A 300 Mbps plan does not download 300 megabytes a second. It downloads about 36, because the plan is sold in bits and the file is sized in bytes, and a little of every packet is spent on the packet itself. This download time calculator does that arithmetic and shows each line of it.
Put in the size of the download and your connection speed. It works out how long the download takes, the speed you would need to finish it in the time you have, and the speed a download manager would show. Every figure it supplies names the standard it comes from, and the list of what it cannot tell you is below the answer.
In this guide
The download time calculator
The size the store or the website lists. 1 TB is 1000 here and 500 MB is 0.5.
Windows calls 2^30 bytes a GB, which is 7.4% more than the decimal GB a store means.
The plan's figure in megabits a second, or a speed test run on the device doing the download.
A plan or a link speed counts every bit on the wire; a speed test counts only the data that arrives, so the overhead is already out of it.
Lower it if others are streaming or downloading at the same time, or if the server sends slower than your line.
For the speed you would need: the time you want the download done in.
How that is worked out, line by line
- The download in megabits800,000 Mb
- Share of the line left for the file94.9 %
- Speed left for the file284.8 Mbps
- The same speed in megabytes a second35.6 MB/s
- The same time in hours0.78 h
On those numbers you are in the 10 minutes to an hour band, which is the section below.
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Where every number here comes from
No part of this was measured on a bench here. This site works from published specifications and from what owners report, and every number below is either one you typed in or a published figure with its source named next to it.
The figures this page supplies
- Bits in a byte: 8. IEC 80000-13 defines the byte as eight bits. Connection speeds are sold in bits a second and files are sized in bytes, which is the eightfold gap between the two.
- Megabytes in a decimal gigabyte: 1,000. The SI prefixes, as NIST publishes them: giga is 10^9 and mega is 10^6, so a gigabyte as a store counts it is 1000 megabytes. Mbps uses the same decimal mega.
- File data in one full packet: 1,460 bytes. RFC 6691: the most data a TCP segment carries is the 1500-byte Ethernet MTU less the fixed 20-byte IPv4 header and 20-byte TCP header, so 1460 bytes.
- What that packet takes on the wire: 1,538 bytes. IEEE 802.3: the 1500-byte payload plus a 14-byte Ethernet header and 4-byte checksum makes a 1518-byte frame, and the 8-byte preamble and the 12-byte gap between frames take the line time of 1538 bytes.
Why each step is the step it is
- The download in megabits:
size_gb * gb_unit * mb_per_gb * bits_per_byte. The size in decimal megabytes times eight, so that it is counted in the same unit as the connection speed. - Share of the line left for the file:
100 - speed_src * (100 - tcp_payload / wire_bytes * 100). 1460 bytes of file in every 1538 on the wire is 94.9%. That is the best case for a full-size packet over Ethernet; a speed test already reports what is left, so it takes 100%. - Speed left for the file:
speed_mbps * share_pct / 100 * eff_pct / 100. Your speed, times the share of it this download gets, times the share left once the packets' own headers are paid for. - The same speed in megabytes a second:
file_mbps / bits_per_byte. Megabits divided by eight. This is the figure a download manager that counts in bytes would show, and why a 300 Mbps plan never shows 300 there. - Minutes to download:
time_s / 60. The seconds above, divided by 60. - The same time in hours:
time_s / 3600. The seconds above, divided by 3600. - Speed needed to finish in your time:
size_mbit / (want_min * 60) / (share_pct / 100 * eff_pct / 100). The megabits divided by the seconds you allowed, then grossed up for the share this download gets and the packets' overhead, so it is a figure of the same kind as the speed you typed.
What this cannot tell you
Every calculator has an edge, and a page that hides its edge is worse than no page at all. Here is this one's.
- It is arithmetic on the figures you give it. No download was timed for it, and this site has not run these connections itself.
- The 94.9% is the best case for TCP over Ethernet. Wi-Fi spends more of its airtime on its own overhead, and a weak signal far more, so on Wi-Fi a speed test on the same device is the figure to use.
- A download is only as fast as the slowest link in the chain: the server, the network between, your router, the Wi-Fi and the drive it is written to. This cannot see any of them, so set the share box if you know one is slower.
- A game launcher often downloads compressed files and then unpacks and installs them, which takes time on top of the download that is not counted here.
- Providers often deliver a little more or less than the plan figure, and speeds at busy evening hours can drop. A speed test at the hour you download is closer than the plan's figure.
- It times downloads. An upload, such as a backup to the cloud or a video sent to a site, runs at the plan's upload speed, which on many plans is a small fraction of the download speed; put that figure in the speed box to time one the same way.
