Download Time Calculator
Estimate how long a download or upload takes from file size and connection speed, allowing for the overhead that makes real transfers slower.
estimated download time
Two deductions, and most people know only one
The familiar one: speeds are quoted in bits, files are measured in bytes, so divide by eight. 100 Mbps is 12.5 MB/s. The second is that the line is not carrying only your file — every packet drags headers with it, and the gaps between packets are real time on the wire:
| Your file | 1460 bytes |
| TCP header | 20 |
| IPv4 header | 20 |
| Ethernet preamble and start delimiter | 8 |
| Ethernet header | 14 |
| Frame check sequence | 4 |
| Inter-frame gap | 12 |
| On the wire | 1538 bytes |
So 94.9% efficiency at best, and over IPv6 — whose header is 40 bytes rather than 20 — it drops to 93.6%.
| Line | Bits ÷ 8 | After framing |
|---|---|---|
| 50 Mbps | 6.25 MB/s | 5.93 MB/s |
| 100 Mbps | 12.5 MB/s | 11.87 MB/s |
| 500 Mbps | 62.5 MB/s | 59.33 MB/s |
| 1000 Mbps | 125 MB/s | 118.66 MB/s |
And that is the optimistic figure, because it assumes every packet is full. A 100-byte payload is only 56% efficient, so a chatty protocol spends nearly half the line on framing. None of it is waste exactly — the headers are how the packet knows where it is going, and the gap is how the receiver keeps sync — but a download running at 11.5 MB/s on a 100 Mbps line is running at essentially full speed.
How to use
- Enter the file size and its unit.
- Enter your connection speed in megabits or megabytes per second.
- Read the estimated time, both theoretical and realistic.
- Test your actual speed rather than trusting the advertised figure.
Frequently asked questions
How is download time calculated?
Divide the file size by the transfer rate, having first put both into the same units. The catch is that file sizes are in bytes and connection speeds are in bits, so the size must be multiplied by eight before dividing — miss that and the answer is eight times too optimistic.
Why is my download slower than this estimate?
Several reasons compound. Protocol overhead takes a few per cent, wifi rarely delivers its rated speed, the server may be limiting your rate, and other devices share your connection. A realistic expectation is 70 to 90 per cent of the theoretical figure on a wired connection, and often less over wifi.
Does upload speed differ from download?
On most home connections, substantially. Cable and DSL are asymmetric by design, often giving a tenth of the download speed for uploads. Fibre is frequently symmetric. This matters for video calls, cloud backup and sending large files, which all depend on the slower direction.
What speed do I actually need?
Streaming high definition takes about 5 Mbps per stream, and 4K around 25. Video calls need 2 to 4 Mbps. General browsing is comfortable at 25 Mbps for a household. Beyond that, more speed shortens large downloads but changes little else — latency matters more than bandwidth for how fast a site feels.
Why do big files sometimes start fast and slow down?
Often a write-cache effect: the receiving drive absorbs the first burst into fast memory, then settles to its sustained speed. It can also be the source server applying a rate limit after an initial allowance, which is common on free file-hosting services.
Does distance to the server matter?
For a single connection, yes. Higher latency limits how much data can be in flight at once, so a server on another continent can transfer more slowly than a nearby one on identical bandwidth. Downloading several files at once, or a protocol that opens parallel connections, works around it.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.