Bandwidth Calculator

Bits, bytes, and why your download looks slow

Network speeds are quoted in bits per second; storage and files are measured in bytes. There are eight bits in a byte, so a 100 Mbps connection tops out at 12.5 MB/s before any overhead. This single factor of eight is responsible for most "my internet is slower than advertised" complaints. A lower-case b means bits, an upper-case B means bytes — Mbps and MB/s differ by a factor of eight even though they look almost identical in a browser tab.

The second unit trap: MB versus MiB

There are two conventions for the prefixes themselves. In the SI convention used by network and drive manufacturers, one megabyte is 10⁶ = 1,000,000 bytes. In the binary convention used by most operating systems when reporting file sizes, what is displayed as one megabyte is really a mebibyte: 2²⁰ = 1,048,576 bytes. The gap is 4.9% at mega and grows to 10% at tera — which is why a "1 TB" drive shows up as about 931 GB in a file manager. Nothing has been lost; the two systems are counting differently. This calculator offers both so you can match whichever source your number came from.

Overhead is real and unavoidable

Raw link speed is not usable throughput. TCP/IP headers, acknowledgements, retransmissions, encryption and framing all consume capacity, and a typical figure is 5 to 10% on a wired connection. Wi-Fi is far worse — shared airtime, collisions and protocol overhead often leave only half of the nominal rate available in practice, which is why a 300 Mbps Wi-Fi label rarely delivers more than 150 Mbps of real data. Server limits, congestion at peak times and the distance to the source can all constrain the transfer further.

Sharing one line between streams

Bandwidth divides among everything using the connection. Four people streaming at once each get roughly a quarter of the line, assuming nothing prioritizes one flow over another. The per-stream figure above is that simple division. In practice, real-time traffic such as video calls suffers far more from latency and jitter than from raw bandwidth — a call needs only a few megabits, but it needs them consistently, which is why a large download in the background can ruin a call on a line that is nominally fast enough for both.

Reading a data cap

Monthly caps are quoted in bytes, usually gigabytes. The last output line multiplies the transfer by 30 so you can see what a daily habit costs against a cap. Streaming video is the dominant consumer for most households: standard definition is roughly 1 GB per hour, high definition around 3 GB, and 4K can exceed 7 GB per hour, so a single evening of 4K viewing can consume more than a week of ordinary browsing.

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