“How long will this take to copy?” is a question every engineer gets asked: a backup to a remote site, a VM migration, a firmware image pushed to a hundred devices, logs pulled from an airport overnight. The answer depends on more than the link speed: bits versus bytes, decimal versus binary units, protocol overhead and, over long distances, latency. This page works it out instantly and shows where the time goes.
Transfer time
Enter how much data you need to move and the speed of the link. Choose how much of the link the transfer can really use; the default allows for normal TCP/IP and Ethernet overhead. Switch to Required speed to work backwards from a deadline.
The same transfer on other links
Unit converter
Networks are sold in bits per second; files and disks are measured in bytes. Enter a rate in any unit to see it in all the others, and how much data it moves in a day or a month.
Latency and TCP
A single TCP connection can only have one window of data in flight before it waits for an acknowledgement, so over a long round trip it may never reach the link speed. Packet loss slows it further. Enter the path details to see the real ceiling for one stream and what limits it.
Why transfers are slower than the link
Bits and bytes
One byte is eight bits. A 1 Gbps link carries at most 125 MB/s, not 1,000 MB/s. Most “why is my copy so slow?” conversations start here.
Decimal and binary units
Network speeds and disk makers use decimal units: 1 GB is 1,000,000,000 bytes. Windows and many tools report sizes in binary units: 1 GiB is 1,073,741,824 bytes, about 7% more, and the gap grows to about 10% at the terabyte scale. A “500 GB” folder in Explorer is really 500 GiB, so choose the matching unit in the calculator.
Protocol overhead
Every 1,500-byte packet on Ethernet also carries a 14-byte header, a 4-byte checksum, an 8-byte preamble and a 12-byte gap before the next frame: 1,538 bytes on the wire. Inside it, the IPv4 and TCP headers take another 40 bytes, leaving 1,460 bytes of your data. That is 94.9% of the line rate at best. Jumbo frames (9,000 MTU) raise it to about 99%, which is why storage and backup networks use them.
Latency, windows and loss
TCP sends a window of data and waits for acknowledgements. A single stream can never exceed window ÷ round-trip time; to fill a link, the window must be at least the bandwidth-delay product (speed × round-trip time). Packet loss makes TCP back off: the Mathis formula estimates the ceiling as roughly (MSS ÷ RTT) × (1.22 ÷ √loss). Over a long, slightly lossy path, that limit can be far below the link speed.
These figures are estimates for planning. Real transfers also depend on disks, CPU, encryption, other traffic and the application itself. Everything is calculated in your browser.