The formula, once

Everything on this site — and every download-time question anywhere — reduces to:

time = (size × 8) ÷ (speed × efficiency)

• size in bytes, × 8 to get bits (the 8:1 trap from bits vs bytes)
• speed in bits per second, so Mbps values plug in directly as megabits
• efficiency between 0 and 1 — 0.9 is the honest default, 1.0 is the brochure

A shortcut worth keeping: with size in MB and speed in Mbps, theoretical seconds = MB × 8 ÷ Mbps. For GB, that’s GB × 8,000 ÷ Mbps — so 1 GB on 100 Mbps is 8,000 ÷ 100 = 80 s. Even quicker: GB × 8 ÷ Gbps gives the same seconds (1 × 8 ÷ 0.1 = 80). The calculator exists because this mental math gets old fast — and because the efficiency correction is where every estimate goes wrong.

Worked example 1 — the 50 GB game on 100 Mbps

50 GB = 50 × 8 = 400 gigabits. At 100 Mbps theoretical: 400,000 Mbit ÷ 100 Mbps = 4,000 s = 1 h 6 min 40 s. At 90% efficiency: ÷ 90 Mbps = 4,444 s ≈ 1 h 14 min.

And if the launcher means 50 GiB (the binary units from bits vs bytes): 53.7 GB → ~1 h 19 min. The unit choice alone is worth five minutes.

Worked example 2 — a 2 GB movie on hotel Wi-Fi

Hotel “high-speed” is often 10–15 Mbps shared. Say the test shows 12 Mbps and you fold congestion into 75% efficiency: 2 GB = 16 Gbit ÷ (12 × 0.75 = 9 Mbps) = 1,778 s ≈ 29 min 38 s. The same file at home on 300 Mbps lands in about a minute — the file didn’t change, the pipe did.

Worked example 3 — an 800 MB file on gigabit fiber

800 MB = 6.4 Gbit ÷ 1,000 Mbps = 6.4 s theoretical; ~7.1 s at 90%. This is the regime where the formula over-promises — on sub-10-second transfers, TCP ramp-up and server response time matter more than line rate. Expect “a few seconds” either way.

Worked example 4 — the upload people forget: 20 GB backup

Uploads use the identical formula with the upload rate. Cable plans are asymmetric: a line sold as 500/35 Mbps pushes 20 GB = 160 Gbit out at 35 × 0.85 ≈ 29.8 Mbps effective → 5,376 s ≈ 1 h 30 min. The download of the same 20 GB would take ~6 minutes. Backup math is why “my internet is fast” and “my sync is slow” coexist.

Worked example 5 — reverse: the deadline

4 GB of footage must leave before a 10-minute deadline: 32 Gbit ÷ 600 s = 53.3 Mbps sustained. At 90% efficiency that’s a ~59 Mbps plan minimum — and if your upload is the typical 20 Mbps, it can’t be done; the honest answer is 27 minutes. Deadline math saves more panic than any speed test.

Worked example 6 — what did I actually get?

You timed it: 5 GB arrived in 7 minutes 30 seconds. 40 Gbit ÷ 450 s = 88.9 Mbps achieved — at 90% efficiency, consistent with a ~100 Mbps plan doing exactly what it should. Run your own number in the calculator’s “what speed did you get” panel.

Two traps that break every estimate

1. MB vs Mbps in the same breath. “I get 50” is meaningless until you name the unit — 50 Mbps moves 6.25 MB per second, 50 MB/s means a 400 Mbps line. Mixing them is a 10× error, not a rounding error.
2. Theoretical math on a real network. Every number above has a “÷ 0.9” hiding in it. On congested Wi-Fi, use 0.6–0.75. For the full tax list see why real speed is lower; for turning speed into monthly budget, data-cap math.

Frequently asked questions

What is the formula for download time?

seconds = (file size in bytes × 8) ÷ (speed in bits per second × efficiency). At full theoretical efficiency that's simply MB × 8 ÷ Mbps; multiply the result by ~1.11 to correct for the real-world 90%.

How long does 1 GB take on 100 Mbps?

1 GB = 8 gigabits. At 100 Mbps theoretical: 80 seconds. At the 90% efficiency a real wired link manages: ~89 seconds. On a '100 MB/s' connection — if you meant megabytes — it's a 10-second file, so unit confusion is a 10× error here.

How long does a 100 GB game take on gigabit fiber?

100 GB = 800 gigabits ÷ 1,000 Mbps = 13 min 20 s theoretical, ~15 min at 90%. If the store means 100 GiB (107.4 GB), add another minute — about 16 minutes total.

Why did my download take longer than the formula says?

The formula assumes a steady rate for the whole transfer. Real links ramp up, servers throttle per-connection, Wi-Fi dips under contention, and the last percent often slows while the file is assembled. The math gives the honest middle; the calculator's efficiency field is where you fold in your network's reality.

How do I compute upload time?

Identically — but use the upload speed, which on cable and DSL is often 5–10% of the download rate. A '500 Mbps' plan that uploads at 20 Mbps pushes a 20 GB backup out in ~2.5 hours, not the 6 minutes the download would have taken.

What's a quick mental-math shortcut?

GB to minutes at 100 Mbps: multiply GB by ~1.5 (at 90% efficiency, 1 GB ≈ 89 s ≈ 1.5 min). Scale from there — 25 Mbps is 4× slower (~6 min per GB), gigabit is 10× faster (~9 s per GB).