The math (and the trap)
Transfer time is size ÷ speed — the trap is units. Storage sizes come in two flavors: decimal (1 GB = 10⁹ bytes — what drive makers and networks use) and binary (1 GiB = 2³⁰ bytes ≈ 1.074 GB — what your OS often reports). And speeds come in bits per second on networks but bytes per second for disks — an 8× difference. This calculator makes every one of those choices explicit, so a "250 GB at 100 Mbps" answer is actually right: 250 × 10⁹ × 8 bits ÷ 10⁸ bit/s = 20,000 s ≈ 5 h 33 min.
Why real transfers run slower
Link rates are physical-layer numbers. TCP/IP headers, acknowledgments, encryption, and protocol chatter typically eat 5–10% on a clean network — that's the Overhead field. Wi-Fi and USB can lose far more to contention and bus sharing; a "5 Gbps" USB 3.0 disk enclosure rarely sustains half that. For honest estimates, benchmark your actual link once and type that number in instead of the marketing rate.
Handy reference rates
| Interface | Nominal rate | ≈ bytes/s |
|---|---|---|
| Fast Ethernet | 100 Mbps | 12.5 MB/s |
| USB 2.0 | 480 Mbps | 60 MB/s |
| Gigabit Ethernet | 1 Gbps | 125 MB/s |
| Wi-Fi 6 (typical link) | 1.2 Gbps | 150 MB/s |
| USB 3.0 | 5 Gbps | 625 MB/s |
| SATA III | 6 Gbps | 750 MB/s |
| 10G Ethernet | 10 Gbps | 1.25 GB/s |
| NVMe Gen4 SSD | — | ~7 GB/s |
Frequently asked questions
- Why does my OS say the file is smaller than the seller claimed?
- Decimal vs binary again: a "1 TB" drive is 10¹² bytes, which Windows reports as ~931 GiB (often mislabeled "GB"). Nothing is missing.
- Is Mbps megabits or megabytes?
- Bits — the lowercase b matters. MB/s is megabytes. Mixing them up is a factor of 8, the single most common transfer-math mistake.
- How long to fill my drive over my internet plan?
- Put the drive size in the size field and your plan's rate in the speed field — with ~8% overhead you'll be within a few percent of reality on a wired link.
- What about latency?
- Latency dominates small transfers (a 1 KB request is all round-trip, no bandwidth) but for bulk transfers over a healthy connection, bandwidth × overhead is what matters — which is what's modeled here.