Units of information

OCR J277 Paper 1 uses the base-1000 (decimal) convention for units in the spec — kilobyte = 1,000 bytes, megabyte = 1,000 KB, etc. (Some textbooks still use base-1024 — OCR mark schemes accept either if the working is clear, but the spec answer is base-1000.)

The hierarchy

Unit	Symbol	Size
Bit	b	1 binary digit (0 or 1)
Nibble		4 bits
Byte	B	8 bits
Kilobyte	KB	1,000 bytes
Megabyte	MB	1,000 KB = 1,000,000 bytes
Gigabyte	GB	1,000 MB
Terabyte	TB	1,000 GB
Petabyte	PB	1,000 TB

Converting between units

Each step up multiplies by 1,000 (or 8 for bit↔byte). Each step down divides.

• 16 bits ÷ 8 = 2 bytes.
• 5,000 KB ÷ 1,000 = 5 MB.
• 2 GB × 1,000 = 2,000 MB = 2,000,000 KB = 2,000,000,000 bytes.

A common pattern in OCR is "calculate the file size", e.g. an image 600 × 400 pixels at 24 bits per pixel:

• pixels = 600 × 400 = 240,000
• bits = 240,000 × 24 = 5,760,000 bits
• bytes = 5,760,000 ÷ 8 = 720,000 bytes = 720 KB.

Why bits and bytes?

A computer stores everything as binary. The smallest unit is the bit — high or low voltage representing 1 or 0. Bytes are the smallest addressable unit in most architectures — RAM addresses one byte at a time.

Common OCR exam mistakes

• Confusing b (bit) and B (byte). Network speeds quote bits/sec; file sizes quote bytes.
• Using ÷ 1,024 when the question expects ÷ 1,000.
• Forgetting to divide by 8 when converting bits → bytes.
• Quoting "8 bits = 1 nibble" — a nibble is 4 bits.