What that wait means for your setup
These follow from the answer above and nothing else. Each heading below covers one span of results, so read the one the calculator pointed you at and ignore the rest.
Ten minutes or less
Your connection is not what is holding you back on a download this size. If a launcher still takes longer, look at unpacking and installing, which happen after the download and are not counted here.
HDD or SSD for Games and Backups Worth Buying 2026 covers this range in full.
10 minutes to an hour
A wait you can plan around. If the speed you typed is the plan's and the device is on Wi-Fi, run a speed test on that device: a weak signal is one common reason a download runs slower than the plan, and a cable or a better-placed router fixes it without a faster plan.
How to Optimize Your WiFi in 2026, 4 Fixes Worth Doing covers this range in full.
Wireless vs Wired Network 2026: The Best Setup Is Both covers this range in full.
1 to 8 hours
Start it overnight. If a speed test on the device comes in well under the plan, the bottleneck is inside the house, and the router guides below cover that. If the speed test matches the plan, only a faster plan shortens the wait.
Best Gaming Router Under $200, 5 Honest Picks for 2026 covers this range in full.
Best Wi-Fi 7 Mesh System 2026: 3 Picks Worth the Money covers this range in full.
Mesh WiFi System vs Regular Router 2026: Is Mesh Worth It covers this range in full.
More than 8 hours
The line itself is the limit at this size, and no router makes a line faster than its plan. Download overnight across more than one night, or check whether your provider offers a faster tier; the speed needed figure above says what tier would do it in your time.
Wireless vs Wired Network 2026: The Best Setup Is Both covers this range in full.
If the download is a game, the storage calculator works out whether the drive has room for it once it is installed. A download time calculator can only be as right as the speed you give it, so a speed test on the device doing the download beats the number on the plan. The speed needed line works the other way round, for when you already know how long you have and want to know which plan would do it.
Questions about the download time calculator
How long does it take to download 100 GB?
Turn the size into megabits and divide by the speed that reaches the file. 100 GB is 800,000 megabits. On a 300 Mbps plan, 94.9% of the line carries file data once the packets' own headers are paid for, which leaves 284.8 Mbps, so this download time calculator gives 46.8 minutes. On a 1000 Mbps plan the same download takes 14 minutes, and on 50 Mbps it takes 4.68 hours.
What download speed do I need?
Divide the size in megabits by the seconds you have, then allow for the overhead. To finish 100 GB in 30 minutes takes 468.2 Mbps of plan speed. To finish it in 10 minutes takes 1,404.6 Mbps, and overnight, in 480 minutes, 29.3 Mbps does it. Run the download speed calculator above with your own size and time; if others share the connection while it runs, lower the share box and the speed needed goes up.
Why is my download slower than my internet speed?
Mostly because the two are counted in different units. A plan is sold in megabits a second and a download manager that counts in bytes shows megabytes a second, which is eight times smaller. A 300 Mbps plan leaves 284.8 Mbps for the file, which shows as 35.6 MB/s. The rest of the gap is the packets' headers: RFC 6691 and IEEE 802.3 put 1,460 bytes of file in every 1,538 on the wire. Anything slower than that is the Wi-Fi, the server or the share others are using, which the download time calculator cannot see.
Should I use my plan speed or a speed test?
A speed test on the device doing the download, when you can. A speed test times the data that arrives, so the packets' overhead is already out of its figure; the plan figure counts every bit on the line. A speed test of 300 Mbps gives 44.4 minutes for 100 GB, against 46.8 minutes from a 300 Mbps plan figure. On Wi-Fi the difference is usually larger, because the radio link has its own overhead and a weak signal slows it further.
Does a size from File Explorer take longer to download?
Yes, by about 7%, because Windows counts a GB as 2^30 bytes where a store counts 10^9. 100 GB as File Explorer shows it is 858,993 megabits against 800,000, and on a 300 Mbps plan it takes 50.3 minutes against 46.8. Pick how the size was counted in the second box and the download time calculator does the conversion.
Sources
- RFC 6691: TCP options and maximum segment size: Says the most data in one TCP segment is the MTU less only the fixed IP and TCP headers, which on Ethernet's 1500 bytes is 1460.
- IEEE 802.3-2022: IEEE Standard for Ethernet: The Ethernet standard, which sets the frame's header, checksum, preamble and the minimum gap between frames that make up the 1538 bytes of line time.
- NIST: metric (SI) prefixes: Lists mega as 10^6 and giga as 10^9, the decimal meaning of the MB and GB a store lists and the M in Mbps.
- IEC 80000-13: quantities and units, information science and technology: Defines the byte as eight bits and the gibibyte as 2^30 bytes, the unit Windows labels GB.
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