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Download Time Calculator

How long a file takes to transfer, with the bits-and-bytes trap handled correctly.

Enter the file size and your connection speed. Watch the units — MB and Mbps differ by a factor of eight.

Protocol overhead, latency and shared bandwidth. Set to 0 for the theoretical best case.

Exact calculation. Given your inputs, this is the answer — no estimation involved.

How this calculator works

Divide the file size in bits by the connection speed in bits per second. The whole calculation turns on two distinctions that are routinely muddled, and getting either wrong changes the answer by a factor of eight or by 7%.

**Bits versus bytes.** Connection speeds are quoted in mega*bits* per second; file sizes are in mega*bytes*. There are 8 bits in a byte, so a 100 Mbps connection moves 12.5 MB per second, not 100. This is the single most common error in download-time estimates.

**Decimal versus binary prefixes.** A "GB" means 1,000,000,000 bytes to a drive manufacturer and to the SI standard, but Windows reports file sizes using 1,073,741,824 bytes and still labels it "GB". The correct name for the binary unit is GiB. Both are offered here, because the number on your screen depends on which program printed it.

Real transfers are slower than the arithmetic — protocol overhead, latency, server limits and shared bandwidth all take a cut — so the overhead allowance defaults to a realistic 8% rather than pretending a link runs at exactly its rated speed.

The formula

Exactly what happens to your numbers, step by step.

  1. The core calculation

    file bits = file size × bytes per unit × 8
seconds   = file bits ÷ bits per second

  2. With overhead

    effective speed = rated speed × (1 − overhead %)

  3. The units

    1 MB  = 1,000,000 bytes      (decimal, SI)
1 MiB = 1,048,576 bytes      (binary, what Windows calls "MB")
1 byte = 8 bits

A worked example

A 1 GB file on a 100 Mbps connection.

What you enter

File size
1 GB (decimal)
Speed
100 Mbps
Overhead
8%

The working

File in bits
1,000,000,000 × 8 = 8 gigabits
Best case
8,000 ÷ 100 = 80 seconds
Effective speed
92 Mbps
Realistic
87 seconds
The common wrong answer
10 seconds

About 1 minute 27 seconds

Dividing 1 GB by 100 Mbps directly gives 10 seconds, which is wrong by a factor of eight. That is the bits-and-bytes trap, and it is why speeds always sound faster than they feel.

Assumptions

Every result here rests on these. Change your inputs and the result changes with them.

  • The connection runs at its rated speed less the overhead you set.
  • Decimal units are powers of 1,000; binary units are powers of 1,024, per IEC 80000-13.
  • The transfer is not limited by the server, the disk or anything else at the far end.

What this cannot tell you

  • Real-world speeds vary constantly with congestion, Wi-Fi quality and the server you are downloading from.
  • Many downloads are limited by the source rather than your connection — a fast link does not help against a slow server.
  • Very small files are dominated by latency rather than bandwidth, so this over-predicts speed for them.

Questions people ask

Why is my download slower than my internet speed suggests?

Usually the bits-and-bytes confusion: 100 Mbps is 12.5 MB per second. Beyond that, protocol overhead, Wi-Fi, congestion and the server’s own limits all reduce real throughput.

What is the difference between Mbps and MB/s?

Mbps is megabits per second, MB/s is megabytes per second, and there are 8 bits to a byte. Divide Mbps by 8 to get MB/s: 100 Mbps is 12.5 MB/s.

Why does Windows show a different file size than the website did?

Because Windows uses binary units but labels them with decimal names. A file a site calls 1 GB (1,000,000,000 bytes) shows as 0.93 GB in Windows, which is really 0.93 GiB. Both describe the same file.

How long should a 4K movie take to download?

A 4K film is often 15 to 25 GB. At 100 Mbps that is roughly 20 to 35 minutes with realistic overhead. Enter your own figures above for a more precise answer.

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Sources and review

Sources

Methodology version 1.0.0 · Last reviewed