taap.digitalnetwork engineering toolkit

How long does it take to transfer 4 TB at 2.5 Gbps?

Answer
3 h 57 min 2 s with 10% overhead

4 TB is 32,000 Gbit. At 2.5 Gbps minus 10% overhead (2,250 Mbps effective) that takes 3 h 57 min 2 s. The theoretical line-rate minimum is 3 h 33 min 20 s.

xfer-time · adjust the values
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The calculation

4 TB × 8 = 32,000 Gbit
32,000 Gbit ÷ (2.5 Gbps × 0.9) = 3 h 57 min 2 s

Sizes and rates use decimal units (1 GB = 109 bytes, 1 Gbps = 109 bit/s). If your source reports TiB or GiB, the data is about 10% or 7% larger respectively.

Effect of overhead

OverheadTransfer time
0%3 h 33 min 20 s
5%3 h 44 min 34 s
10%3 h 57 min 2 s
20%4 h 26 min 40 s
30%5 h 4 min 46 s

When it takes longer

A single TCP flow over a long-RTT path is capped at window ÷ RTT, so check the bandwidth-delay product before blaming the link. Disk throughput, encryption on one CPU core (scp, SMB signing) and shared links are the other usual suspects. Parallel streams (rclone --transfers 8, iperf3 -P 4) usually close the gap.

More questions like this

100 GB @ 1 Gbps1 TB @ 1 Gbps1 TB @ 10 Gbps10 TB @ 10 Gbps50 GB @ 100 Mbps500 GB @ 1 Gbps1 TB @ 100 Mbps

Frequently asked questions

What is 2.5 Gbps in MB/s?

312.5 MB/s at line rate, about 281.2 MB/s with 10% overhead.

Why use 10% overhead?

Ethernet, IP and TCP headers take about 5% at a 1500-byte MTU; the rest covers retransmissions, protocol chatter and imperfect pipelining. It is a realistic default for a healthy path.