How this calculator works
The calculator converts between data sizes and transfer rates and estimates how long a transfer takes. It runs in your browser. The most common mistake it prevents is mixing bits and bytes: link speeds are quoted in bits per second (Mbps), while file sizes are in bytes (MB). One byte is 8 bits, so a 100 Mbps link moves at most 12.5 MB per second.
Units
- Decimal units (kB, MB, GB) use powers of 1,000. Network speeds and disk makers use them.
- Binary units (KiB, MiB, GiB) use powers of 1,024. Operating systems often use them while still labelling them "KB", "MB" and "GB", which is why a "1 TB" disk shows as about 931 GB in Windows.
Why real transfers are slower
The estimate is a best case at full line rate. Real throughput is lower because of:
- Protocol overhead. Ethernet framing plus IP and TCP headers take about 5 percent of each full-size 1,500-byte packet, and more when packets are small.
- Latency and TCP windows. A single TCP stream can't exceed its window size divided by the round-trip time. Over long distances one stream may use only part of a fast link; parallel streams or tuned windows help.
- Shared links and contention. Other traffic, Wi-Fi conditions and provider oversubscription all reduce what you get.
- The slowest link wins. A transfer runs at the speed of the narrowest point between the two ends, which may be the far server.
Sizing a site's bandwidth
Multiply the average page weight by page views for the period, then allow headroom for peaks: traffic in the busiest hour is often several times the daily average. A content delivery network serves most static files from its own edge and reduces what your origin needs.
Updated 26 October 2026