How The Bandwidth Calculator Works
The calculator rearranges the single relationship time = size / speed in three directions so you can solve for any missing value. Internally, all file sizes are converted to bytes and all speeds to bits per second before doing the math, which automatically handles the most common mistake — forgetting to multiply by 8 when going from MB (megabytes) to Mbps (megabits per second). A 100 MB file on a 100 Mbps link is not "one second" — it's (100 × 8) / 100 = 8 seconds of raw transfer time, before any protocol overhead. The calculator shows both the derived result and the underlying megabits value so you can sanity-check the arithmetic.
Theoretical vs Actual Transfer Times
The calculator reports ideal transfer time at 100% line efficiency. Real-world transfers lose 5-15% to TCP/IP overhead (headers, acks, slow-start), and more if the link has high round-trip latency, packet loss, or encryption. A fair rule of thumb: multiply the calculator's time by 1.15 for HTTPS downloads over a well-provisioned link, 1.3-1.5 for VPN, and 2× or more over cellular or congested Wi-Fi. For LAN transfers (SMB over gigabit Ethernet, for instance) the calculator is close to reality — modern NIC drivers push 95-98% of line rate routinely. Cloud storage uploads from a home ISP are usually bottlenecked by the upstream link, not the ISP's download speed, which is why "gigabit" plans with 30 Mbps upstream feel slow when pushing backups.
Common Link Speeds And Their Real Transfer Rates
For quick reference, at 100% efficiency: 25 Mbps downloads ~3 MB/s (a 4 GB game takes ~22 minutes); 100 Mbps does ~12.5 MB/s (same game in ~5.5 minutes); 300 Mbps does ~37.5 MB/s (~1.8 minutes); 1 Gbps does ~125 MB/s (~33 seconds); 2.5 GbE does ~312 MB/s (~13 seconds); 10 GbE does ~1.25 GB/s (~3 seconds). Over real Internet paths, expect 85% of these numbers. If your ISP benchmark (like speedtest.net) reports less than 85% of the advertised speed during sustained transfer, either your local network is the bottleneck or the ISP is overprovisioned on peak.
Binary vs Decimal Units
This calculator uses binary units: 1 KB = 1024 bytes, 1 MB = 1,048,576 bytes, matching how Windows and most file managers display sizes. Storage manufacturers use decimal units (1 TB = 1,000,000,000,000 bytes), which is why a "1 TB" drive shows as roughly 931 GB formatted. For file transfer calculation, binary is the correct convention because the file size you see in your OS is binary. If you want to match a drive manufacturer's advertised capacity, mentally subtract ~7% at GB scale and ~10% at TB scale from what you see in Windows. Network bandwidth is always decimal (1 Mbps = 1,000,000 bits per second), and the calculator handles this correctly when mixing size and speed.
