Bandwidth / Transfer Time Calculator
Calculate how long a transfer will take at a given link speed, or how much bandwidth you need to move data inside a target window. Bits and bytes, decimal and binary units are all handled explicitly rather than silently assumed.
Protocol overhead and losses. 95% is a realistic starting point for a clean path.
Unit conventions used
- 1 byte = 8 bits, always. Network speeds are quoted in bits per second, file sizes in bytes.
- KB, MB, GB and TB use powers of 1000. KiB, MiB, GiB and TiB use powers of 1024.
- Kbps, Mbps and Gbps are decimal, so 1 Gbps = 1,000,000,000 bits per second.
The core calculation
Every transfer time calculation is the same division. Get the data size into bits, get the link rate into bits per second, and divide.
Time (seconds) = Data size (bits) ÷ Link rate (bits per second)
Data in bits = Data size × 8 (if the size is in bytes)
= Data size × 8 × 10^9 (if the size is in GB)
Worked: 100 GB at 1 Gbps
100 GB = 100 × 8 × 10^9 = 800,000,000,000 bits
1 Gbps = 1,000,000,000 bps
Time = 800,000,000,000 ÷ 1,000,000,000 = 800 s = 13 m 20 s
With 95% efficiency: 800 ÷ 0.95 = 842 s = 14 m 2 sWorking backwards for required bandwidth is the same relationship rearranged:
Required rate (bps) = Data size (bits) ÷ Time (seconds) With efficiency: Required rate ÷ efficiency Worked: 1 TB in 1 hour at 95% efficiency 1 TB = 1 × 10^12 bytes = 8,000,000,000,000 bits 1 hour = 3,600 s Rate = 8,000,000,000,000 ÷ 3,600 = 2,222,222,222 bps = 2.22 Gbps With 95% efficiency: 2.22 ÷ 0.95 = 2.34 Gbps
Example: is a 1 Gbps link enough for a nightly backup?
You need to move 400 GB every night inside a 6-hour window. The raw rate is 400 × 8 × 10^9 ÷ 21,600 = 148 Mbps. With 95% efficiency that becomes 156 Mbps.
A 1 Gbps link is more than six times the requirement, so the window is not the constraint. But check the storage: 400 GB in 6 hours needs a sustained 18.5 MB/s of read and write throughput, which is comfortably within a single spinning disk but worth verifying if the backup target is a NAS with many concurrent jobs.
Bits, bytes and the factor of eight
Network engineers measure capacity in bits per second and storage in bytes. Both conventions are well established, and the mismatch accounts for more confused capacity planning than any other single factor.
| Link speed | Bits per second | Bytes per second | Realistic throughput at 95% |
|---|---|---|---|
| 10 Mbps | 10,000,000 | 1.25 MB/s | 1.19 MB/s |
| 100 Mbps | 100,000,000 | 12.5 MB/s | 11.9 MB/s |
| 1 Gbps | 1,000,000,000 | 125 MB/s | 119 MB/s |
| 2.5 Gbps | 2,500,000,000 | 312.5 MB/s | 297 MB/s |
| 10 Gbps | 10,000,000,000 | 1.25 GB/s | 1.19 GB/s |
| 25 Gbps | 25,000,000,000 | 3.125 GB/s | 2.97 GB/s |
| 40 Gbps | 40,000,000,000 | 5 GB/s | 4.75 GB/s |
| 100 Gbps | 100,000,000,000 | 12.5 GB/s | 11.9 GB/s |
Decimal and binary units
There are two competing definitions of a kilobyte, and vendors choose whichever makes their product look better. Standards bodies resolved this by introducing the KiB, MiB, GiB and TiB names for the 1024-based units, but the older convention is still widely used in casual writing.
| Decimal name | Value | Binary name | Value | Difference |
|---|---|---|---|---|
| KB | 1,000 B | KiB | 1,024 B | 2.4% |
| MB | 1,000,000 B | MiB | 1,048,576 B | 4.9% |
| GB | 1,000,000,000 B | GiB | 1,073,741,824 B | 7.4% |
| TB | 1,000,000,000,000 B | TiB | 1,099,511,627,776 B | 10.0% |
| PB | 1,000,000,000,000,000 B | PiB | 1,125,899,906,842,624 B | 12.6% |
The gap grows with each magnitude, which is why a 16 TB drive reports as about 14.6 TiB and why a "100 GB" transfer can take 7% longer than the naive calculation suggests if the source is really 100 GiB.
