The bits-versus-bytes gotcha, the transfer-time formula, real-world overhead, and the binary-versus-decimal trap, explained with worked examples.
- Internet speed is sold in bits; files are saved in bytes.
- Divide your Mbps by eight to get MB/s.
- Transfer time equals file size in bits divided by your speed in bits per second.
- Add 10 to 25 percent for real-world overhead.
Want to know how long to transfer a file before you start? Your file manager guesses two minutes, then the download runs for twelve. The gap is not your imagination. It comes from one unit mix-up that trips up almost everyone. Internet speed is sold in bits. Files are measured in bytes. Those two units differ by a factor of eight. Once you see that, the math gets simple.
Why 100 Mbps is not 100 MB/s
Internet plans are measured in megabits per second, written Mbps with a lowercase b. Files are measured in megabytes, written MB with a capital B. One byte equals eight bits. So a 100 Mbps line moves about 12.5 megabytes each second, not 100.
That single letter is the whole trick. Lowercase b means bits. Capital B means bytes. Marketing teams love the bigger number, so they advertise in bits. A “1 Gbps” plan sounds eight times faster than “125 MB/s,” yet they describe the same line. Your operating system, meanwhile, reports file sizes in bytes. The two camps never switched to a shared unit, so the mismatch lives on.
The one conversion that clears up the confusion
Convert speed to bytes before you estimate anything. Take the megabits-per-second number and divide by eight. That gives megabytes per second. A 300 Mbps plan becomes 37.5 MB/s. A 1 Gbps plan becomes 125 MB/s. Multiply by eight to go back the other way.
Keep a few anchors in your head. Divide by eight, and the scary speed number shrinks to something usable. 25 Mbps is roughly 3 MB/s. 100 Mbps is 12.5 MB/s. 500 Mbps is 62.5 MB/s. Gigabit is 125 MB/s. From there, every transfer estimate is just size divided by speed.
The transfer-time formula, step by step
The core formula is short. Transfer time in seconds equals the file size in bits divided by the speed in bits per second. In practice, take the file size in megabytes, multiply by eight, then divide by your Mbps. The answer is your best-case time.
Here is the order of operations:
- Find the file size in megabytes (MB).
- Multiply that size by eight to get megabits.
- Divide by your plan speed in Mbps.
- The result is the transfer time in seconds.
- Multiply by 1.1 to 1.25 for real-world overhead.
That last step matters more than people expect. We cover it next.
A worked example you can copy
Picture a 4.7 GB movie download on a 100 Mbps connection. That is 4,700 MB, or 37,600 megabits. Divide by 100, and you get 376 seconds. That is about six minutes and 16 seconds at best. Add overhead, and seven minutes is realistic.
Try a bigger one. A 50 GB game install on a 300 Mbps plan works the same way. That is 50,000 MB, times eight, for 400,000 megabits. Divide by 300, and you get about 1,333 seconds. Best case is roughly 22 minutes. With overhead, plan for 25 to 27 minutes. The pattern never changes: size to bits, then bits divided by speed.
Local copies follow the same rule. A Samsung T7 SSD reads near 1,000 MB/s over USB. So a 50 GB file moves in under a minute on a good cable. The slow part is almost always the network, not the drive.
Why real transfers run slower than the math
Your best-case number assumes a perfect link, which never happens. TCP/IP headers wrap every packet and eat a slice of bandwidth. Wi-Fi loses signal through walls and distance. Servers cap their upload speed. Together these trim real throughput by about 10 to 25 percent.
A few forces stack up against your advertised speed:
- Protocol overhead: TCP and IP headers add roughly 5 to 10 percent.
- Wi-Fi loss: a Wi-Fi 6 router slows down with distance and interference.
- Shared lines: your neighborhood splits one pipe at busy hours.
- Server limits: the host may throttle each connection on purpose.
- Up-to speeds: ISPs advertise a ceiling, not a guarantee.
Numbers make this concrete. A Wi-Fi 6 router rated at “gigabit” often delivers 200 to 600 Mbps in a real home. Walls, neighbors, and old client devices each take a cut. The same laptop on Cat 6 Ethernet holds near the full line rate.
For a quick reality check, multiply your best-case time by 1.15. A wired Cat 6 connection gets closest to the math. Switch stationary devices to Ethernet when a transfer really matters. For more on the link itself, see how bandwidth is defined.
