Download Times Explained: Protocol Overhead, TCP Windowing, and Peering

1 Gbps = 125 MB/s. Every textbook says it. So does your ISP's invoice. The truth: in real life you'll see maybe 95 MB/s — and that's only for HTTP downloads of large files. SMB transfers over VPN shrink that to 25. rsync over a transatlantic link drops it to 5.

This article explains where the missing 90% of your bandwidth goes. With the bitcalc Download Time Calculator to get realistic numbers for your situation.

Layer 1 vs. Layer 7: why 1 Gbps isn't 1 Gbps

Your ISP sells Layer 1 bandwidth: raw bit rate on the wire. What you can use is Layer 4/7 throughput: what's left after all the protocol headers. Between them are several layers, each taking a cut.

TCP windowing: the silent killer

Max throughput = Window Size / RTT. 64K window at 10ms = 52 Mbps max. Window scaling (now standard) lets windows reach 1 GB — theoretically 800 Gbps at 10ms. But that only works for long uninterrupted streams. Synchronous I/O ("read this 4K block, then the next") collapses throughput regardless of window size.

The bitcalc Download Time Calculator

The Download Time Calculator is deliberately simple: file size and bandwidth in, time out. No overhead presets — because overhead factors vary so wildly by environment that any fixed factor would be wrong. Instead: multiply your bandwidth by the overhead factor from the table above and plug that into the calculator.

Peering: when the internet itself is the bottleneck

Your ISP sells you 1 Gbps — to their own network. Once your data leaves it, it enters the peering fabric. Provider A peers with Provider B over a single 10 Gbps link shared by 50,000 customers. Your 1 Gbps becomes 200 Mbps because the peering link is congested. That's why your speedtest shows 900 Mbps but streaming buffers — the speedtest server is inside your ISP's network. Netflix isn't.