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TechA 5 GB game update on a "100 Mbps" connection should take under 7 minutes by the naive math — yet the progress bar disagrees. The formula for download time is genuinely simple, but two traps (bits vs. bytes, and advertised vs. real speed) make estimates miss by wide margins.
Here's the exact calculation, the traps, and how to estimate like someone who won't be surprised.
Try it yourself: Enter file size and connection speed for an instant estimate. Download Time Calculator →
The formula
Download time is file size divided by transfer speed — with one critical catch: sizes are usually in bytes, speeds in bits. Convert first (1 byte = 8 bits):
Time (seconds) = (file size in MB × 8) ÷ speed in Mbps
Worked example
5 GB file (5,000 MB) on a 100 Mbps connection: (5,000 × 8) ÷ 100 = 40,000 ÷ 100 = 400 seconds ≈ 6 min 40 s. That's the theoretical best case — the laws of physics version.
Why real downloads are slower
- Advertised speed is a ceiling. ISPs sell "up to" speeds; congestion, routing, and your Wi-Fi typically deliver 70–90% of it.
- Protocol overhead. TCP/IP headers, encryption, and retransmissions eat roughly 5–10% of raw throughput.
- The server's limits. Your connection can only download as fast as the other end uploads. Small sites and free tiers throttle hard.
- Wi-Fi losses. Distance, walls, and interference cut wireless speed well below what your plan provides at the router.
Rule of thumb: add 10–30% to the theoretical time for a realistic estimate. Our 6:40 file becomes ~8 minutes in practice.
The GB vs. GiB footnote
Storage makers define 1 GB as 1,000 MB; operating systems often use 1 GiB = 1,024 MiB. A "5 GB" file might be 5,000 or 5,120 MB depending on who's counting — a ~2.4% swing per GB level that compounds on big downloads. For estimates it barely matters; for exact planning, check which unit your source uses.
Uploads use the same math
The formula is symmetric — uploading a 500 MB video on a 10 Mbps upload link takes (500 × 8) ÷ 10 = 400 seconds. Note that most home connections have much slower upload than download (a 100 Mbps plan might upload at 10 Mbps), which is why sending large files feels glacial.
Common downloads, real times
Theoretical times at three connection speeds (add 10–30% for reality):
| Download | Size | 25 Mbps | 100 Mbps | 500 Mbps |
|---|---|---|---|---|
| Album (MP3) | 100 MB | 32 sec | 8 sec | 2 sec |
| HD movie | 5 GB | 27 min | 6.7 min | 80 sec |
| 4K movie | 25 GB | 2.2 hr | 33 min | 6.7 min |
| AAA game | 100 GB | 8.9 hr | 2.2 hr | 27 min |
What actually limits your speed
Ranked by typical impact: your plan's cap (the hard ceiling) → Wi-Fi quality (the most common real-world bottleneck — a wired connection often doubles wireless speeds) → server-side throttling (free tiers, small hosts) → network congestion (evenings are slower) → protocol overhead (~5–10%, unavoidable). If downloads feel slow, test wired vs. wireless first — it diagnoses the problem in 30 seconds.
Speed tests: read them right
Run speed tests wired, with nothing else using the network, and test against a nearby server. A single test is a snapshot — run three at different times of day. And remember the units: speed tests report in Mbps, so divide by 8 to compare with the MB/s your browser shows.
Resume support saves big downloads
For multi-gigabyte files, use a download manager or client that supports resuming. A dropped connection at 90% of a 100 GB game download is a minor annoyance with resume — and a full restart without it. Browsers increasingly resume automatically, but dedicated tools handle segmented downloading (multiple connections to the same file) for a modest speed boost on well-provisioned servers.
Frequently asked questions
Advertised speeds are maximums, not guarantees. Network congestion, Wi-Fi losses, protocol overhead (5–10%), and the server's own upload limits all reduce real throughput — expect 70–90% of the headline number in practice.
MB is megabytes (file sizes), Mb is megabits (speeds), and 1 byte = 8 bits. A 100 Mbps connection moves at most 12.5 MB/s — divide any speed in Mbps by 8 to get MB/s.
Yes — it's the same math. Just use your (usually much slower) upload speed. A 100/10 Mbps plan downloads at 100 but uploads at 10.
No. Parallel downloads split the same connection — the total stays capped by your slowest bottleneck, usually your plan speed or the server.
Disclaimer: Estimates assume ideal conditions. Real-world speeds vary with network congestion, Wi-Fi quality, server capacity, and ISP throttling.