Binary versus decimal: why your 1 TB drive shows 931 GB
Two counting systems share the same prefixes, which breeds confusion. Networking uses decimal units, so 1 megabit is exactly 1,000,000 bits. Storage drives use decimal too. Windows, though, counts in binary, where 1 “GB” is really 1,073,741,824 bytes. That is why a 1 TB drive reads as about 931 GB.
Standards bodies tried to end the mess. The IEC defined binary prefixes with an “i” inside them. A kibibyte (KiB) is 1,024 bytes. A mebibyte (MiB) is 1,048,576 bytes. A gibibyte (GiB) is 1,073,741,824 bytes. NIST publishes these names so software can label sizes clearly. Most apps still ignore them, so you decode the gap yourself.
Bandwidth-to-transfer-time reference table
This table converts common plan speeds into byte rates and transfer times. It assumes decimal units and a perfect link. Real transfers run about 10 to 25 percent slower. Use the figures as a fast sanity check, then add overhead for a realistic estimate.
| Plan speed | Byte rate | 1 GB file | 5 GB movie | 50 GB game |
|---|---|---|---|---|
| 25 Mbps | 3.1 MB/s | 5 min 20 s | 26 min 40 s | 4 hr 27 min |
| 100 Mbps | 12.5 MB/s | 1 min 20 s | 6 min 40 s | 1 hr 7 min |
| 300 Mbps | 37.5 MB/s | 27 s | 2 min 13 s | 22 min 13 s |
| 500 Mbps | 62.5 MB/s | 16 s | 1 min 20 s | 13 min 20 s |
| 1 Gbps | 125 MB/s | 8 s | 40 s | 6 min 40 s |
| 2.5 Gbps | 312.5 MB/s | 3 s | 16 s | 2 min 40 s |
Times are theoretical maximums using decimal units, where 1 GB equals 1,000 MB. Add 10 to 25 percent for real-world overhead.
Skip the math: use the bandwidth calculator
You do not have to run these numbers by hand every time. Our bandwidth calculator takes a file size and a plan speed, then returns a transfer time. It handles the bits-to-bytes step and the overhead factor for you. Enter your numbers and read the answer.
It is free, needs no signup, and runs in your browser.
Frequently asked questions
Short answers to the questions readers ask most about bandwidth math. Each one uses the same divide-by-eight logic from this guide. For your own files, the bandwidth calculator gives an exact number in seconds.
Why isn’t 100 Mbps the same as 100 MB/s?
Because Mbps counts bits and MB/s counts bytes. One byte is eight bits. So 100 Mbps equals about 12.5 MB/s, eight times smaller than it sounds.
How do I convert Mbps to MB/s?
Divide the Mbps number by eight. A 300 Mbps plan becomes 37.5 MB/s. A 1 Gbps plan becomes 125 MB/s. Multiply by eight to reverse it.
What is the formula for file transfer time?
Transfer time in seconds equals file size in bits divided by speed in bits per second. In practice, multiply the size in MB by eight, then divide by your Mbps.
Why is my download slower than my plan speed?
Overhead, Wi-Fi loss, shared lines, and server caps all trim real speed. Expect about 10 to 25 percent less than the advertised rate. Wired Ethernet gets you closest to the maximum.
Why does my 1 TB drive show as 931 GB?
The maker counts in decimal, where 1 TB is 1,000,000,000,000 bytes. Windows counts in binary, where 1 GB is 1,073,741,824 bytes. The same space just gets a smaller label.
Does Wi-Fi or Ethernet transfer files faster?
Ethernet is faster and steadier for large transfers. A cable adds little overhead and ignores walls and interference. Use Ethernet for stationary devices when a transfer really matters.
Related tools and resources
These guides and calculators pair well with the math above. Each one solves a related networking question in plain language. Start with the bandwidth calculator to turn any file size into a transfer time.
- Bandwidth calculator: estimate any file in seconds.
- How to troubleshoot network connectivity: the full diagnostic walkthrough.
- Network tools: the full set of free checkers and calculators.
- IPv4 vs IPv6: how addressing affects your connection.
- How to read ping results: latency, packet loss, and timeouts.
References
The figures in this guide trace back to primary sources. NIST defines the binary prefixes that explain the 931 GB drive. RFC 9293 specifies the TCP header that adds overhead. Cloudflare covers how bandwidth is measured.