Why is my 100 Mbps connection only downloading at 12 MB/s?

Because those are the same speed in different units. There are eight bits in a byte, so 100 megabits per second is 12.5 megabytes per second. Internet plans are advertised in bits and file managers report in bytes, which is why the numbers look so different.

What is the difference between MB and MiB?

A megabyte is one million bytes and a mebibyte is 1,048,576 bytes — about 5% more. Storage vendors and macOS use the decimal MB while Windows displays binary sizes labelled as MB, which is why a 1 TB drive shows as roughly 931 GB.

What efficiency should I assume for a real download?

Somewhere between 80 and 95% of the advertised rate for a wired connection to a fast server. WiFi, a busy network or a distant server can push it much lower, so the figure is adjustable rather than fixed.

Does the Download Time Calculator work for uploads too?

Yes — the arithmetic is identical. Enter your upload speed instead of your download speed, remembering that most home connections are asymmetric and upload far more slowly.

Is anything I enter transmitted?

No. The file size and speed are used only for arithmetic in your browser, and no request is made to any server.

Download Time Calculator

Estimate how long a file takes to transfer at a given connection speed, with bits and bytes handled correctly.

The Download Time Calculator runs entirely in your browser. The sizes and speeds you enter are used only for the estimate on your device.

Open the Data Size Converter

About Download Time Calculator

Download Time Calculator answers a question people get wrong by a factor of eight. File sizes are quoted in bytes — MB, GB — while connection speeds are quoted in bits — Mbps, Gbps. A 100 Mbps line therefore moves 12.5 megabytes per second, not 100, which turns a five-minute estimate into forty. Enter a size and a speed in whatever units they were given to you in and the transfer time comes out in days, hours, minutes and seconds. Decimal units where a megabyte is a million bytes and binary units where a mebibyte is 1,048,576 sit side by side, because storage vendors and operating systems disagree about which they mean. An efficiency factor accounts for protocol overhead.

Features

Handles the bits-versus-bytes distinction that trips most estimates up
Decimal units (KB, MB, GB, TB) and binary units (KiB, MiB, GiB, TiB)
Speeds in bit/s or byte/s, from bit/s up to Gbit/s
Efficiency factor for protocol overhead and links that underperform
Readable duration broken into days, hours, minutes and seconds
Sub-second results keep three decimals rather than rounding to zero
Comparison table showing the same file on four common connections
Effective throughput reported in both bits and bytes per second

How to use the Download Time Calculator

Enter the file size and choose its unit — check whether it is MB or MiB
Enter the connection speed and choose bit/s or byte/s
Optionally lower the efficiency percentage to model a link that underdelivers
Read the transfer time and compare it against other connection speeds

Example

Input

1 GB file over a 100 Mbps connection

Output

80 seconds · effective throughput 12.50 Mbit/s (12,500,000 B/s)

One gigabyte is eight gigabits, so a 100 megabit line needs eighty seconds rather than ten.

Common errors & troubleshooting

The estimate is eight times longer than expected. — That is the bits-versus-bytes gap, and the estimate is right. Your connection's Mbps figure is megabits; divide by eight to get the megabytes per second a file transfer actually sees.
The real download took noticeably longer than the estimate. — Lower the efficiency figure. Protocol overhead, a slow server, shared bandwidth and WiFi contention all mean a link rarely sustains its advertised rate — 80 to 90% is a realistic starting point.
Unsure whether a file size is in MB or MiB. — Windows reports MB but means MiB; macOS and storage vendors use decimal MB. The difference is about 5% at gigabyte scale, so try both if the estimate needs to be tight.
Entering a speed of zero returns nothing. — A transfer at zero speed never completes, so the calculator needs a speed above zero to produce a finite time.

Frequently asked questions

Why is my 100 Mbps connection only downloading at 12 MB/s?: Because those are the same speed in different units. There are eight bits in a byte, so 100 megabits per second is 12.5 megabytes per second. Internet plans are advertised in bits and file managers report in bytes, which is why the numbers look so different.
What is the difference between MB and MiB?: A megabyte is one million bytes and a mebibyte is 1,048,576 bytes — about 5% more. Storage vendors and macOS use the decimal MB while Windows displays binary sizes labelled as MB, which is why a 1 TB drive shows as roughly 931 GB.
What efficiency should I assume for a real download?: Somewhere between 80 and 95% of the advertised rate for a wired connection to a fast server. WiFi, a busy network or a distant server can push it much lower, so the figure is adjustable rather than fixed.
Does the Download Time Calculator work for uploads too?: Yes — the arithmetic is identical. Enter your upload speed instead of your download speed, remembering that most home connections are asymmetric and upload far more slowly.
Is anything I enter transmitted?: No. The file size and speed are used only for arithmetic in your browser, and no request is made to any server.

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