Bandwidth Calculator

The one conversion that underpins everything is that a byte is eight bits. File sizes are quoted in bytes while connection speeds are almost always quoted in bits per second, so the calculator converts the file to bits before dividing by the speed. That single factor of eight is why a 100 Mbps line does not deliver 100 MB every second.

Units matter twice over. Decimal units treat a kilobyte as 1,000 bytes and a gigabyte as 1,000,000,000, which is how storage and network hardware are marketed. Binary units treat a KiB as 1,024 bytes and a GiB as 1,073,741,824, which is closer to how operating systems report sizes. Both sets are available so you can match whichever figure you are working from.

Overhead is applied as a multiplier on the payload. The slider sets a percentage from 0 to 20, defaulting to 5, and the total bits to move are multiplied by one plus that percentage before the division. In data mode the multiplier is applied in reverse, dividing the raw capacity to give the usable payload. The effective throughput figure shows the resulting real-world rate, which is always lower than the nominal line speed.

Choose transfer time mode, pick the HD Movie preset at 4 GB, set the speed to 100 Mbps and leave the overhead at 5 per cent.

Using decimal units, 4 GB is 4,000,000,000 bytes. Multiplied by eight, that is 32,000,000,000 bits. A 100 Mbps connection carries 100,000,000 bits per second, so without overhead the transfer would take 320 seconds.

Applying the 5 per cent allowance multiplies the payload by 1.05, giving 33,600,000,000 bits and a transfer time of 336 seconds, which the tool displays as 5 minutes and 36 seconds. Put another way, effective throughput is 100 divided by 1.05, or about 95.2 Mbps, which is roughly 11.9 MB per second.

Now switch to data mode with the same 100 Mbps line and a duration of one hour. The raw capacity is 100,000,000 bits multiplied by 3,600 seconds, which is 360,000,000,000 bits, or 45,000,000,000 bytes. Dividing by 1.05 for overhead leaves about 42.86 GB of usable payload in the hour, alongside the per-second, per-minute and per-day figures the tool lists.

Treat the result as a best case. The headline speed an ISP advertises is a maximum under good conditions, not a guarantee, and the figure you get on a speed test at midday may be well below it in the evening when the local segment is busy. The overhead slider covers protocol overhead from TCP and IP headers, acknowledgements and retransmissions, but it cannot model congestion.

Several other ceilings can bind before the line does. The server you are downloading from may cap per-connection speed or be throttled by its own load. Wi-Fi typically delivers a fraction of its rated speed once distance, walls and competing devices are accounted for, so a gigabit line reached over Wi-Fi rarely behaves like a gigabit. A slow hard disk, or antivirus scanning each file, can also become the bottleneck.

The most common misreading is confusing Mbps with MB per second. Dividing by eight is the first step, so 100 Mbps is 12.5 MB per second before overhead and about 11.9 MB per second after a 5 per cent allowance. If a download manager shows roughly a twelfth of your advertised number, the line is behaving normally rather than underperforming.