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File transfer time, with latency.

Enter the file size and link speed. Optionally add round-trip latency and the TCP window to see when latency, not the link, sets your speed.

Header overheadTCP window limitLatency

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Inputs

Decimal units: 1 GB = 1,000 MB.
1 Gbps = 1,000 Mbps.
Use 0 to ignore latency. A ping time to the other machine is a good guide.
65,535 bytes is the limit without window scaling. Modern systems usually scale it up; raise this to model that. Only used when latency is above 0.

Result

Enter your numbers.

File Transfer Time Calculator: how it works

Two things keep a transfer below the link speed. First, headers: with a 1,500-byte payload (the Ethernet MTU), a 20-byte IPv4 header and a 20-byte TCP header (the minimum sizes), each frame carries 1,460 bytes of file data. On the wire each frame also adds 38 bytes of Ethernet preamble, header, checksum and interpacket gap (7 + 1 + 14 + 4 + 12), so 1,460 / 1,538 = 94.9% of the link speed is file data. The tool applies that figure.

Second, latency. A TCP sender can have at most one window of unacknowledged data in flight, so throughput cannot exceed window size divided by round-trip time. Without window scaling the window field is limited to 65,535 bytes, per the TCP article; the bandwidth-delay product article explains the link-capacity side.

Worked example. 50 GB over a 1 Gbps link with no latency: 1,000 x 0.9493 = 949.3 Mbps, so 50 x 8,000 Mbit / 949.3 = 421 s, or 7 min 1 s. Add 50 ms round-trip latency with a 65,535-byte window: 65,535 x 8 / 0.05 s = 10.49 Mbps, and the same file takes about 10 h 36 min. With window scaling on, a real system avoids most of that penalty. Other costs like disk speed, encryption and Wi-Fi are not modelled.

Frequently asked questions

How long does it take to transfer a file over the network?

Divide the file size in bits by the effective speed. This tool also removes about 5% for Ethernet, IP and TCP headers and applies a latency limit if you enter a round-trip time.

Why is my transfer slower than my link speed?

Header overhead, Wi-Fi quality, disk speed, and a small TCP window combined with high latency all reduce speed. The tool models the headers and the window.

What is the TCP window limit?

Without window scaling the TCP window is limited to 65,535 bytes, so throughput is at most window divided by round-trip time.

Is 1 Gbps the same as 125 MB/s?

Yes, 1 Gbps is 125 MB/s before overhead; after about 5% overhead it is about 119 MB/s.

Sources

Checked

Runs entirely in your browser. Nothing you type is sent or stored.

Not professional advice

Estimates only, from the inputs you enter. Real transfers vary with hardware, software and network conditions.

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