Mbps vs MB: convert to MB/s and time downloads

A plan sold as 100 Mbps often shows 12.5 MB/s in the download window, and the file being downloaded carries a third label in MB. The three numbers look unrelated, yet the first two describe one connection in two different units. Reading one as the other throws every estimate off by a fixed factor, and that factor is easy to remove once it is named.

Mbps vs MB: a speed against a file size

One is a rate counted in bits per second, and the other is an amount counted in bytes. Mbps, short for megabits per second, says how much data crosses a connection each second, so it is the figure on an internet plan. MB, short for megabytes, says how much data a file holds, so it is the figure next to a photo, an app or a video.

A rate and an amount are different kinds of quantity, so the two cannot be compared directly. The link between them is the size of the unit: a byte is eight bits, which is why a speed written in Mbps looks eight times larger than the same speed written in MB/s. Spelling it MBps, as some sites do, changes nothing, because the capital B still stands for bytes.

Plans are quoted in Mbps, and download windows often show MB/s. A reader who sees 100 on the plan and 12.5 in the window is looking at the same speed twice.

Bits vs bytes: eight bits make one byte

The standards body NIST defines the byte as a group of eight bits, which fixes the factor between the two units for every speed and every file. A bit is a single one or zero, and eight of them together make a byte. Lowercase b means bit and uppercase B means byte, so the case of one letter separates a rate in bits from a rate in bytes.

Two rows describe a rate and one row describes an amount. Mbps and MB/s answer the same question in different units, while MB answers a different question altogether.

Megabit vs megabyte

The prefix is identical in both words, and only the unit it multiplies differs. A megabit counts bits and a megabyte counts bytes, so one megabyte equals eight megabits. A 1 MB file therefore holds eight times what one second of a 1 Mbps link carries.

The same relation gives an easy reference point: 8 Mbps is exactly 1 MB/s. Every other speed converts by the same factor of eight.

Convert Mbps to MB/s by dividing by eight

One division turns the speed on a plan into the rate a download window reports. Divide the Mbps figure by eight to get MB/s, and multiply MB/s by eight to go back. A 100 Mbps line works out to 12.5 MB/s, and 8 Mbps lands on exactly 1 MB/s.

MB/s = Mbps ÷ 8
Mbps = MB/s × 8

The table lists the speeds plans are usually sold at, already divided. Find the row nearest the plan and carry its MB/s figure into the time estimate.

A different prefix changes nothing. Write the speed in Mbps first, then divide by eight: 1 Gbps is 1,000 Mbps, so it gives 125 MB/s, and 10 Gbps gives 1,250 MB/s. One kbps is a thousandth of one Mbps, so 1 Mbps is the same as 1,000 kbps.

How long does a download take at your speed?

A 1,000 MB file needs 80 seconds on a 100 Mbps line, which moves 12.5 MB every second. The wait is the file size in MB divided by the speed in MB/s, so the only prerequisite is the conversion above.

seconds = file size in MB ÷ MB/s

At 12.5 MB/s a 500 MB file takes 40 seconds. At 10 Mbps, which is 1.25 MB/s, the same file takes 400 seconds, or 6 minutes 40 seconds. Speed and time move in opposite directions: one tenth of the speed means ten times the wait.

The table below fills in the common sizes and speeds, so no calculator is needed. Find the file size in the left column and the plan speed along the top.

The large cells read better in minutes, so divide them by 60. That turns 4,000 seconds into 66 minutes 40 seconds.

Sending a file uses the same division with the plan's upload speed in place of the download speed. When the upload figure is the lower one, the same file takes longer to send than to receive, and a smaller file is the fix that does not depend on the line. Attachment limits are covered in "Email Attachment Size Limits: Fit Your PDF".

Why measured speed falls short of the plan figure

Every wait in the table assumes the line runs at the plan figure throughout, so the result is a best case. A download window that shows 12.5 MB/s on a 100 Mbps plan is not falling short: that is the plan's full speed written in bytes. To compare a measured speed with the plan, multiply the MB/s in the window by eight.

The FCC broadband label rule, published in the Federal Register on 2022-12-16, treats the plan figure and the typical figure as different things. The Commission requires US providers to display typical upload and download speeds and typical latency, not only the top figure. A measured result well below the table should therefore be compared with the typical speed on the label.

Decimal and binary prefixes: 1,000 against 1,024

Two counting systems share the same letters, so a megabyte can mean 1,000,000 bytes or 1,048,576 bytes. Decimal prefixes step by 1,000 at every level, and binary prefixes step by 1,024. A binary megabyte, written MiB, is 1,048,576 bytes, and a binary gigabyte, written GiB, is 1,073,741,824 bytes, against 1,000,000,000 bytes for a decimal GB.

NIST states that the binary prefixes are not part of the International System of Units, and that makers of storage devices use 1,000,000 bytes for a megabyte. Software that divides by 1,073,741,824 but prints "GB" therefore shows a smaller number than the label on the drive.

At the gigabyte step the two definitions differ by 73,741,824 bytes, a ratio of about 1.074. That margin disappears in a download estimate and matters only where capacity itself is the subject. Conversion between the size units is covered in "File size units explained: how many GB in a TB?".

Why a 1 TB drive shows about 931 GB

The drive and the file manager count the same bytes with different divisors. A 1 TB drive holds 1,000,000,000,000 bytes, and dividing that by 1,073,741,824 gives about 931. Nothing is missing, and neither number is wrong.

Common slips: 100 Mbps is not 100 MB/s

Each slip comes from reading one letter or one prefix as another, and each has a one-step check. The table pairs the three slips with the correction, and each note that follows gives the reasoning for one slip.

• Reading a plan speed as bytes. A 100 Mbps line delivers 12.5 MB/s, so a window showing 12.5 MB/s is doing exactly what the plan says. Check the unit before suspecting the line.
• Treating b and B alike. A lowercase b is bits and an uppercase B is bytes, and a swapped case changes the answer eightfold. A figure with "per second" or a slash is a rate, while a bare B names an amount.
• Letting G stand for three things. Gbps is a speed, GB on a file is a size, and GB on a mobile plan is a monthly cap. Write the full unit before doing any arithmetic.

Summary: divide by eight, then divide the file size

Two divisions give the wait: the plan speed over eight gives MB/s, and the file size over MB/s gives seconds. A 100 Mbps line moves 12.5 MB/s, so a 1,000 MB file takes 80 seconds. Treat that as a best case, because the FCC label rule asks providers for typical speeds rather than the top figure.

The file size is the one input that does not depend on the connection. A smaller file finishes sooner at any speed, and the steps for shrinking images are in "Compress Images Online, Free: JPEG, PNG, WebP".