How Long Will My Download Take?

Divide by eight

A 25 GB game on a 100 Mbps connection: about 34 minutes, at best.

But the number worth internalising is this one:

100 Mbps = 12.5 MB/s.

If your download manager shows roughly an eighth of the speed you're paying for, nothing is broken. You're looking at two different units.

The capital letter is the whole distinction

• Mb — megabits. How internet plans are sold.
• MB — megabytes. How files are sized and downloads are displayed.

There are 8 bits in a byte, so the two differ by a factor of eight. That's it. That's the entire confusion.

It isn't really a marketing trick, though it does flatter the number. Networking has counted in bits since long before home broadband existed; storage has always counted in bytes. Two industries settled on different units and never reconciled them.

Your plan	Best real speed
25 Mbps	3.1 MB/s
100 Mbps	12.5 MB/s
500 Mbps	62.5 MB/s
1 Gbps	125 MB/s

When someone says their gigabit connection is "only downloading at 100," they're usually reading 100 megabytes per second — which is 80% of the line, and rather good.

The arithmetic

25 GB × 1024 = 25,600 MB

100 Mbps ÷ 8 = 12.5 MB/s

25,600 ÷ 12.5 = 2,048 seconds = 34 minutes

The other ambiguity: why 1 TB shows as 931 GB

There's a second, smaller unit disagreement hiding in the file sizes themselves.

Your operating system counts 1 GB as 1024 MB. Drive manufacturers count it as 1000 MB.

The gap is about 2.4% per step, and it compounds across steps — so by the time you reach a terabyte you've lost roughly 7%. That's where 931 comes from.

This calculator uses the 1024 convention, matching what your OS reports, because that's the number you'll see when the file lands.

This answer is a floor

The calculation assumes you receive your full advertised speed for the entire transfer. You won't.

• Protocol overhead takes 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
• Everyone else in the house shares the same connection

Use the answer to know whether a download is a coffee break or an overnight job — not to set a timer by.

Why one site is fast and another crawls

Same connection, same moment, wildly different speeds. This confuses people into blaming their ISP.

Usually it's the server, not your line. A slow or distant or overloaded server can't send you data faster than it sends it, however much bandwidth you've bought. Distance matters too — a transfer from the other side of the world is slower than a local one at identical advertised speeds.

A speed test measures the path to a nearby test server under ideal conditions, which is why it flatters your connection relative to real downloads.

The one fix worth trying

A cable instead of wifi. For most people it's the single biggest change, and it's often dramatic — especially on a fast plan, where wifi is far more likely to be the bottleneck than the line is.

After that: check nothing else in the house is saturating the connection, and accept that some downloads are slow for reasons at the other end entirely.

Print the bits, bytes and bandwidth sheet if you're the person who keeps having to explain the divide-by-eight thing.