// FREE IT TOOLS

Bandwidth & Transfer Time Calculator

Work out how long a transfer will take at any speed, or the bandwidth you need to hit a deadline — with decimal and binary units handled properly.

// FILE SIZE

presets:

// TRANSFER SPEED

Nothing is sent to the server — all calculations happen in your browser.

// TRANSFER TIME

13 minutes 20 seconds

ETA 00:13:20

Sustained throughput

12.5 MB/s

Total data

80,000,000,000 bits

This is the theoretical figure at full line rate. Real transfers rarely reach it — TCP overhead, disk speed, latency and competing traffic all take a bite. Expect roughly 90–95% of these numbers for a large sequential transfer, and considerably less for many small files.

10 GB at 100 Mbps = 13 minutes 20 seconds (00:13:20)

Bits, Bytes, and Why the Number Never Matches

Network speeds are sold in bits per second. File sizes are measured in bytes. There are eight bits in a byte, so a 100 Mbps connection tops out at 12.5 MB/s — and that single missing factor of eight is behind more "my internet is broken" tickets than any other cause.

The calculator keeps the distinction explicit. Every speed you enter is converted to bits per second internally, so a rate given as Mbps and one given as MB/s are handled by the same arithmetic and produce the same answer when they describe the same link.

Decimal and Binary Units

There is a second mismatch, and it is the reason a "1 TB" drive shows up as 931 GB. Drive manufacturers and network engineers use powers of ten. Most operating systems use powers of two — but label the result with the decimal name.

Decimal	Bytes	Binary	Bytes
KB	1,000	KiB	1,024
MB	1,000,000	MiB	1,048,576
GB	1,000,000,000	GiB	1,073,741,824
TB	1,000,000,000,000	TiB	1,099,511,627,776

Both sets are available in the calculator. Pick the one your source actually used — choosing the wrong column introduces a 2.4% error at KB and 10% by TB.

What Real Transfers Actually Achieve

A link running at line rate is not moving your data at line rate. Ethernet frames carry a 14-byte header, a 4-byte checksum, and 20 bytes of preamble and inter-frame gap. Inside the 1,500-byte payload, TCP and IP consume another 40 bytes. That leaves 1,460 bytes of application data out of 1,538 bytes on the wire — about 95%, before disk speed, latency or any other traffic is considered.

Link speed	Theoretical max	Typical sustained
100 Mbps	12.5 MB/s	~11.8 MB/s
1 Gbps	125 MB/s	~118 MB/s
10 Gbps	1.25 GB/s	~1.18 GB/s

These figures assume a single large sequential transfer and storage that can keep up. Many small files perform dramatically worse, because each one costs a latency round trip rather than a slice of bandwidth — which is why archiving a directory before sending it is often several times faster than copying it file by file.

Frequently Asked Questions

Why is my real download slower than the calculator predicts?: The calculator gives the theoretical time at full line rate, which assumes every bit on the wire is your data. In practice Ethernet framing, TCP/IP headers, disk speed, latency and competing traffic all take a slice. A well-tuned large sequential transfer over gigabit Ethernet typically achieves around 95% of line rate — roughly 118 MB/s on a 1 Gbps link rather than 125 MB/s.
What is the difference between Mbps and MB/s?: A factor of eight. Mbps means megabits per second and MB/s means megabytes per second. Network links are almost always sold in bits per second, while file sizes and download bars are in bytes. So a 100 Mbps connection moves at most 12.5 MB/s. This is the single most common reason people think their connection is broken.
Why does my 1 TB drive only show 931 GB?: Because drive manufacturers use decimal units and operating systems use binary ones. A 1 TB drive holds 1,000,000,000,000 bytes. Windows divides that by 1024 three times, reporting 931 GiB — but labels it "GB". The drive is exactly the size it was sold as; only the unit label differs. Linux and macOS report the same number but usually call it GiB.
How long does it take to transfer 1 TB over a 1 Gbps link?: About 2 hours 13 minutes at full line rate — 1,000,000,000,000 bytes is 8,000,000,000,000 bits, and dividing by 1 Gbps gives 8,000 seconds. Allow closer to 2 hours 20 minutes in practice once Ethernet and TCP overhead are accounted for.
Does the number of files change the transfer time?: Yes, substantially, and this calculator cannot model it. It assumes one large stream. Thousands of small files run far slower than the same number of bytes in one archive, because every file costs a round trip of latency for open, close and metadata. If you are moving many small files, archive them first — the difference is often several times faster.
What speed do I need to back up 1 TB overnight?: Switch the calculator to "What speed do I need?" and enter your window. An eight-hour window needs about 278 Mbps sustained to move 1 TB — comfortably within gigabit Ethernet, but well beyond a typical 100 Mbps link. This is the calculation that decides whether a backup window is realistic.
Why is my upload slower than my download?: Most consumer connections are asymmetric — ADSL, VDSL and DOCSIS cable all allocate far more bandwidth downstream than upstream. A "100 Mbps" line might offer only 10 Mbps upstream. Enter the slower figure when calculating an upload or a cloud backup, not the headline speed on your bill.
Is my data sent to a server?: No. All calculation happens in your browser using JavaScript. The sizes and speeds you enter never leave your device and are not stored or transmitted anywhere.

