Bits, bytes and why the numbers differ
Internet speeds are sold in megabits per second (Mbps), while file sizes are shown in megabytes (MB) and gigabytes (GB). One byte is 8 bits, so a 100 Mbps connection moves at most 12.5 megabytes per second. That factor of 8 is the main reason downloads seem slower than the advertised speed.
Time (seconds) = file size in bits ÷ speed in bits per second ÷ efficiency
File size in bits = size in bytes × 8
Worked examples
- 10 GB at 100 Mbps: 10 × 8,000 megabits ÷ 100 = 800 seconds = 13 min 20 s at full speed, or about 14 min 49 s at a realistic 90% efficiency.
- 50 GB game at 1 Gbps: 400 seconds, about 6.7 minutes at full speed.
- 25 GB 4K film at 50 Mbps: about 67 minutes.
- 1 GiB file at 100 Mbps: a GiB is 1,073,741,824 bytes, so about 86 seconds.
Typical download times
| File | 25 Mbps | 100 Mbps | 500 Mbps | 1 Gbps |
|---|---|---|---|---|
| 5 MB song / photo set | 1.6 s | 0.4 s | 0.08 s | 0.04 s |
| 1 GB HD film | 5 min 20 s | 1 min 20 s | 16 s | 8 s |
| 10 GB | 53 min | 13 min 20 s | 2 min 40 s | 1 min 20 s |
| 100 GB game | 8 h 53 min | 2 h 13 min | 27 min | 13 min 20 s |
These are ideal times at 100% efficiency; real downloads usually take 5–20% longer.
Why real downloads are slower
- Protocol overhead: TCP/IP and encryption add a few percent of extra data.
- Wi-Fi: distance, walls and other devices can cut speeds by half or more; a wired connection usually gets close to the plan speed.
- The server: the other end may limit the speed per user.
- Sharing: other devices and people on the same connection share the bandwidth.
- Upload vs download: many home plans have much lower upload speeds, so uploading a backup takes far longer than downloading it.
Uploads and backups
Use the same calculation for uploads with your upload speed. A 500 GB initial cloud backup over a 20 Mbps upload takes about 55 hours at full speed, which is why many backup services offer seeding by shipping a disk. See the backup storage calculator to size the backup first.
Common mistakes
- Confusing Mbps with MB/s (a factor of 8). See the Mbps to MB/s converter.
- Mixing GB and GiB: operating systems often show GiB labelled as "GB".
- Using the download speed for uploads.
Typical file sizes
| File | Typical size |
|---|---|
| Photo from a phone | 2–6 MB |
| Song (compressed) | 3–10 MB |
| Mobile app | 50–500 MB |
| HD film (streaming quality, about 2 hours) | 1–4 GB |
| Operating-system update | 1–6 GB |
| Modern video game | 50–150 GB |
Sizes vary a lot between apps, games and quality settings; check the size shown by the store or download page.
Bandwidth versus latency
Bandwidth (Mbps) decides how fast large files download. Latency, or ping, is the delay for a single round trip, measured in milliseconds. Video calls and online games care more about low latency than about high bandwidth, while large downloads care mostly about bandwidth. A connection can be fast for downloads and still feel slow in games if latency is high.
Streaming versus downloading
Streaming services adapt quality to the connection. As a rough guide, HD video needs around 5 Mbps and 4K around 15–25 Mbps per stream, so a household with several simultaneous streams needs a correspondingly faster plan.
How to make a slow download faster
- Use a cable for big downloads. Wired Ethernet usually delivers close to the plan speed; Wi-Fi through walls can lose half or more.
- Prefer the 5 GHz or 6 GHz Wi-Fi band near the router; 2.4 GHz reaches further but is slower and more crowded.
- Pause other heavy users such as cloud backups, game updates and 4K streams on the same connection.
- Try off-peak hours. Busy servers and shared neighbourhood links are often faster late at night.
- Check the server, not just your line. If a speed test shows the full plan speed but one site is slow, the limit is on the other end.
Frequently asked questions
How long does it take to download 10 GB at 100 Mbps?
About 13 minutes 20 seconds at full speed, or roughly 15 minutes in practice.
How many MB per second is 100 Mbps?
12.5 MB/s, because one byte is 8 bits.
Why is my download slower than my internet plan?
Bits versus bytes, protocol overhead, Wi-Fi quality, server limits and other users on the connection all reduce the real speed.
How long does it take to download 100 GB?
About 2 hours 13 minutes at 100 Mbps, or 13 minutes 20 seconds at 1 Gbps, before overhead.
How long does a 1 GB download take?
About 80 seconds at 100 Mbps and 8 seconds at 1 Gbps at full speed; allow 5–20% more in practice.
What speed do I need for 4K streaming?
Streaming services commonly recommend roughly 15–25 Mbps per 4K stream.
Does ping affect download time?
Hardly for large files; bandwidth matters most. Ping matters for games, calls and many small requests.
Sources
- IEC 80000-13: quantities and units in information science (bits, bytes and binary prefixes).
- US Federal Communications Commission (FCC): broadband speed guide and household broadband guide.
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.