240 download times — 1 GB over 100 Mbps takes 85 seconds
A time for every meeting of 24 file sizes and 10 line speeds. Divide the advertised number by eight, then lose another 6% to addresses and sequence numbers.
Small b, capital B
The b your provider sells in 100 Mbps is a bit; the B on a file size is a byte. Eight bits to the byte, so a 100 Mbps line tops out at 12.5 MB/s. Half of every "why isn’t my gigabit line giving me 100 MB/s" ends right here.
The other 6% is packaging
Your file leaves in 1,448-byte slices, and each slice carries a source and destination address, a sequence number and a checksum. Between slices the wire must also idle. One slice occupies 1,538 bytes of wire for 1,448 bytes of file, so only 94.1% of the advertised speed is ever your download.
The size your file browser shows
Put a 4.7 GB DVD in a Windows machine and it reads 4.38 GB. Discs and providers count a GB as a billion bytes; the file browser counts 2³⁰ — 1,073,741,824 — and still calls it GB. That is where the 7% gap comes from.
Cells that take the same time
Scale the size and the speed by the same factor and the clock does not move. 1 GB over 100 Mbps takes exactly as long as 10 GB over 1000 Mbps.
Narrower than the line
However fast the line, a narrow hop between router and device caps everything. Wi-Fi 5 tops out at 866 Mbps on paper, so it cannot carry a gigabit line, and a gigabit port ends at 1000 Mbps. The figures below are the standards’ maximums; real links run lower.
Find it by size and speed
What happens when the line is shared. Each figure divides the real speed, packaging already removed, by what the service recommends.
10MB
25MB
50MB
100MB
200MB
350MB
500MB
700MBa CD
1GB
1.5GB
2GB
3GB
4.7GBa DVD
6GB
8GB
10GB
15GB
20GB
25GBa Blu-ray
30GB
50GBa dual-layer Blu-ray
70GB
100GB
150GB
Line speed
- throttled data5Mbps · 0.59MB/s
- typical LTE20Mbps · 2.35MB/s
- 100 Mb plan100Mbps · 11.77MB/s
- 500 Mb plan500Mbps · 58.84MB/s
- gigabit plan1000Mbps · 117.69MB/s
- 2.5 Gb plan2500Mbps · 294.21MB/s
How to read this
- These times assume the line stays full from start to finish. A slow server or a shared connection stretches them.
- Small b is bits, capital B is bytes. Divide by eight to get MB/s.
- The speed you actually see is 94.1% of the advertised one — addresses and sequence numbers take the rest.
- Sizes here are decimal. Windows Explorer’s GB is 7% larger than this one.
Frequently asked questions
Q. How long does 1 GB take on 100 Mbps?
85 seconds. Plain division says 80, and the addresses and sequence numbers on every slice add the other five.
Q. Why doesn’t my gigabit line give 100 MB/s?
1000 Mbps caps at 125 MB/s and lands at 117 MB/s once packaging is removed. Pass it through Wi-Fi 5 or a gigabit port and it drops further.
Q. What is the difference between Mbps and MB/s?
Small b is bits, capital B is bytes. Divide by eight: 100 Mbps is 12.5 MB/s.
Q. Why does a 4.7 GB DVD show as 4.38 GB?
Same file, different ruler. Discs count a billion bytes to the GB; the file browser counts 1,073,741,824.
Q. Mine is slower than the table says.
The table assumes a full line. The serving side, your router, Wi-Fi and every other device sharing the connection all add time.