This example uses 100 GB with the decimal GB definition and a sustained rate of 100 Mbit/s with the decimal mega prefix. It demonstrates ideal line-rate arithmetic, not an observed download or an ISP promise. DownloadTime does not test the connection, apply an overhead percentage, inspect a server, or add the duration to a device clock. Keeping both unit definitions explicit is essential: choosing 100 GiB instead would describe a larger quantity and produce a different duration even though the leading number remains 100.
Convert 100 GB to an exact number of bits
One decimal gigabyte is exactly 1,000,000,000 bytes. Therefore 100 GB is 100,000,000,000 bytes. One byte is exactly eight bits, so the transfer size is 800,000,000,000 bits. The entered rate is 100 Mbit/s, equal to 100,000,000 bit/s. Writing both quantities in bits removes the visual similarity between GB and Mbit/s and exposes the factor of eight. No intermediate value needs binary floating-point rounding: all multipliers are exact integers.
Divide total bits by sustained bits per second
The exact calculation is 800,000,000,000 bits ÷ 100,000,000 bit/s = 8,000 seconds. Because the quotient is already a whole number, conservative whole-second rounding does not change it. Convert 8,000 seconds by taking two complete hours, which use 7,200 seconds. The remaining 800 seconds contain 13 complete minutes, using 780 seconds, with 20 seconds left. The readable ideal duration is therefore 2 hours 13 minutes 20 seconds.
Compare the result only with the same stated units
If the source actually reports 100 GiB, the size is 107,374,182,400 bytes and the ideal duration at 100 Mbit/s is 8,589.934592 seconds, conservatively shown as 8,590 whole seconds. If a transfer application reports 100 MB/s rather than 100 Mbit/s, the rate is eight times the numeric bit rate. Those are different scenarios, not corrections to this one. Real progress may still take longer because the entered sustained rate may not be maintained. The example establishes a transparent mathematical baseline and nothing more.