The arithmetic, and the overhead we assume
Speeds are sold in bits per second and files are measured in bytes, and one byte is 8 bits. So time = size in bytes × 8 ÷ speed in bits per second. 1 GB at 100 Mbps: 8,000,000,000 ÷ 100,000,000 = 80 seconds, if every bit on the wire were your file.
They aren't. Data travels in packets, and each packet carries headers. On ordinary Ethernet a full packet carries 1,460 bytes of your file but takes up 1,538 bytes of line time once you count the TCP, IP and Ethernet headers, the checksum, the preamble and the gap between frames. That is 94.9 % efficiency, the best a download can do. We call it "TCP/IP (5 %)" and use it as the default.
Real downloads usually do worse: Wi-Fi, a busy server, other people on the line, or a slow disk at either end. "Typical (20 %)" assumes you get 80 % of the plan speed. That figure is our estimate, not a measurement of your line. If your speed test shows less than your plan, type the measured number in instead.
Download times at a glance
Best case, with TCP/IP overhead (94.9 % efficiency). Multiply by about 1.19 for the typical case.
| Size | 10 Mbps | 25 Mbps | 50 Mbps | 100 Mbps | 300 Mbps | 500 Mbps | 1 Gbps |
|---|---|---|---|---|---|---|---|
| 1 GB | 14 min 3 s | 5 min 37 s | 2 min 49 s | 1 min 24 s | 28 s | 17 s | 8 s |
| 4 GB | 56 min 11 s | 22 min 28 s | 11 min 14 s | 5 min 37 s | 1 min 52 s | 1 min 7 s | 34 s |
| 10 GB | 2 h 20 min | 56 min 11 s | 28 min 5 s | 14 min 3 s | 4 min 41 s | 2 min 49 s | 1 min 24 s |
| 50 GB | 11 h 42 min | 4 h 41 min | 2 h 20 min | 1 h 10 min | 23 min 25 s | 14 min 3 s | 7 min 1 s |
| 100 GB | 23 h 25 min | 9 h 22 min | 4 h 41 min | 2 h 20 min | 46 min 49 s | 28 min 5 s | 14 min 3 s |
| 150 GB | 35 h 7 min | 14 h 3 min | 7 h 1 min | 3 h 31 min | 1 h 10 min | 42 min 8 s | 21 min 4 s |
| 1 TB | 9 days 18 h | 3 days 22 h | 46 h 49 min | 23 h 25 min | 7 h 48 min | 4 h 41 min | 2 h 20 min |