// NETWORK INFRASTRUCTURE

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DMC IT Services designs network and cloud infrastructure for SMBs across London, Cambridge, Hertfordshire, and Bedfordshire — including sizing links for backup and migration workloads before they run.

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Bits, Bytes, and Why the Number Never Matches

Decimal and Binary Units

Decimal	Bytes	Binary	Bytes
KB	1,000	KiB	1,024
MB	1,000,000	MiB	1,048,576
GB	1,000,000,000	GiB	1,073,741,824
TB	1,000,000,000,000	TiB	1,099,511,627,776

Both sets are available in the calculator. Pick the one your source actually used — choosing the wrong column introduces a 2.4% error at KB and 10% by TB.

What Real Transfers Actually Achieve

Link speed	Theoretical max	Typical sustained
100 Mbps	12.5 MB/s	~11.8 MB/s
1 Gbps	125 MB/s	~118 MB/s
10 Gbps	1.25 GB/s	~1.18 GB/s

Frequently Asked Questions

Why is my real download slower than the calculator predicts?

The calculator gives the theoretical time at full line rate, which assumes every bit on the wire is your data. In practice Ethernet framing, TCP/IP headers, disk speed, latency and competing traffic all take a slice. A well-tuned large sequential transfer over gigabit Ethernet typically achieves around 95% of line rate — roughly 118 MB/s on a 1 Gbps link rather than 125 MB/s.

What is the difference between Mbps and MB/s?

A factor of eight. Mbps means megabits per second and MB/s means megabytes per second. Network links are almost always sold in bits per second, while file sizes and download bars are in bytes. So a 100 Mbps connection moves at most 12.5 MB/s. This is the single most common reason people think their connection is broken.

Why does my 1 TB drive only show 931 GB?

Because drive manufacturers use decimal units and operating systems use binary ones. A 1 TB drive holds 1,000,000,000,000 bytes. Windows divides that by 1024 three times, reporting 931 GiB — but labels it "GB". The drive is exactly the size it was sold as; only the unit label differs. Linux and macOS report the same number but usually call it GiB.

How long does it take to transfer 1 TB over a 1 Gbps link?

About 2 hours 13 minutes at full line rate — 1,000,000,000,000 bytes is 8,000,000,000,000 bits, and dividing by 1 Gbps gives 8,000 seconds. Allow closer to 2 hours 20 minutes in practice once Ethernet and TCP overhead are accounted for.

Does the number of files change the transfer time?

Yes, substantially, and this calculator cannot model it. It assumes one large stream. Thousands of small files run far slower than the same number of bytes in one archive, because every file costs a round trip of latency for open, close and metadata. If you are moving many small files, archive them first — the difference is often several times faster.

What speed do I need to back up 1 TB overnight?

Switch the calculator to "What speed do I need?" and enter your window. An eight-hour window needs about 278 Mbps sustained to move 1 TB — comfortably within gigabit Ethernet, but well beyond a typical 100 Mbps link. This is the calculation that decides whether a backup window is realistic.

Why is my upload slower than my download?

Most consumer connections are asymmetric — ADSL, VDSL and DOCSIS cable all allocate far more bandwidth downstream than upstream. A "100 Mbps" line might offer only 10 Mbps upstream. Enter the slower figure when calculating an upload or a cloud backup, not the headline speed on your bill.

Is my data sent to a server?

No. All calculation happens in your browser using JavaScript. The sizes and speeds you enter never leave your device and are not stored or transmitted anywhere.