What actually limits real transfers
The theoretical calculation is only the starting point. In practice, transfer speed is capped by the slowest link in the chain, and the network is often not it.
- Storage throughput. A single spinning disk does 150–200 MB/s sequentially. That is about 1.2–1.6 Gbps, so it saturates a 1 Gbps link but not a 10 Gbps one.
- TCP window limits on long paths. A single TCP flow needs a window of bandwidth × RTT to fill the link. Over a 100 ms path at 10 Gbps that is 125 MB in flight, which requires window scaling. See the TCP throughput calculator for the numbers.
- Latency and loss. Every retransmission costs a round trip. Above about 0.1% loss, throughput collapses regardless of available bandwidth.
- Encryption and compression. AES-NI keeps encryption overhead low, but software-only crypto can become the bottleneck long before the network does.
- Application protocol. A file transfer tool that uses many parallel streams will outperform a single-stream one on a high-latency path, because each stream's window limit applies independently.
Frequently asked questions
Why does a 1 Gbps link only give me about 125 MB/s?
Because network speeds are measured in bits per second while file sizes are measured in bytes, and there are 8 bits in a byte. 1 Gbps is 1,000,000,000 bits per second, which is 125,000,000 bytes per second, or 125 MB/s. Add protocol overhead and you typically see 110–118 MB/s in practice. This 8:1 relationship is the single most common source of confusion in bandwidth calculations.
How long does it take to transfer 100 GB at 1 Gbps?
100 GB is 800,000,000,000 bits at 1000-based units. Dividing by 1,000,000,000 bits per second gives 800 seconds, or 13 minutes 20 seconds. Applying a 95% efficiency factor for protocol overhead and losses gives about 14 minutes 2 seconds. If the 100 GB were actually 100 GiB (1024-based), the transfer would take about 7% longer.
What is the difference between Mbps and MB/s?
Mbps means megabits per second and MB/s means megabytes per second. The capital B is the only difference and it changes the value by a factor of eight. 100 Mbps equals 12.5 MB/s. ISPs advertise in Mbps because the number is larger, while file managers report in MB/s because that is the size unit users see in a file listing.
What efficiency factor should I use?
For a clean LAN path with modern hardware, 95% is realistic. For a WAN link crossing several hops, 90% is safer. For long-distance or lossy paths, 85% or lower. Wireless links can drop well below 50% once interference and retransmissions are counted. The efficiency factor is a rough allowance for TCP/IP headers, retransmissions, contention with other traffic and protocol overheads such as PPPoE.
What is the difference between GB and GiB?
A GB is 10^9 bytes (1,000,000,000) while a GiB is 2^30 bytes (1,073,741,824). Disk manufacturers use decimal units, so a '1 TB' drive holds 1,000,000,000,000 bytes, which operating systems display as about 931 GiB. The same 7.4% difference appears in every bandwidth calculation, so it is worth knowing which unit your source used.
How do I work out what bandwidth I need for a backup window?
Divide the total data volume by the available time to get the raw bit rate, then divide by your efficiency factor to get the link speed you actually have to buy. For example, 1 TB in a 4-hour window is 8,000,000,000,000 bits ÷ 14,400 seconds = 556 Mbps raw, which is about 585 Mbps once you allow 95% efficiency. The second mode of this calculator does exactly that calculation.
Will my disk keep up with the network?
Frequently it will not. A single 7,200 rpm spinning disk manages around 150–200 MB/s sequentially, which is roughly 1.2–1.6 Gbps. A 1 Gbps link is comfortably within that, but a 10 Gbps link is not. NVMe SSDs reach several GB/s, so they are not the constraint until you get past 25 Gbps. Always check the slowest component rather than assuming the network is it.