Gbps and Mbps — the network speeds behind every ISP promise
1 Gbps = 125 MB/s of raw payload. After protocol overhead, real download speed lands around 110 MB/s at best.
ISPs advertise link capacity in bits per second because the number is 8× larger than the equivalent bytes-per-second. Consumers download in bytes. The 8× factor is the source of every 'my 500 Mbps plan only downloads at 60 MB/s' complaint.
The exact factors
1 byte = 8 bits, always > 1 Mbps = 10⁶ bits/s = 125,000 bytes/s = 122.07 KiB/s > 1 Gbps = 10⁹ bits/s = 125 MB/s = 119.2 MiB/s > 10 Gbps = 1.25 GB/s = 1.164 GiB/s
What eats the rest
Ethernet frame overhead: 38 B per 1500 B payload = 2.5% loss. IP + TCP headers: 40 B per packet = 2.7% at 1500 B MTU. HTTPS: 2–5% extra for TLS. Real payload throughput on a 1 Gbps LAN: ~940 Mbps = 117 MB/s. On the internet, latency and packet loss can drop this to 30–60 MB/s even on 1 Gbps plans.
Anchors
DSL: 5–100 Mbps. Cable: 100 Mbps – 1 Gbps. Fibre-to-home: 100 Mbps – 10 Gbps. WiFi 6: up to ~1 Gbps real-world. Cellular 5G: 100–500 Mbps typical. Local NVMe SSD: 3–7 GB/s = 24–56 Gbps. USB 3.2 Gen 2: 10 Gbps. Thunderbolt 4: 40 Gbps.
Worked example — downloading a 10 GB game
10 GB game (10¹⁰ bytes) on a 500 Mbps plan. 500 Mbps = 62.5 MB/s theoretical. Realistic sustained: 55 MB/s. Time: 10,000 / 55 = 182 s = 3 minutes. On a 100 Mbps plan: 900 s = 15 minutes. On a 1 Gbps plan: 91 s. Diminishing returns above 500 Mbps for most single-flow downloads because the game server tops out first.
Symmetric vs asymmetric
Cable and DSL are asymmetric — download much faster than upload. FTTH is often symmetric (1 Gbps up and down). Consumer expectations of upload speed have risen sharply since videoconferencing became universal; asymmetric plans with 10 Mbps upload are now visibly bottlenecked in real use.
The five-second recap
Divide Mbps by 8 for MB/s. Real throughput is 90–95% of advertised on a clean link. Latency and packet loss can drop it much further.
Try the converters mentioned in this article
FAQ
Is 1 Gbps enough for a family of 4?+
More than enough. Even 4K Netflix streams at 25 Mbps; a family of 4 streaming simultaneously uses 100 Mbps peak. 1 Gbps is future-proofing.
Why is my WiFi slower than my ISP plan?+
WiFi has its own overhead. A '1200 Mbps' WiFi 6 router typically delivers 500–700 Mbps real throughput. Distance, walls and interference reduce this further.
What is 'up to' in ISP marketing?+
The peak line rate under ideal conditions. Regulatory bodies (FCC, Ofcom) increasingly require published median speeds instead.
This article was written by The Turbo Unit Converter engineering desk (Networking) and last reviewed on 2026-06-30 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.