How to use the Download Time Calculator
- Enter the file size and choose megabytes, gigabytes or terabytes.
- Enter your connection speed and pick the unit your provider quotes, usually megabits per second.
- Read the theoretical time at full line rate.
- Use the realistic time, which allows for protocol overhead and typical real-world throughput.
- Compare the effective megabytes-per-second figure with what your download manager reports.
How the calculation works
The core arithmetic is a unit conversion. File sizes are quoted in bytes, connection speeds in bits per second, and there are eight bits in a byte, so a 100 Mbps line moves at most 12.5 MB/s. That factor of eight is the single most common source of the complaint that a '100 megabit' connection only downloads at 12 megabytes a second. It is working exactly as specified.
Real transfers never hit line rate. TCP/IP headers consume a few percent, TCP slow start means the first seconds run below capacity, and the server, the path and your own Wi-Fi link all impose their own ceilings. The realistic estimate here assumes 85% of rated throughput, which matches typical wired broadband; Wi-Fi, especially on 2.4 GHz or at range, can fall well below that.
This calculator uses decimal units throughout. 1 GB is 10⁹ bytes, which is how storage vendors and ISPs quote figures. Operating systems often report binary units, where a gibibyte is 2³⁰ bytes, about 7.4% larger. That mismatch is why a '1 TB' drive shows as 931 GB in Windows, and why a download's reported size can differ slightly from the vendor's stated size.
seconds = (bytes × 8) ÷ bits per second; realistic = seconds ÷ 0.85Source: IEEE 802.3 Ethernet standard on bit-rate notation; IEC 80000-13 for binary versus decimal prefixes; RFC 5681 on TCP congestion control and slow start.
Worked example
A 25 GB game download on a 100 Mbps connection.
- 25 GB = 25 × 10⁹ bytes = 2 × 10¹¹ bits.
- Theoretical time = 2 × 10¹¹ ÷ 10⁸ = 2,000 seconds, or 33 minutes 20 seconds.
- At 85% throughput = 2,353 seconds.
About 33 minutes in theory, closer to 39 minutes in practice.
Frequently asked questions
Why is my download slower than my speed test?+
Speed tests use nearby optimised servers. A real download is limited by the source server, the route and your local Wi-Fi as well as your line.
Is Mbps the same as MBps?+
No. Mbps is megabits per second; MBps is megabytes, eight times larger. ISPs quote the smaller unit.
Does uploading use the same maths?+
Yes, but substitute your upload speed. Most consumer connections are asymmetric with far slower upload.
Why does my 1 TB drive show 931 GB?+
Vendors count 10¹² bytes; the operating system displays binary tebibytes of 2⁴⁰ bytes. The data is all there.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.