What the download speed calculator does
Enter a file size in MB or GB and a connection speed in Mbps, and the calculator tells you how long the download will take. It exists because of a units trap: internet speeds are sold in megabits per second, file sizes are shown in megabytes, and there are eight bits in a byte — so a “100 Mbps” line moves about 12.5 MB per second, not 100. Runs in your browser.
The calculation
Time (s) = file size (MB) × 8 ÷ speed (Mbps)
4.7 GB (4,700 MB) at 100 Mbps: 4,700 × 8 ÷ 100 = 376 s ≈ 6 min 16 s
50 GB game at 500 Mbps: 50,000 × 8 ÷ 500 = 800 s ≈ 13 min
1 GB at 10 Mbps (mobile): 1,000 × 8 ÷ 10 = 800 s ≈ 13 min
Rule of thumb: MB per second ≈ Mbps ÷ 8The calculator uses 1 GB = 1,000 MB. Operating systems often report sizes in binary units (1 GiB = 1,024 MiB), which makes files look about 7% smaller than their decimal size; the difference is within the real-world margin anyway.
Download times at common speeds
| File | 10 Mbps | 50 Mbps | 100 Mbps | 500 Mbps | 1 Gbps |
|---|---|---|---|---|---|
| 5 MB photo | 4 s | 1 s | < 1 s | < 1 s | < 1 s |
| 100 MB app | 80 s | 16 s | 8 s | 2 s | 1 s |
| 1 GB video | 13 min | 2.7 min | 80 s | 16 s | 8 s |
| 4.7 GB DVD | 63 min | 13 min | 6 min | 75 s | 38 s |
| 25 GB Blu-ray | 5.6 h | 67 min | 33 min | 6.7 min | 3.3 min |
| 100 GB game | 22 h | 4.4 h | 2.2 h | 27 min | 13 min |
Why real downloads are slower
- Protocol overhead: TCP/IP headers and acknowledgements consume 5–10% of raw bandwidth.
- The server: many servers cap per-connection speed well below your line; a 1 Gbps connection often sees 50–200 Mbps from a single source.
- Wi-Fi: real throughput is typically a third to a half of the advertised link rate, and drops with distance and interference. Use a cable for large downloads.
- Shared lines: other devices streaming or updating share the same bandwidth.
- Peak-time congestion on cable and mobile networks.
- Disk speed rarely matters now, but an old hard drive can bottleneck a gigabit download.
Plan on 70–80% of the calculated speed for a wired connection to a fast server, and less on Wi-Fi.
Upload is usually slower
Most consumer connections are asymmetric: a “100/20” plan uploads at a fifth of its download speed. Backing up 100 GB of photos to the cloud at 20 Mbps takes about 11 hours, not the 2.2 the download figure suggests. Enter the upload speed to time uploads; fibre plans with symmetric speeds are the exception.
Common uses
- Deciding whether to start a large game or OS download now or overnight.
- Estimating how long a cloud backup or a video upload will take.
- Checking whether a connection can handle a 4K stream (about 25 Mbps) alongside other use.
- Comparing broadband plans in practical terms rather than headline numbers.
- Working out mobile data consumption: a 1 GB download uses 1 GB of allowance regardless of speed.