Why is my transfer time longer than expected?

ISP-advertised speeds are in megabits per second (Mbps), not megabytes (MBps). Because 1 byte = 8 bits, a 100 Mbps connection can transfer at most 12.5 MB per second. Real-world throughput is also reduced by protocol overhead, network congestion, and distance to the server.

What is the difference between bits and bytes?

A bit is the smallest unit of data (0 or 1). A byte is 8 bits. Network speeds are measured in bits per second (bps, Kbps, Mbps, Gbps), while file sizes are measured in bytes (KB, MB, GB). This calculator converts between them automatically.

What speeds are used in the comparison table?

Dial-up (56 Kbps), DSL (10 Mbps), 4G LTE (50 Mbps), Cable Broadband (200 Mbps), and Gigabit Fiber (1000 Mbps). These represent typical real-world speeds for each technology.

Does this account for overhead?

The calculator shows theoretical transfer time based on raw bandwidth. Real transfers are slower due to TCP/IP overhead, latency, server limits, and other factors. A practical rule of thumb is to multiply the result by 1.1 to 1.3.

Network Speed Calculator

このツールについて

Estimate how long it takes to transfer any file over any network connection. Enter the file size with its unit (bytes, KB, MB, GB, or TB) and the connection speed with its unit (Kbps, Mbps, or Gbps). The calculator instantly shows the transfer time in seconds, minutes, and hours, and explains the difference between bits and bytes. A reference table shows your file across five common connection speeds — Dial-up, DSL, 4G LTE, Cable Broadband, and Gigabit Fiber — so you can compare at a glance. Useful for planning backups, uploads, video streaming, and anything where bandwidth matters.

使い方

1 Enter the file size and select the appropriate unit (B, KB, MB, GB, or TB).
2 Enter the connection speed and select the unit (Kbps, Mbps, or Gbps).
3 Click 'Calculate' — the transfer time appears in seconds, minutes, and hours.
4 Check the comparison table to see how long the same file takes over Dial-up, DSL, 4G, Cable, and Fiber.

The formula behind transfer time

Estimating how long a download or upload takes comes down to one division: time = file size ÷ speed. The catch is that the two quantities are usually measured in incompatible units, so the calculation only works once everything is converted to a common base. This tool converts both sides to bits, divides, and reports the result in seconds, minutes, or hours, whichever reads most naturally. It runs entirely in your browser.

Bits versus bytes — the unit that trips everyone up

File sizes are quoted in bytes (KB, MB, GB), but network speeds are quoted in bits per second (Kbps, Mbps, Gbps). There are 8 bits in a byte, so the two are not interchangeable. A "100 Mbps" connection does not move 100 MB per second — it moves 100 ÷ 8 = 12.5 MB per second at best. The tool makes this explicit by showing the effective MB/s alongside every result, which is why a download always feels about eight times slower than the advertised number suggests.

One more subtlety: this calculator treats file-size units as binary multiples (1 KB = 1,024 bytes, 1 MB = 1,024 KB) while speed units are decimal (1 Mbps = 1,000,000 bits per second), matching how storage and bandwidth are conventionally labeled. That mismatch adds a few percent to real transfer times beyond the bits/bytes factor.

A worked example

Take a 100 MB file on a 100 Mbps connection. First convert the file: 100 MB × 8 × 1,024 × 1,024 = 838,860,800 bits. Then convert the speed: 100 Mbps × 1,000,000 = 100,000,000 bits per second. Divide: 838,860,800 ÷ 100,000,000 ≈ 8.39 seconds. The effective rate is 12.5 MB/s, so a casual estimate of "100 MB ÷ 12.5 MB/s = 8 seconds" lands close — the small extra comes from the binary-versus-decimal unit mismatch described above. Change either field and the result, the effective MB/s, and the comparison table all update instantly.

The comparison table

For whatever file size you enter, the tool also shows how long the same transfer would take across five reference connections, so you can see at a glance what your bandwidth buys you:

Connection	Speed	Effective rate
Dial-up	56 Kbps	~7 KB/s
DSL	10 Mbps	1.25 MB/s
4G LTE	50 Mbps	6.25 MB/s
Cable broadband	200 Mbps	25 MB/s
Gigabit fiber	1,000 Mbps	125 MB/s

That same 100 MB file takes over three hours on dial-up, about 17 seconds on 4G, and well under a second on gigabit fiber — a vivid illustration of why bandwidth matters for large downloads.

Common use cases

Sizing a backup or upload window. Before pushing a 50 GB archive to cloud storage, check whether it fits in an overnight window on your upload speed.
Planning a media download. Estimate how long a game, a video file, or an OS image will take so you know whether to wait or walk away.
Comparing plans. See concretely what upgrading from 100 Mbps to gigabit does to your real transfer times before paying for it.
Teaching the bits/bytes distinction — the side-by-side units make the eight-to-one relationship click.

Why real transfers are slower — and common mistakes

This calculator gives the theoretical best case: file size divided by raw link speed. Actual transfers are slower for reasons it cannot model, so treat the number as a floor, not a promise.

Protocol overhead. TCP/IP headers, handshakes, and acknowledgements consume part of the link; usable throughput is typically 85–95% of the raw rate.
The slowest link wins. Your transfer is capped by whichever segment is slowest — often the server's upload speed or a congested router, not your own connection.
Latency and small files. For many tiny files, round-trip delay dominates and the size-based estimate is wildly optimistic. The formula is most accurate for one large, continuous transfer.
Confusing your plan's units. A "1 Gbps" plan is 1,000 Mbps of bits — entering it as bytes will overstate your speed eightfold. Match the unit selector to how the figure was advertised.