Download Time Calculator
Estimate how long a download or upload takes from the file size and your connection speed, accounting for the bits-versus-bytes gap.
How to use this calculator
- 1Enter the file size and its unit.
- 2Enter your connection speed — usually quoted in Mbps.
- 3Optionally lower the efficiency to reflect real-world overhead.
- 4Read the transfer time at both your realistic and full advertised speed.
How it works
Transfer time
file (bits) = file (bytes) × 8 time = file (bits) ÷ speed (bits per second) 1 byte = 8 bits, so 100 Mbps ≈ 12.5 MB/s real speed = advertised speed × efficiency (~85%)
The calculation is a single division — file size divided by speed — but two unit traps make real downloads slower than expected. First, speeds are in bits while files are in bytes, and a byte is eight bits, so the headline megabits-per-second figure must be divided by eight to get megabytes per second. Second, no connection delivers its full advertised rate: protocol overhead, network congestion, and the server's own speed typically leave you with 80–90% of it.
Worked example
A 5 GB file on a 100 Mbps connection: 5 GB is 40 billion bits, and 100 Mbps moves 100 million bits per second, so at full speed the download takes 400 seconds — about 6 minutes 40 seconds. At a realistic 85% efficiency it's closer to 8 minutes.
Download Time Calculator: the complete guide
Bits, bytes, and the factor of eight
The most common surprise in download times comes from a units clash that the industry does little to clarify. Internet speeds are advertised in megabits per second (Mbps), with a lowercase 'b' for bits. File sizes are measured in megabytes (MB), with an uppercase 'B' for bytes. There are eight bits in a byte, so a 100 Mbps connection can download at most 12.5 megabytes per second — an eighth of the number that sounds like the speed.
This is why a '1 gigabit' connection does not download a 1 GB file in one second, but in about eight. The mismatch is not a scam exactly — bits are the natural unit for a data link, and bytes for a file — but it reliably makes people feel their connection is slower than promised. Keeping the b/B distinction straight is the single most useful thing to know about transfer speeds.
Why you never get the full advertised speed
Even after the bits-to-bytes conversion, real transfers fall short of the theoretical rate. Every network protocol carries overhead — headers, acknowledgements, error correction — that consume a slice of the bandwidth on housekeeping rather than your file. Congestion, Wi-Fi interference, and the distance to the server all take further bites.
The single biggest limiter is often the other end. A download can only go as fast as the slowest link in the chain, and that is frequently the server, not your connection — a busy website or an overloaded file host will throttle you far below your line speed. This is why the same broadband gives wildly different speeds on different sites, and why the 'efficiency' slider here matters: 85% is a fair everyday assumption, but a slow server can drop you far lower.
Decimal and binary units, one more wrinkle
There is a second units subtlety beneath the first. Networking uses decimal units — a megabit is exactly 1,000,000 bits — while operating systems traditionally report file sizes in binary units, where a 'megabyte' is 1,048,576 bytes. This calculator uses the decimal convention throughout, matching how speeds are defined, which is the right basis for transfer-time estimates.
The practical effect is small — a few percent — and it is dwarfed by the overhead discussed above, so it rarely changes the answer meaningfully. But it is why a downloaded file's size in your file manager may not exactly match the figure the download page quoted. For estimating how long something takes, the bits-versus-bytes factor of eight and the real-world efficiency are what actually matter; the decimal-versus-binary question is a rounding detail.
Frequently asked questions
Why is my download slower than my internet speed?
Mostly because of bits versus bytes. Speed is in megabits per second, file size in megabytes, and there are 8 bits per byte — so a 100 Mbps connection downloads at most 12.5 MB/s. Overhead and slow servers reduce it further, typically to 80–90% of the advertised rate.
How many megabytes per second is 100 Mbps?
12.5 MB/s at most. Divide the megabits figure by 8 to get megabytes, because a byte is 8 bits. So 100 Mbps ÷ 8 = 12.5 MB/s in theory, and a little less in practice after overhead.
How do I calculate download time?
Convert the file size to bits (bytes × 8) and divide by the connection speed in bits per second. A 5 GB file on 100 Mbps is 40 billion bits ÷ 100 million per second = 400 seconds. This calculator handles the unit conversions for you.
Does the server's speed matter?
Yes — often more than your connection. A download is only as fast as its slowest link, and a busy or throttled server can hold you well below your line speed. This is why the same broadband downloads quickly from one site and slowly from another.