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Network bandwidth and transfer time calculator

How long will this data take over this link? Enter the size and line speed to see the time, including the TCP limit set by efficiency, link share and latency (RTT). In reverse mode, work out the line speed needed to finish within a given time.

Free tool · Infrastructure
Direction
Data

Decimal units (MB, GB, TB: 10^6, 10^9, 10^12 bytes) differ from binary units (MiB, GiB, TiB: 2^20, 2^30, 2^40 bytes); pick whichever your operating system shows.

Link

The nominal speed of the link (bits per second). Provider figures are usually decimal: 1 Gbit/s = 10^9 bit/s.

%

Loss from protocol headers, encryption and retransmissions; 85–95% is common. Enter a measured net rate with 100%.

%

The percentage this transfer can take if the link carries other traffic. 100% on a dedicated link.

TCP and latency
ms

Round-trip time between the two ends (ping). Leave empty or 0 to skip the TCP limit.

KiB

How much data may be in flight without acknowledgement. The classic value without window scaling is 64 KiB; modern systems usually grow it to a few MiB.

streams

Number of simultaneous connections; each stream has its own window.

Results

Transfer time
1.2 h
Usable speed 900 Mbit/s (line × 90% efficiency × 100% share)
Achieved data rate
112.5 MB/s
107.29 MiB/s
Effective speed
900 Mbit/s
Limiting factor
—
The link is the limit: the TCP cap is 1.68 Gbit/s, the usable line speed is 900 Mbit/s.
Window needed to fill the link
2.15 MiB
2.15 MiB per stream

Calculation and units

  • 1 byte = 8 bits. Decimal: 1 GB = 10^9 bytes. Binary: 1 GiB = 2^30 bytes (≈ 1.074 × 10^9). Line speed: 1 Gbit/s = 10^9 bit/s.
  • Usable speed = line speed × efficiency × link share.
  • TCP cap (bit/s) = streams × window (bytes) × 8 / RTT (s).
  • Time = bytes × 8 / min(usable speed, TCP cap). Reverse: line speed = bytes × 8 / (time × efficiency × share).

The values you enter are processed in your browser; they are not sent to a server and not stored.

This is a preliminary estimate: it assumes a constant speed; TCP slow start, packet loss, congestion control, disk speed and the application protocol can lengthen the real time. Verify important transfers by measuring a small sample.

Shall we size your site-to-site links, backup transfer windows or cloud migration together?

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01

How to use

  1. A

    Enter the data size with its unit (decimal or binary) and the line speed; set the efficiency and the link share for this transfer.

  2. B

    Enter the latency between the two ends (RTT) and the TCP window; see whether the link or the TCP window is the limit.

  3. C

    If needed, switch to reverse mode and enter the target time: the speed shown is the nominal line speed required, including efficiency and share.

02

How is transfer time calculated?

Line speed is in bits per second and data size in bytes, so time is bytes × 8 / speed. The data actually carried is not the whole nominal speed: Ethernet, IP and TCP headers, encryption and retransmissions reduce it. The efficiency field represents this loss as one percentage; if you have a measured net rate, enter it with 100%.

If the link carries other traffic, enter the share the transfer will get separately. The effective speed in the result is the smaller of the usable speed and the TCP cap described below.

03

The TCP window and latency limit

TCP can keep at most one window of data in flight before an acknowledgement arrives. The speed of a single stream therefore cannot exceed window / RTT: with a 64 KiB window and 20 ms RTT the cap is about 26 Mbit/s, even on a 1 Gbit/s link. As latency grows (a remote site, satellite, an overseas cloud) the cap falls.

The window needed to fill the link equals the bandwidth-delay product (speed × RTT); the tool shows that too. The remedy is window scaling and larger buffers, or parallel streams; many file transfer tools run multi-stream for this reason.

04

Decimal and binary units

Network providers and disk makers use 1 GB = 10^9 bytes; some operating systems show GiB (2^30 bytes) and may label it GB. The difference is about 7.4% at GB scale and about 9–10% at TB scale. Choose the unit in which you see the data size; that is why the tool keeps the two apart.

When planning a cloud migration, a remote-site backup or a large software rollout, read the result with a safety margin; real transfers rarely run at a constant speed.

FAQ

Is my input sent anywhere?
No. The calculation runs entirely in code in your browser; values are not sent to any server and are not stored.
Why does a 1 Gbit/s link give 125 MB/s on paper but my download is slower?
1 Gbit/s = 125 MB/s is the nominal value. Protocol headers, encryption, sharing, disk speed, server-side limits and the TCP window/RTT limit all lower the real rate. The efficiency field covers the first two, the RTT and window fields the last.
What is the TCP window and why does it matter?
It is the amount of data the sender may send without waiting for an acknowledgement. A single stream is limited to window / RTT; on high-latency links a small window leaves the link idle.
How do I convert between Mbit/s and MB/s?
Divide by eight: 1 Mbit/s = 0.125 MB/s, 100 Mbit/s = 12.5 MB/s, 1 Gbit/s = 125 MB/s. A lowercase b means bit, an uppercase B means byte.
What is the difference between link share and efficiency?
Efficiency stands for the transfer's own protocol losses; link share stands for sharing the line with other traffic. The two are multiplied to give the usable speed.

Does the transfer time not fit your plan?

Let us design lines, WAN and transfer paths together for site-to-site links, backup windows, cloud migration and OT/SCADA networks. In a free discovery call we discuss your current state and targets.

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