TechCompare LogoTechCompare

Data migration calculator: how long will your transfer really take over the WAN?

By TechCompare · Updated · How we check

The 35.3-day estimate is a constrained window example. The 11.1-hour figure is an ideal link-rate floor, not a guarantee. Use measured throughput to plan the migration.

A 5 TB migration over 1 Gbps at 40 ms RTT takes about 35.3 days in this fixed unscaled window scenario. The ideal link-rate floor is 11 hours 6 minutes 40 seconds with a window and storage path that can sustain the link. This is a deliberately constrained example with scaling disabled and a fixed window of about 64 KiB, not a general OS default.

Windows normally enables receive-window autotuning, and Linux enables receive-buffer autotuning by default. A small initial advertised window need not remain a persistent cap.

Transfer time
35d 7h 43m 24s
Window-limited to 13.1 Mbps
File size
5 TB
5000 GB
Link speed
1 Gbps
Gigabit Ethernet
Effective throughput
13.1 Mbps
at 40 ms RTT
Link utilized
1.31%
Bottleneck: TCP window
This exact 5 TB copy over Gigabit Ethernet, pre-loaded. Change the file size, link, or model a longer round trip.
Edit in the full tool

How this is calculated

A fixed unscaled window capped at 65,535 bytes gives about 13.1 Mbps at 40 ms RTT. A 1 Gbps link at that RTT needs 5 MB in flight, its bandwidth-delay product, to avoid a receive-window bottleneck. Scaling allows a larger advertised window, while autotuning or explicit buffer sizing determines its effective size. Turning on the calculator's scaling option without increasing the entered 64 KiB value leaves the window bottleneck in place.

Verdict

Use both scenarios to understand the window constraint, then plan around a probe transfer. At 40 ms RTT, a persistent unscaled window near 64 KiB limits a 1 Gbps path to about 13.1 Mbps, giving roughly 35.3 days for 5 TB.

In the calculator, enable scaling and enter at least 5 MB (about 4,883 KiB) to remove the modeled window limit. Even then, protocol overhead, storage, packet loss, encryption, and shared traffic can make the migration slower than the ideal 11 hours 7 minutes.

More WAN scenarios

100 GB over USB 3.0
A 100 GB transfer over USB 3.0 works out to 2 minutes 40 seconds at the interface's 5 Gbps peak rate.
View details
1 TB over USB 3.0
A 1 TB transfer over USB 3.0 works out to 26 minutes 40 seconds at the interface's 5 Gbps peak data rate.
View details
4 GB over USB 2.0
A 4 GB transfer over USB 2.0 works out to 1 minute 6 seconds at the interface's 480 Mbps peak rate.
View details

Related guides

10 TB data migration to cloud
The larger 10 TB cloud migration scenario at the same 40 ms RTT.
View guide
Bandwidth-delay product explained
The concept page that derives why the window caps throughput at 1.3% of the link.
View guide

Frequently asked questions

How do I calculate data migration time over a WAN?
Use measured sustained throughput for the planning estimate. To explore a window constraint, the model uses min(link rate, receive window times 8 divided by RTT). A fixed unscaled window near 64 KiB at 40 ms gives about 13.1 Mbps, or 35.3 days for 5 TB. That cap is a scenario assumption, not a general OS default.
How long does a 5 TB data migration take over a 1 Gbps WAN?
About 35.3 days in this fixed unscaled window scenario at 40 ms RTT. A receive window of at least 5 MB removes the modeled window limit and gives an ideal link-rate floor of 11 hours 6 minutes 40 seconds. Scaling must be negotiated and the effective buffer must be large enough. Other bottlenecks can still lengthen the transfer.
What is the best tool for planning a data migration?
Use a calculator that makes its link rate, effective receive window, and RTT assumptions explicit, then validate them with a representative probe transfer. Compare the ideal link-rate floor with a constrained-window scenario. Use actual sustained throughput for the final schedule.
Should I use a physical transfer appliance for a data migration?
Compare measured network transfer time with the appliance's shipping, ingest, and processing time. This 5 TB fixed unscaled window example takes about 35.3 days at 40 ms RTT, while a sustained 1 Gbps path with a large enough window has an ideal 11.1-hour floor. Run a representative probe before choosing.

Related tools

Display Bandwidth Calculator

Check display bandwidth for HDMI, DisplayPort, USB-C, Thunderbolt, and USB4.

Use tool

RAID Calculator

Calculate usable capacity and fault tolerance for RAID 0, 1, 5, 6, and 10.

Use tool

RAM Latency Calculator

Convert DDR3/DDR4/DDR5 timings (CL, tRCD, tRP, tRAS) into true latency in nanoseconds.

Use tool

Memory and Storage Throughput Visualizer

Visualize the large speed gap between CPU cache, RAM, and storage.

Use tool