Bandwidth Transfer Time Calculator | File Size and Speed to ETA
Drop in a file size and a connection speed to work out roughly how long a transfer will take, shown in seconds, minutes, and hours. An overhead slider trims the headline link rate down to a realistic effective speed.
💡 About this tool
If a 10 GB backup over a "100 Mbps" line takes far longer than you guessed, the culprit is usually a units mismatch. Link speed is quoted in megabits per second (Mbps), while file size is measured in bytes (GB). Since one byte is eight bits, 100 Mbps tops out around 12.5 MB/s — and protocol overhead shaves a few more percent off that.
This calculator handles the bit/byte conversion and the overhead haircut in one shot, then divides the total bits by the effective speed to give you an ETA. It is handy for sizing up cloud upload windows, planning a video delivery schedule, or estimating downtime for a server migration — any time you need to answer "how long to move this much data over this pipe?" before you start. The breakdown panel also shows total bits, effective speed, and the exact second count so you can sanity-check the result.
🧐 Frequently Asked Questions
Q. What is the difference between Mbps and MB/s? A. Mbps is megabits per second; MB/s is megabytes per second. With 8 bits to a byte, 100 Mbps equals about 12.5 MB/s. ISPs advertise in Mbps while files are sized in MB or GB, so mixing the two produces an eightfold error.
Q. What overhead value should I use? A. Overhead represents bandwidth lost to TCP/IP headers, Ethernet framing, and similar control data. On a typical wired link it runs about 5–15%; Wi-Fi or congested links lose more. If you are unsure, start at the default 10% and adjust.
Q. Why is my real download slower than this estimate? A. The tool assumes a steady link speed. In practice, server-side caps, network congestion, Wi-Fi signal strength, and distance all make throughput fluctuate. Entering your measured throughput instead of the advertised plan speed gives a closer estimate.
Q. Can this estimate the time to copy a file to a USB or external drive? A. No. This tool only models transfers over a network connection. A local copy between drives is limited by the disk read/write speed, not link speed, so this calculation does not apply.
Q. Does KB mean 1000 or 1024 here? A. For storage units the tool uses powers of two (1 KB = 1024 bytes), matching how operating systems report file sizes. For link speeds it uses powers of ten (1 Kbps = 1000 bps), matching networking convention.
📚 Why bits and bytes never line up
Networks have always measured throughput in bits per second because a link's smallest unit is the bit, while storage has long been counted in bytes. The transfer-time calculation is one of the few places these two worlds share a screen — and that is exactly where the confusion starts.
The word "kilo" makes it worse. Storage commonly treats 1 KB as 1024 bytes, while networking treats 1 Kbps as 1000 bps. To untangle this, the IEC defined the binary prefixes KiB and MiB (kibi, mebi) for the 1024-based values, but in everyday use KB and MB still cheerfully stand for both meanings.