Dev & Computing · 字节与位（Byte vs Bit） · Updated 2026-10-25
What Is Byte vs Bit?
Definition
A bit (lowercase b) is the smallest unit of information, 0 or 1; a byte (uppercase B) is 8 bits and can encode 256 states — the base unit for file sizes and memory. A "300M broadband" plan means 300 Mbps (300 million bits per second), which tops out at 300÷8 = 37.5 MB/s in download speed. That is the entire reason a 300M plan shows only ~37 MB/s.
Formula
Download speed (MB/s) = bandwidth (Mbps) ÷ 8
Worked Example
A 300 Mbps line peaks at 37.5 MB/s; downloading a 2.25 GB movie takes about 2.25×1024÷37.5 ≈ 61 seconds. Misreading it as 300 MB/s would wrongly predict 8 seconds.
Common Misconception
Reading a 100M plan as 100 MB/s (actually 12.5 MB/s, 8x off). Also, drive makers label in decimal (1 GB = 10⁹ bytes) while OSes show binary (1 GiB = 2³⁰ bytes), so a "1 TB" drive showing ~931 GB is normal, not missing capacity.
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