Mbps vs MB/s — the factor of 8 that makes your gigabit feel slow
ISPs quote link speed in megabits per second. Your browser, Steam, and file manager show megabytes per second. 1 byte is 8 bits, so a '100 Mbps' plan tops out around 12.5 MB/s in Chrome — not '100 anything' as most users read the number.
The bits-vs-bytes split is the single most confused consumer-tech unit. The rule is fixed and universal: **lowercase b is bits, uppercase B is bytes**. 1 Byte = 8 bits, no ambiguity. Mbps = megabits/sec (ISP contract). MB/s = megabytes/sec (browser progress bar). The factor of 8 is why every second person on Reddit thinks their gigabit fibre is broken.
The exact factors and the SI-vs-binary trap
1 Byte = 8 bits (exact, always) > 1 Mbps = 1,000,000 bits/s (SI decimal) > 1 Mbps ÷ 8 = 125,000 Bytes/s = 125 kB/s > 100 Mbps ÷ 8 = 12.5 MB/s theoretical peak > Windows/macOS display 1 MB = 1,048,576 Bytes (binary MiB, mislabelled) > IEC correct: MiB = 2²⁰ Bytes, MB = 10⁶ Bytes
Real-world throughput vs link rate
The 12.5 MB/s figure above is the physical-layer maximum. Actual TCP throughput is lower because of protocol overhead: Ethernet framing ~2%, TCP/IP headers ~5%, ACK round-trips ~5–20% depending on latency. Expect 10–11.5 MB/s from a 100 Mbps link on a well-tuned TCP session, and 8–9 MB/s from a mediocre one. Wi-Fi adds another 15–30% penalty on top.
Download-time cheat sheet
| File size | 25 Mbps | 100 Mbps | 1 Gbps | |---|---|---|---| | 100 MB photo dump | 32 s | 8 s | <1 s | | 1 GB game update | 5 min 20 s | 1 min 20 s | 8 s | | 10 GB game install | 53 min | 13 min | 80 s | | 50 GB AAA game | 4 h 27 min | 1 h 7 min | 6 min 40 s | | 4K movie (25 GB) | 2 h 13 min | 33 min | 3 min 20 s |
The four places bits-vs-bytes bites
**ISP marketing.** '1 Gbps fibre' = 125 MB/s peak = ~110 MB/s real. Selling it as 'gigabit' is technically accurate; users read it as 'a gigabyte per second' and are disappointed.
**Streaming bitrates.** Netflix 4K peaks at 15 Mbps = 1.9 MB/s. Rated bandwidth needs a 25 Mbps buffer for handshakes and rebuffering. A '15 Mbps stream' downloads a 3-hour 4K movie file worth ~20 GB.
**Cloud storage upload.** Consumer plans typically have 1/10 the upload of download (100 Mbps down / 10 Mbps up). A 50 GB Time Machine backup at 10 Mbps = 1.25 MB/s = 11 hours 6 minutes — overnight, not lunch break.
**Video calls.** Zoom HD needs ~1.5 Mbps up and down = 190 kB/s. A '5 Mbps' plan handles three simultaneous HD calls without stutter. This is why work-from-home rarely bottlenecks on modern residential plans.
LAN speeds — Gigabit, 2.5G, 10G
Gigabit Ethernet: 1000 Mbps = 125 MB/s theoretical, ~117 MB/s real over TCP. A single mechanical HDD (150 MB/s sequential) saturates gigabit; a SATA SSD (550 MB/s) does not. 2.5 Gbps Ethernet is the current sweet spot for NAS: 313 MB/s peak matches SATA SSD ceiling. 10 Gbps (1.25 GB/s) only pays off with NVMe storage on both ends.
Where the 'MB vs MiB' rounding shows up
Windows File Explorer labels 1,048,576 Bytes as '1.0 MB' — that is actually 1 MiB. Your 500 GB SSD (500,000,000,000 Bytes decimal) reports as 465.7 GB in Windows (that many GiB). No storage is missing; the drive vendor uses SI decimal and Windows uses binary with the wrong label. macOS switched to decimal (correct MB label) in 10.6 in 2009.
The five-second recap
Divide any Mbps number by 8 to get MB/s. 100 Mbps ≈ 12 MB/s in the browser. Uppercase B is bytes, lowercase b is bits — this rule has no exceptions.
Try the converters mentioned in this article
FAQ
Is 100 Mbps enough for 4K Netflix?+
More than enough. Netflix caps 4K at 15 Mbps, or 1/6 of a 100 Mbps plan. The 'need gigabit for 4K' claim is marketing, not physics.
Why does my speed test show more than my plan?+
ISPs provision 5–10% overhead so the plan speed is met after headers. A '100 Mbps' plan often measures 105–115 Mbps to a well-peered speed-test server.
What is 'GB' in AWS billing?+
AWS charges decimal GB (10⁹ Bytes). Your OS shows GiB (2³⁰). 100 GB uploaded shows as 93.1 GiB in Windows. Both are correct; different labels for different bases.
Can Wi-Fi hit gigabit?+
Wi-Fi 6 rated 1.2 Gbps PHY delivers 700–850 Mbps real in ideal conditions. Wi-Fi 5 (1300 Mbps rated) delivers ~400 Mbps real. Marketing rates always inflate real throughput by 30–50%.
Is Mbps the same everywhere?+
Yes. Both SI and IEC agree 1 Mbps = 10⁶ bits/s. Only storage sizes have the SI/binary ambiguity; transfer rates do not.
This article was written by The Turbo Unit Converter engineering desk (Networking & applied data units) and last reviewed on 2026-07-10 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.