CalcOpenly

Download time calculator

Calculate how long a file takes to download or upload from its size in MB, GB or TB and your internet speed in Mbps, allowing for protocol overhead.

Updated Checked against 5 worked examples

%
TCP/IPv4 over Ethernet carries 1,460 bytes of data in every 1,538 bytes sent, about 5 % overhead. Use 0 for the raw line rate.
Try
Download time
1 h 10 min 10 s
Download time: 1 h 10 min 10 s
Time in seconds
4,210.53s
Effective speed
95Mbit/s / 11.875MB/s
Real downloads also depend on the server, Wi-Fi signal and other traffic, so actual times are often longer.

At 100 Mbit/s with 5% overhead, the useful rate is 11.88 MB/s, so 50 GB takes 1 h 10 min 10 s.

Same file at common connection speeds

Your speed (100 Mbit/s)1 h 10 min 10 s10 Mbit/s11 h 41 min 45 s50 Mbit/s2 h 20 min 21 s300 Mbit/s23 min 23 s1000 Mbit/s7 min 1 s
How it's calculated S

  1. Size in bits

    50,000,000,000 bytes×8=400,000,000,000 bits50{,}000{,}000{,}000\ \text{bytes} \times 8 = 400{,}000{,}000{,}000\ \text{bits}

    1 byte = 8 bits. GB means 10⁹ bytes; GiB means 2³⁰ bytes.

  2. Speed after overhead

    100,000,000 bit/s×(1−5%)=95,000,000 bit/s100{,}000{,}000\ \text{bit/s} \times (1 - 5\%) = 95{,}000{,}000\ \text{bit/s}

  3. Time

    t=400,000,000,00095,000,000=4,210.526316 st = \frac{400{,}000{,}000{,}000}{95{,}000{,}000} = 4{,}210.526316\ \text{s}

    That is 1 h 10 min 10 s.

About the download time calculator

Download time is file size divided by the useful data rate. The calculator turns the size into bits (1 byte = 8 bits; GB means 10⁹ bytes and GiB means 2³⁰ bytes), takes the connection speed in bits per second, and removes the share of each packet taken by protocol headers: time = bits ÷ (speed × (1 − overhead)).

People use it before downloading a game or a cloud backup, and to judge whether a faster plan is worth paying for. The default, a 50 GB file on a 100 Mbit/s line with 5% overhead, takes 1 h 10 min 10 s; at 1,000 Mbit/s the same file takes about 7 minutes.

The result is a best case. The 5% default is the framing cost of TCP/IPv4 over Ethernet; the server's own limit, Wi-Fi signal and other traffic are not modelled. For an upload, enter your upload speed, which on asymmetric lines such as ADSL is lower than the download speed.

Worked examples

50 GB at 100 Mbit/s with 5 % overhead

File size
50 GB
Connection speed
100 Mbit/s
Protocol overhead
5%
Time in seconds
4,210.53 s
Effective speed
95 Mbit/s

Checked against: 50 × 8 × 10⁹ bits ÷ (100 × 10⁶ × 0.95) bit/s = 4210.526… s (Python fractions)

1 GB at 100 Mbit/s, no overhead

File size
1 GB
Connection speed
100 Mbit/s
Protocol overhead
0%
Download time
1 min 20 s
Time in seconds
80 s

Checked against: 8 × 10⁹ / 10⁸ = 80 s

1 GiB at 1 Gbit/s (binary size, decimal speed)

File size
1 GiB
Connection speed
1 Gbit/s
Protocol overhead
0%
Time in seconds
8.58993 s

Checked against: 2³⁰ × 8 bits / 10⁹ bit/s = 8.589934592 s

4.7 GB DVD image at 25 Mbit/s with 10 % overhead

File size
4.7 GB
Connection speed
25 Mbit/s
Protocol overhead
10%
Time in seconds
1,671.11 s

Checked against: 4.7 × 8 × 10⁹ / (25 × 10⁶ × 0.9) = 1671.11 s (Python fractions)

Questions

How long does it take to download 1 GB at 100 Mbps?

About 84 seconds with 5% protocol overhead, or 80 seconds at the raw line rate. 1 GB is 8,000 megabits, and 8,000 ÷ 100 = 80 s. Halving the speed doubles the time: 160 s raw at 50 Mbps. At 1 Gbps the same gigabyte takes 8 s raw, about 8.4 s with overhead.

What is the difference between Mbps and MB/s?

Mbps (megabits per second) is how connection speeds are sold; MB/s (megabytes per second) is how browsers and download managers show progress. A byte is 8 bits, so divide Mbps by 8: a 100 Mbps line moves at most 12.5 MB/s, and about 11.9 MB/s after 5% overhead. Both use decimal prefixes, so 1 Mbit = 10⁶ bits (NIST).

Why is my download slower than my internet speed?

Part of every packet is headers, not your file. With a 1,500-byte Ethernet MTU, TCP/IPv4 carries 1,460 bytes of data in every 1,538 bytes on the wire (IEEE 802.3 framing plus 20-byte IP and TCP headers), a 5.1% loss; the 12-byte TCP timestamp option cuts the data to 1,448 bytes, 5.9%. Beyond that, the server's upload limit, Wi-Fi signal and other devices sharing the line slow it further.

Why does a 50 GB download show as 46.6 GB on my computer?

Because Windows File Explorer counts in binary units but labels them GB: it divides by 2³⁰ (1,073,741,824) bytes, so 50 × 10⁹ bytes appears as 46.6 GB. Stores and ISPs use decimal gigabytes (10⁹ bytes, IEC 80000-13), and macOS has shown decimal sizes since Mac OS X 10.6 in 2009. Choose GiB in the size field for a size copied from Windows.

How accurate is the download time calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “50 GB at 100 Mbit/s with 5 % overhead” is checked against 50 × 8 × 10⁹ bits ÷ (100 × 10⁶ × 0.95) bit/s = 4210.526… s (Python fractions).

Where does the method come from?

NIST — Prefixes for binary multiples (IEC 80000-13): 1 byte = 8 bits, GB vs GiB; IEEE 802.3 Ethernet framing (1,500-byte MTU + 38 bytes of preamble, header, FCS and gap) with RFC 791 / RFC 793 20-byte IPv4 and TCP headers.

About this calculator

t=size (bits)speed (bit/s)×(1−overhead)t = \frac{\text{size (bits)}}{\text{speed (bit/s)} \times (1 - \text{overhead})}

Sources

  1. NIST — Prefixes for binary multiples (IEC 80000-13): 1 byte = 8 bits, GB vs GiB
  2. IEEE 802.3 Ethernet framing (1,500-byte MTU + 38 bytes of preamble, header, FCS and gap) with RFC 791 / RFC 793 20-byte IPv4 and TCP headers

Checked against references

5 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.

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