Transfer time calculator
How long a file takes at a given line speed, with protocol overhead.
This screen is entirely about bits and bytes. Line speeds are quoted in bits (100Mbps) and file sizes in bytes (700MB) — a factor of eight, and missing it makes the answer eight times wrong. That is the answer to "why only 12MB/s on a 100Mbps line". Whether a kilo is 1,000 or 1,024 shifts things by another 5%, so that is selectable too.
File and connection
What you enter is saved in this browser, so it is still here next time
MB
Mbps
What networking and drive labels use (1MB = 1,000,000B).
Headers, retransmission and shared capacity. 5–10% is close to reality.
Time
Realistic
0h 1m 1s
Time
- Theoretical
- 56 s
- Effective rate
- 60.87 s
- Per second
- 12.5 MB/s
- In bytes
- 700,000,000
Line speeds are in bits and file sizes in bytes — a factor of eight, so 100Mbps is 12.5MB/s.
Based on line speed alone. A slower server or disk sets the real pace instead.
Turning bits into bytes, in order
File size arrives in bytes and line speed in bits, so one of them is converted. 700 MB is 700 × 1,000,000 = 700,000,000 bytes, and 100 Mbps is 100,000,000 bits per second. Dividing the bits by 8 gives 12,500,000 bytes per second, or 12.5 MB/s, and 700,000,000 ÷ 12,500,000 = 56 seconds is the ideal transfer time. Switching to GB or Gbps only multiplies by another 1,000; the order stays the same.
Overhead comes off the speed
An overhead of 8% means 8% of what is sent is headers and retransmission, so the bytes per second are multiplied by 0.92. 12.5 MB/s becomes 11.5 MB/s, and 700,000,000 ÷ 11,500,000 = 60.87 seconds. The large time at the top is this effective figure split into hours, minutes and seconds, so it reads 1 min 1 s, with the seconds rounded. The rows below show the ideal and effective times side by side, so you can see exactly how many seconds the overhead cost. Enter 0 and the two coincide; nothing above 90% is accepted.
Choosing 1,024 only makes the file bigger
Picking binary changes the file size alone to powers of 1,024. 700 MB becomes 700 × 1,048,576 = 734,003,200 bytes, which takes 58.72 seconds on the same line — a 4.9% difference. The line speed is always decimal whichever you choose, because that is how networking standards define it; no equipment reads 100 Mbps as 104,857,600 bps. The MB/s in the per-second row is likewise always divided by 1,000,000.
What this page does not do
It is the arithmetic of one file with a line to itself. Several downloads at once, a server-side speed cap, a weak Wi-Fi signal, and asymmetric lines where upload is slower than download are not modelled separately. All of those losses have to be folded into the single overhead percentage, which is why the number is best read as "at least this long".
Common questions
QWhy does 100Mbps give 12.5MB/s?
100Mbps is 100 million bits per second; divided by eight that is 12.5 million bytes per second. Connections are advertised in bits and downloads displayed in bytes, hence the factor of eight.
QIt always takes longer than this.
TCP/IP headers, retransmission and shared capacity all reduce the effective rate. Entering 5–10% overhead brings it closer to what you see; a server-side limit can push it lower still.
QIs it 1,000 or 1,024?
Both are in use. Networking and drive labelling are decimal (1MB = 1,000,000B), while operating systems often display binary units (1MiB = 1,048,576B). That is why a 1TB disk shows as 931GB.
QHow long does a 20 GB film take at 500 Mbps?
500 Mbps is 62.5 MB per second and 20 GB is 20,000,000,000 bytes, so ideally 320 seconds, or 5 min 20 s. With 8% overhead the rate falls to 57.5 MB/s and the time becomes 347.8 seconds, about 5 min 48 s. In practice a server rarely delivers the full rate, so longer is the usual outcome.
QCan I use it for uploads?
The formula is the same, so enter your upload speed in the speed field. Home connections are usually asymmetric, though: 100 Mbps down often comes with 10–20 Mbps up. Entering the big number from the plan gives an upload time five to ten times too short.