What bandwidth actually measures
Bandwidth is the ceiling on how much data a connection can carry per second — think of it as the width of a pipe, not the pressure pushing water through it. A 100 Mbps connection can, in theory, move 100 megabits of data every second. Whether it actually does depends on the server on the other end, how many devices are sharing your Wi-Fi, and the protocol wrapping your data.
This tool covers the six questions people usually have about that pipe: how long a transfer will take, how much bandwidth you'd need to hit a target time, how big a file you could move in a given window, what speed you'll actually get once overhead is factored in, how today's common connection types compare, and how much bandwidth a given use case really calls for.
Bits, bytes, and why the math never lines up at first glance
Here's the mix-up that catches almost everyone at least once: internet plans are sold in megabits per second (Mbps), but file sizes are shown in megabytes (MB). One byte is 8 bits, so a "100 Mbps" connection moves a maximum of 12.5 megabytes per second, not 100. Forget that, and every transfer estimate you make by eye will be off by roughly 8x.
There's a second, quieter mismatch buried in file sizes themselves. This calculator treats KB, MB, GB, and TB the way operating systems typically do — as binary units where 1 KB = 1,024 bytes — while bandwidth units (Kbps, Mbps, Gbps) follow the decimal convention networking equipment uses, where 1 Kbps = 1,000 bps. Both conventions are standard within their own domain; the trick is not to mix them up when converting by hand.
| Unit | Equals |
|---|---|
| 1 byte | 8 bits |
| 1 KB | 1,024 bytes |
| 1 MB | 1,024 KB |
| 1 GB | 1,024 MB |
| 1 Kbps | 1,000 bps |
How to work out download time by hand
The core formula is just division, provided you convert both sides to the same unit first — bits, since that's what bandwidth is quoted in:
Transfer time = File size (bits) ÷ Bandwidth (bits per second)
Take a 100 MB file over a 100 Mbps connection. Convert the file to bits: 100 MB × 8 = 800 megabits. Divide by the 100 Mbps line speed: 800 ÷ 100 = 8 seconds. That's the theoretical best case — before any protocol overhead is subtracted, which is a separate step covered below.
Flip the same formula around and you get the other two calculation types on this page: divide file size by a target time to find the bandwidth required, or multiply bandwidth by time to find how much data fits in a given window.
Real-world speed: why protocol overhead matters
No connection delivers its full rated bandwidth as usable data. Every protocol spends part of each packet on headers, checksums, and acknowledgements that keep the transfer reliable rather than actually carrying your file. That's the gap between the number on your internet plan and the number you see in a download progress bar.
| Protocol | Typical overhead |
|---|---|
| UDP | ~5% |
| FTP | ~8% |
| TCP/IP | ~10% |
| HTTP | ~12% |
| HTTPS | ~15% |
That 15% for HTTPS is the encryption tax — the extra handshake and encryption layer that keeps your traffic private also adds a bit more overhead than plain HTTP. Run the same 100 MB / 100 Mbps example through HTTPS and the effective speed drops to 85 Mbps, stretching the 8-second ideal to a little over 9.4 seconds. It's a modest difference on one file, but it adds up on large transfers.
How common connection types compare
The comparison mode runs your file size through eight common connection speeds side by side, so you can see the gap in concrete seconds and minutes rather than just the abstract Mbps numbers:
| Connection | Rated speed |
|---|---|
| Dial-up | 56 Kbps |
| DSL | 8 Mbps |
| Cable | 100 Mbps |
| 4G LTE | 50 Mbps |
| Fiber | 500 Mbps |
| Gigabit / 5G | 1,000 Mbps |
| 10 Gig | 10,000 Mbps |
A 1 GB file that takes over a day and a half on Dial-up clears in under a second on a 10 Gig line — the same file, a nearly 180,000x difference in wait time.
How much bandwidth do you actually need?
Rather than guessing, the recommendation mode starts from a baseline Mbps figure per person per activity, multiplies it by how many people and concurrent streams you have, then adds 20% headroom so ordinary background traffic — updates, notifications, a second tab — doesn't push you right to the edge:
| Use case | Baseline (per person, per stream) |
|---|---|
| 1 Mbps | |
| Web browsing | 2 Mbps |
| Video calls | 4 Mbps |
| HD streaming / online gaming / general use | 5 Mbps |
| Remote work | 10 Mbps |
| 4K streaming | 25 Mbps |
A household with 4 people each streaming HD video at once needs roughly 5 × 4 × 1.2 = 24 Mbps to stay comfortable — which rules out DSL and points toward Cable or Fiber. If you're setting up a home network and want to check the addressing side of things too, the IP Subnet Calculator covers that.
Sources and further reading
This page is for general reference only. Real transfer speeds depend on your specific network, hardware, and service provider — always test with your own connection for anything mission-critical.