Download Time Calculator

How this is calculated

File sizes are quoted in BYTES and connection speeds are sold in BITS per second. There are 8 bits in a byte, so a 100 Mbps connection moves at most 12.5 megabytes per second. This is the single biggest source of confusion about download speeds, and it is not marketing sleight of hand so much as two industries having settled on different units a long time ago - networking has always counted bits, storage has always counted bytes. The capital B is the whole distinction. MB means megabytes, Mb means megabits, and the difference is a factor of eight. Your internet plan is in Mb; the file and your download manager are in MB. The time is simply the file size in megabytes divided by the speed in megabytes per second. There is a second, smaller ambiguity in the size units themselves. This calculator treats 1 GB as 1024 MB, which is what operating systems generally report. Drive manufacturers and some software use 1 GB = 1000 MB instead. The gap is about 2.4 percent at the gigabyte level and grows with size, and it is why a '1 TB' drive shows up as roughly 931 GB once formatted. What this cannot model is reality. The figure assumes you get your full advertised speed for the whole transfer, which almost never happens: protocol overhead takes a few percent before anything else, the server at the other end has its own limits, wifi is usually slower than the line behind it, and a household shares one connection. Treat the answer as a floor on the time rather than a prediction - the real transfer will take longer, sometimes considerably.

Common questions

Why is my download so much slower than my plan says?

Most likely because the two are measured in different units. Internet plans are sold in megaBITS per second and downloads are shown in megaBYTES per second, and there are 8 bits in a byte - so a 100 Mbps connection tops out around 12.5 MB/s. If your download manager shows roughly an eighth of what you pay for, nothing is wrong. The calculator shows both numbers side by side for exactly this reason.

Is that a marketing trick?

Not really, though it does flatter the number. Networking has counted in bits since long before home broadband existed, and storage has always counted in bytes - the two industries settled on different units and never reconciled them. The giveaway is the capital letter: Mb is megabits, MB is megabytes. It is a genuinely unhelpful convention and it is not going to change.

Why does my 1 TB drive show as 931 GB?

Because the drive manufacturer counted 1 TB as a trillion bytes and your operating system counts it as 1024 x 1024 x 1024 x 1024. The gap is about 2.4 percent per step and it compounds, so a terabyte loses roughly 7 percent by the time you reach it - which is where 931 comes from. This calculator uses the 1024 convention, matching what your operating system reports.

Will my download really take this long?

It will take longer. The figure assumes you receive your full advertised speed for the entire transfer, which almost never happens. Protocol overhead costs a few percent before anything else; the server at the other end has its own limits and may be throttling you; wifi is usually slower than the line feeding it; and everyone else in the house is sharing the same connection. Read the answer as a floor.

How do I get closer to the full speed?

A cable rather than wifi is the single biggest change for most people, and it is often dramatic. Beyond that: check nothing else in the house is saturating the line, and remember that a slow download frequently means a slow or distant SERVER rather than a slow connection - which is why one site downloads at full speed and another crawls on the same line at the same moment.

What speed should I enter?

Your plan's advertised speed if you want the best case, or the result of a speed test if you want something closer to reality. Bear in mind a speed test measures the path to a nearby test server under ideal conditions, so it is also optimistic relative to a real transfer from a busy server on the other side of the world.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Download Time Calculator at https://www.bfcbrilliance.com/tools/download-time-calculator.

What I entered:
- File size: ___
- Size in: ___
- Connection speed: ___
- Speed in: ___

What it calculated:
- Download takes: ___
- In seconds: ___
- Your speed in megaBYTES per second: ___
- File size in megabytes: ___
- You could move this much in an hour: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

More internet speed tools