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How It Works

The underlying formula is simple: download time = file size ÷ effective bandwidth. The part that trips almost everyone up is units. Internet plans are advertised in megabits per second (Mbps) — a network/ISP speed unit — while file sizes are shown in megabytes (MB), and one byte equals eight bits. So before the division even happens, the advertised Mbps number has to be divided by 8 to get MB/s. A "100 Mbps" connection is not 100 MB/s — it's roughly 12.5 MB/s. A 10 GB file on that connection takes about 800 seconds (~13.3 minutes) at full theoretical speed, not the 100 seconds someone might guess by skipping the unit conversion.

This calculator accepts size in MB or GB and speed in either Mbps or MB/s, and handles the ÷8 conversion internally so you never have to do the mental math — and never fall into the "why is my fast internet downloading so slowly" trap that's really just a units mix-up.

What to Know

• The result is a theoretical ceiling, not a guarantee. Real-world throughput typically runs 5-15% under the advertised rate because of protocol overhead, Wi-Fi signal loss, and server-side limits.
• Wi-Fi introduces its own losses. A weak signal, distance from the router, or interference can cut real speed well below what a wired Ethernet connection would deliver on the same plan.
• Upload and download aren't fully independent. Acknowledgment packets for a download travel over the upload channel, so a saturated upload (a large cloud backup running in the background) can measurably slow a download on an asymmetric connection.
• Advertised speed is a maximum, not an average. ISPs typically market "up to X Mbps," and actual delivered speed can vary by time of day and network congestion.
• The same math applies at every scale. Whether it's a 5 MB PDF on a mobile connection or a 100 GB game library on fiber, the Mbps-to-MB/s conversion and the theoretical-vs-real gap work identically — only the numbers change.