File Transfer Time Calculator
The reason a download seems to take longer than your connection speed promises comes down to units: speeds are quoted in megabits per second while files are measured in megabytes, and a byte is eight bits. This calculator converts both to a common base, divides to find the ideal transfer time, and then lets you add an overhead percentage for the protocol headers, retransmissions, and shared bandwidth that slow real transfers. Enter your file size and connection speed to see the best-case and realistic durations.
File transfer time formula
file bits = size (GB) * 8,000,000,000
speed (bits/s) = speed (Mbps) * 1,000,000
ideal seconds = file bits / speed (bits/s)
with overhead = ideal seconds * (1 + overhead% / 100)
effective MB/s = speed (Mbps) / 8
Decimal units are used: 1 GB is 1,000,000,000 bytes and 1 Mbps is 1,000,000 bits per second. One byte is eight bits, so the file size in bytes is multiplied by eight to get bits.
Transfer speed facts
- A byte is eight bits, so a 100 Mbps line moves about 12.5 megabytes per second at best.
- Internet providers advertise speeds in megabits per second (Mbps), not megabytes.
- Real transfers run slower than the theoretical maximum because of overhead and congestion.
- Decimal gigabytes (1,000,000,000 bytes) are about 7 percent smaller than binary gibibytes.
- Upload speeds are often far lower than download speeds on consumer connections.
File transfer time: frequently asked questions
How do I calculate file transfer time?
Convert the file size to bits and the speed to bits per second, then divide. A file size in gigabytes times 8,000,000,000 gives bits (using decimal gigabytes). A speed in megabits per second times 1,000,000 gives bits per second. Time in seconds equals bits divided by bits per second.
Why is my download slower than my connection speed suggests?
Internet speeds are advertised in megabits per second (Mbps) while files are measured in megabytes (MB), and one byte is eight bits. Real transfers are also slower because of protocol overhead, server limits, and network congestion. This calculator gives the theoretical best case; add an overhead margin for reality.
What is the difference between megabits and megabytes?
A megabit is one million bits; a megabyte is one million bytes, and a byte is eight bits. So a megabyte is eight times larger than a megabit. Connection speeds use bits (Mbps) and file sizes use bytes (MB or GB), which is why a 100 Mbps line downloads at roughly 12.5 megabytes per second.
Should I use 1,000 or 1,024 for the conversions?
This calculator uses decimal units (1 GB equals 1,000,000,000 bytes), which matches how internet providers and storage makers advertise. Some operating systems report binary units (1 GiB equals 1,073,741,824 bytes). The difference is about 7 percent at the gigabyte scale.
Does this account for real-world overhead?
The base figure is the theoretical minimum at full speed. Provide an overhead percentage to model realistic conditions: protocol headers, retransmissions, and shared bandwidth typically add somewhere from 5 to 20 percent or more. The adjusted time multiplies the ideal time by one plus your overhead percentage.
Official sources
- U.S. Federal Communications Commission: Broadband Speed Guide.
- National Institute of Standards and Technology (NIST): Prefixes for binary multiples.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.