Bandwidth Calculator
Calculate how long a file transfer, download or backup will take, given a file size and connection speed.
How to use this calculator
- 1Enter the file size and its unit.
- 2Enter your connection speed as advertised by your ISP — this is almost always in Mbps.
How the calculation works
Transfer time = (File size in bytes × 8) ÷ Connection speed in bits per second- × 8
- Converts bytes (how file sizes are measured) to bits (how connection speeds are measured)
Marketing materials for internet plans always quote speed in bits per second (Mbps), while operating systems display file sizes in bytes (MB) — an 8x factor separates the two, which is why a "100 Mbps" connection downloads at a maximum of about 12.5 MB per second, not 100.
Worked example
5 GB file on a 100 Mbps connection
- 1.File size in bits: 5 GB × 1,000,000,000 bytes/GB × 8 bits/byte = 40,000,000,000 bits.
- 2.Speed: 100 Mbps = 100,000,000 bits/sec.
- 3.Time: 40,000,000,000 ÷ 100,000,000 = 400 seconds ≈ 6 min 40 sec.
Result: 400 sec (6 min 40 sec) at full rated speed
What a transfer-time estimate actually solves
Connection speeds are advertised in one unit and file sizes are almost always shown in another, so working out how long a transfer will take requires converting between the two before any division happens. This calculator exists mainly to make that conversion explicit rather than a source of confusion — plugging in a plan’s advertised speed and a file’s listed size gives a concrete time estimate instead of a vague sense that a "100 Mbps connection" should feel fast.
The bits-versus-bytes conversion at the heart of it
Internet connection speeds are marketed in bits per second — Mbps, Gbps — a convention that goes back to how telecommunications capacity has always been measured. File sizes and download progress, on the other hand, are almost always shown in bytes — MB, GB — because that is the unit operating systems use to measure storage. Since a byte is 8 bits, converting between the two means multiplying or dividing by 8, and skipping that step is the single most common reason someone expects a "100 Mbps" connection to download at 100 megabytes per second, when its actual ceiling is closer to 12.5 megabytes per second.
Where this estimate gets used
The same bits-to-bytes-over-time calculation applies anywhere data has to move across a connection:
- Estimating a download or backup — working out roughly how long a large file, game update or full system backup will take before starting it, so it can be scheduled around other work.
- Checking a streaming plan is fast enough — video services publish a minimum bitrate for each quality tier, and comparing that against a connection’s rated speed shows whether it has headroom to spare or is right at the edge.
- Cloud upload and sync planning — uploading a large dataset or media library to cloud storage is usually far slower than downloading the same amount, since most consumer connections have asymmetric upload and download speeds.
- Comparing ISP plans — translating an advertised Mbps figure into "how long would my typical file take" makes it easier to judge whether a faster, more expensive plan is actually worth it for how the connection gets used.
Why real transfers rarely hit the advertised number
The rated speed on a connection is close to a theoretical ceiling, not a guarantee, for several compounding reasons:
- Protocol overhead — data moving over the internet is wrapped in layers of packet headers, and that bookkeeping consumes some of the connection’s raw capacity before any of the actual file gets through.
- Wi-Fi versus wired — a wired connection can often get close to the rated speed, while Wi-Fi loses throughput to signal strength, interference from other networks, and distance from the router — sometimes substantially.
- Shared and contended capacity — other devices on the same network, other traffic at the ISP level, and the sending server’s own upload capacity can all become the actual bottleneck, regardless of how fast your own connection is rated.
- Latency isn’t throughput — a connection can have plenty of rated bandwidth but still feel slow for small, frequent requests because of latency — the round-trip delay per request — which bandwidth alone does not fix.
What this assumes, and where it stops
Assumptions
- Uses decimal (SI) units throughout — 1 GB = 1,000 MB and 1 Mbps = 1,000 Kbps — matching how ISPs advertise speed and how most file sizes are quoted online.
Limitations
- Does not model a connection that is shared with other simultaneous traffic, Wi-Fi signal quality, or server-side bottlenecks, all of which commonly reduce real-world transfer speed further than the 85% estimate used here.
Common questions
Why does my download take longer than this calculator predicts?
The "theoretical" figure assumes you get the full rated speed with zero overhead, which essentially never happens in practice — Wi-Fi signal strength, other devices sharing the connection, the source server's own upload speed, and general internet congestion all typically bring real-world throughput below the rated maximum, which the "realistic" estimate accounts for.
Why the difference between Mbps and MB/s?
Mbps is megabits per second — how internet speed is marketed. MB/s is megabytes per second — how file sizes and download progress bars are usually shown. Since a byte is 8 bits, a 100 Mbps connection has a theoretical maximum of 12.5 MB/s, which is a common point of confusion when a "100 Mbps" plan seems to download files "too slowly."
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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