Enter a file size and a connection speed. It converts between bits and bytes properly and gives both the theoretical time and a realistic one.
This is the single reason download times confuse people. Internet connections are sold in bits per second. Files are measured in bytes. There are eight bits in a byte, so a 100 Mbps connection delivers at most 12.5 MB per second — an eighth of the number on the marketing material.
The notation is a lowercase b for bits and an uppercase B for bytes, which is a distinction one careless font choice away from invisible. Mbps means megabits per second; MB/s means megabytes per second. A "1 Gbps" fibre line moves 125 MB/s at absolute best.
Storage manufacturers and network engineers use the decimal definition: 1 GB = 1,000,000,000 bytes. Operating systems have historically used the binary one: 1 GiB = 1,073,741,824 bytes, about 7.4% larger. That is why a "1 TB" drive shows up as roughly 931 GB in Windows — nothing has been stolen, the two are counting in different bases. This calculator lets you pick either, because which one is right depends entirely on where your number came from.
A 5 GB file on a 100 Mbps connection. Five gigabytes is 5 × 10⁹ bytes, which is 40 × 10⁹ bits. Divided by 100 × 10⁶ bits per second, that is 400 seconds — 6 minutes 40 seconds.
You will not get 400 seconds. Realistically, expect something closer to eight minutes twenty, and the calculator shows that figure alongside.
Every byte of your file travels wrapped in protocol headers. TCP/IP overhead alone is roughly 3 to 5%. On top of that:
TCP behaviour. A connection does not start at full speed — slow start ramps up over the first seconds, which matters a lot for small files and little for large ones. Packet loss triggers retransmission and a congestion-window reduction, and a single lossy link can halve throughput.
Latency. On a high-bandwidth, high-latency path — a satellite link, or a transfer between continents — the window size can cap throughput well below the line rate regardless of how much bandwidth is available.
The far end. Your connection speed is a ceiling, not a promise. If the server is rate-limiting, overloaded, or simply far away, that is your actual speed. Downloading from a well-provisioned CDN and downloading from someone’s home NAS are different experiences on the identical connection.
Sharing. "Up to 100 Mbps" is contended — with your household, and with your neighbourhood at peak hours. Wi-Fi adds its own losses, and a device two walls away from the router may be the real bottleneck rather than the line.
The 80% figure used here is a reasonable rule of thumb for a healthy wired connection to a good server. A congested Wi-Fi link at 8 pm can be far worse.
| Content | Typical size | At 100 Mbps |
|---|---|---|
| Photo (phone, JPEG) | 3–5 MB | Under a second |
| MP3 album | 80–120 MB | About 8 seconds |
| Hour of 1080p streaming | ~3 GB | 4 minutes |
| Hour of 4K streaming | ~7 GB | 9–12 minutes |
| Feature film, 4K download | 15–25 GB | 20–33 minutes |
| Modern game install | 50–150 GB | 1–3 hours |
| Full system backup | 500 GB–2 TB | 11–45 hours |
The last row is why physically shipping a drive is still, occasionally, the fastest way to move data. A courier carrying 20 TB overnight beats almost any consumer connection.
For streaming, what matters is sustained rate, not total size. Roughly 5 Mbps per 1080p stream and 25 Mbps per 4K stream. A household running two 4K streams, a video call and background updates wants perhaps 80 to 100 Mbps to be comfortable. Beyond that, more bandwidth stops improving anything you can perceive — latency and Wi-Fi quality become the limiting factors, and a faster tier will not fix either.
Most often the bit-versus-byte confusion: a 100 Mbps line delivers 12.5 MB/s at best, and download managers report in MB/s. Beyond that, protocol overhead takes 3 to 5%, the server on the other end may be limiting or overloaded, Wi-Fi loses more than a cable does, and residential lines are contended at peak hours. Getting 70 to 85% of the advertised rate on a wired connection to a good server is normal and healthy.
MB is decimal — 1,000,000 bytes. MiB is binary — 1,048,576 bytes, about 4.9% more. The gap widens with scale: a GiB is 7.4% larger than a GB, and a TiB is 10% larger than a TB. Drive manufacturers and network engineers use decimal; Windows reports binary while labelling it "GB", which is why a 1 TB drive shows as 931 GB. macOS and most Linux tools report decimal, so the same drive looks like 1 TB there.
Roughly 3 to 5 Mbps for 1080p and 15 to 25 Mbps for 4K per simultaneous stream. Add headroom for video calls, cloud backups and automatic updates running in the background. A family of four streaming, working and gaming is comfortable at 100 Mbps. Past a few hundred Mbps, extra bandwidth stops making anything feel faster — latency, Wi-Fi coverage and the far end become the constraints.
More than most people realise, and many connections are heavily asymmetric — 500 Mbps down and 20 Mbps up is a common shape. Upload governs video call quality, cloud backup duration, sending large files and hosting anything. It also matters indirectly for downloads, because TCP acknowledgements travel upstream and a saturated upload link can throttle downloads badly.
Your connection is a ceiling, not a guarantee — the actual rate is whatever the slowest link in the path allows. A large site serving from a CDN edge node in your city can saturate your line; a small server on a modest connection thousands of kilometres away cannot, no matter what you are paying for. Distance also raises latency, which caps TCP throughput independently of available bandwidth.
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