 MB/s when both ends add TCP timestamps, 12 bytes more per packet. Lost and resent packets, Wi-Fi and a busy line take it lower still.

The Units, Spelled Out

Bits per second (the networking world):

Symbol	Name	Equivalent
bps	bits per second	1 bit/s
Kbps	kilobits per second	1,000 bps
Mbps	megabits per second	1,000,000 bps
Gbps	gigabits per second	1,000,000,000 bps
Tbps	terabits per second	1,000,000,000,000 bps

Bytes per second (the file/storage world):

Symbol	Name	Equivalent
B/s	bytes per second	8 bps
KB/s	kilobytes per second	8,000 bps
MB/s	megabytes per second	8,000,000 bps
GB/s	gigabytes per second	8,000,000,000 bps

The conversion formula is always:

MB/s = Mbps ÷ 8
Mbps = MB/s × 8

SI (Decimal) vs Binary Prefixes

This is the second source of confusion, especially for storage devices.

Networking uses decimal (SI) prefixes:

• 1 Mbps = 1,000,000 bits per second (exactly)

Some storage contexts use binary prefixes:

• 1 MiB (mebibyte) = 1,048,576 bytes = 2²⁰ bytes

Windows counts file and drive sizes in these binary units but labels them KB, MB and GB, which is why a "1 TB" drive shows as 931 GB there: 1,000,000,000,000 bytes ÷ 1,073,741,824 bytes per GiB. macOS has counted in decimal since Mac OS X 10.6 in 2009, so the same drive shows as 1 TB on a Mac.

Network speeds are always decimal: Mbps means 1,000,000 bits per second. Download speeds in bytes are another matter. Chrome and Firefox count a download's size and speed in units of 1,024 too, while they label them KB, MB and GB, so a download at 12.5 MB/s shows as about 11.9 "MB/s" in Chrome and 11.9 "MB/sec" in Firefox.

Common Real-World Speeds

Connection	Speed	Max Download Rate
Dial-up (classic)	56 Kbps	~7 KB/s
Basic DSL	5 Mbps	~625 KB/s
4G (typical)	50 Mbps	~6.25 MB/s
Standard cable	100 Mbps	~12.5 MB/s
5G (typical)	300 Mbps	~37.5 MB/s
Gigabit fiber	1 Gbps	~125 MB/s
10 Gigabit LAN	10 Gbps	~1.25 GB/s
Wi-Fi 6 (theoretical maximum)	9.6 Gbps	~1.2 GB/s

The mobile rows are typical speeds, which vary by country, network and year; the others are rated speeds. IMT-2020, the ITU's requirements for 5G, sets a peak of 20 Gbps: the most one device could receive with a cell's radio resources to itself. Wi-Fi 6's 9.6 Gbps needs eight antenna streams at once; a phone with two streams links at up to 2.4 Gbps, and real Wi-Fi throughput is often half the link rate or less.

Transfer Time Estimates

With a known speed and file size, transfer time is:

Time (seconds) = File Size (bits) ÷ Speed (bps)

Or more practically:

Time (seconds) = File Size (MB) × 8 ÷ Speed (Mbps)

Examples at 100 Mbps:

File	Size	Transfer Time
MP3 song	5 MB	0.4 seconds
RAW photo	25 MB	2 seconds
Software installer	500 MB	40 seconds
HD movie	4 GB	~5.3 minutes
4K movie	50 GB	~67 minutes

These are at the full rate; over Ethernet, add about 5% for the headers. At gigabit speed, divide all times by 10.

Bandwidth vs Throughput vs Latency

These three terms are often conflated but measure different things:

Bandwidth — the maximum rate a connection can carry. A pipe's diameter.

Throughput — the rate data actually transfers in a given session. Affected by packet loss, TCP window size, server speed, and distance. Always ≤ bandwidth.

Latency — the round-trip delay between sending a packet and receiving an acknowledgement. Measured in milliseconds. A low-latency connection feels responsive even at modest bandwidth. High latency makes even a fast connection feel sluggish for interactive use (video calls, gaming, web browsing).

A geostationary satellite link might offer 50 Mbps with about 600 ms of latency, since each signal climbs to an orbit 35,786 km up and back down, twice per round trip. Low-orbit services such as Starlink cut that to tens of milliseconds. A 4G mobile connection might offer 30 Mbps with 30 ms of latency. For most interactive use, the 4G connection feels much faster than the geostationary link despite the lower bandwidth number.

Reading Speed Tests

Speed test sites measure throughput, not raw bandwidth. Results are affected by:

• Server distance — servers farther away introduce more latency, which limits TCP throughput
• Time of day — shared network congestion during peak hours
• Wi-Fi vs wired — half-duplex Wi-Fi collisions and interference reduce effective speed
• Your device — a slow CPU or saturated disk can be the bottleneck, not the network

For accurate results: use a wired connection, test to a nearby server, and average 3–5 runs.

Tools

Convert between any bit or byte speed unit — bps, Kbps, Mbps, Gbps, MB/s, GB/s, and the binary KiB/s to TiB/s, which count in 1,024s as Chrome and Firefox do — with the Bandwidth Converter. It shows every value in full, the data a speed moves in an hour, a day and 30 days, and a table of common connection speeds. For how long one file takes, use the Download Time Calculator; for file sizes, the Data Storage Converter.