Why your transfer takes 8x longer than it "should"
This is the single most common mistake in transfer-time estimates, and it isn't a rounding error — it's an 8× difference. Internet providers advertise speed in megabits per second (Mbps), but file sizes and transfer progress bars are almost always shown in megabytes (MB). There are 8 bits in a byte, so a "100 Mbps" connection actually moves data at about 12.5 MB per second, not 100. Dividing a 10 GB file directly by "100" gives 100 seconds — the wrong answer, off by a factor of 8. Dividing correctly (accounting for the bits-to-bytes conversion) gives the real answer: 800 seconds, or about 13 minutes and 20 seconds. This calculator handles that conversion automatically — just pick whichever unit matches what you actually have (Mbps for an advertised internet speed, MB/s for a transfer progress bar), and the math takes care of itself.
Decimal vs. binary: the other common discrepancy
A second, smaller source of confusion: a "gigabyte" can mean two slightly different things. Storage manufacturers and most networking contexts use the decimal definition (1 GB = 1,000³ bytes), while some operating systems have historically used the binary definition (1 GiB = 1,024³ bytes) while still labeling it "GB." The difference is about 7.4% at the gigabyte scale — not enormous, but enough to matter for a large transfer, and it compounds at larger scales (terabyte-scale transfers see the same percentage gap, just on bigger numbers). Use the Size Convention toggle above to match whichever your source is actually using.
Frequently asked questions
How long will it take to transfer 10 GB over a 100 Mbps connection?
About 13 minutes and 20 seconds (800 seconds total). 100 Mbps equals 12.5 MB/s once you convert from bits to bytes, and 10 GB (10,000,000,000 bytes) divided by 12,500,000 bytes per second gives 800 seconds.
What's the difference between Mbps and MB/s?
Mbps is megabits per second (how ISPs advertise internet speed); MB/s is megabytes per second (how file transfer progress is usually shown). There are 8 bits in a byte, so a given Mbps number is 8 times larger than the equivalent MB/s number — a "100 Mbps" connection is a 12.5 MB/s connection, not a 100 MB/s one.
How much data can I transfer in a given time?
Use the Calculate File Size tab: enter your transfer speed and how much time you have, and it works out the maximum file size that fits. A 100 Mbps connection over 15 minutes can move about 11.25 GB.