Bandwidth Usage Calculator

Bits and bytes: the factor of eight

Connection speeds are quoted in bits per second and file sizes in bytes. A 100 Mbps connection transfers at most about 12.5 MB per second, because there are eight bits in a byte. The lowercase b means bits and the uppercase B means bytes, a distinction that carries the entire difference and is routinely lost in writing. Almost every complaint that a download is slower than the advertised speed is this conversion.

Then subtract the overhead

Ethernet, IP and TCP headers, acknowledgements and retransmissions consume real capacity. Effective throughput on a healthy connection is typically 90 to 95 percent of the line rate, and lower on wireless or high-latency links. A 100 Mbps line realistically delivers 11 to 12 MB/s, not 12.5.

Latency limits a single transfer more than bandwidth does

TCP can only have so much data unacknowledged at once. Throughput on one connection is roughly the window size divided by the round-trip time, so a transatlantic link with 150 ms latency may deliver a fraction of its capacity to a single stream regardless of how much bandwidth exists. This is why parallel connections speed up large downloads and why adding bandwidth to a high-latency path often changes nothing.

Storage vendors and operating systems disagree by design

Drive manufacturers use decimal units where a terabyte is 10 to the 12th bytes. Windows reports in binary units while labelling them with decimal names, so the same drive shows as about 931 GB. Nothing is missing; two definitions are in use. macOS and most Linux tools report decimal, which is why the same drive appears larger there.

Estimating monthly usage

Streaming dominates household consumption: roughly 1 GB per hour for HD, 3 GB for 4K, and around 7 GB per hour for the highest-bitrate 4K. Video calls run 0.5 to 1.5 GB per hour. Browsing and email are negligible by comparison. A household that streams 4K for four hours a day will pass a terabyte a month without doing anything else, which is precisely where data caps start to bite.

Speed is shared, and the bottleneck moves

Quoted speed is what the line can carry, not what one device receives while others are active. And the slowest link governs: a gigabit connection behind a Wi-Fi access point delivering 200 Mbps to the far bedroom is a 200 Mbps connection there, which is why upgrading the plan often changes nothing.

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