In GCSE Computer Science, calculations use 1 KB = 1,000 bytes unless the question says otherwise. Some systems count in powers of 2 instead, because computers work in binary (base 2). The closest power of 2 to 1,000 is 210 = 1,024. This creates two conventions:
- Decimal system (1,000): 1 KB = 1,000 bytes (the GCSE rule used by AQA and OCR, and by hard drive manufacturers)
- Binary system (1,024): 1 KiB = 1,024 bytes. OCR accepts 1,024 in conversions, and Edexcel names the 1,024 units kibibyte (KiB), mebibyte (MiB) and so on. Edexcel: 1 KiB = 1,024 bytes and 1 MiB = 1,024 KiB.
This is why your "1 TB" hard drive shows as only 931 GB in Windows: the manufacturer used decimal (1,000^4 bytes) but Windows uses binary (1,024^4 bytes). You didn't lose 69 GB, it's just a measurement difference. Neither value is "wrong": always check which one your question uses.
This deep dive covers Deep Dive: 1,000 or 1,024 Bytes? within Storage Units for GCSE Computer Science. Revise Storage Units in 3.3 Data Representation for GCSE Computer Science with 13 exam-style questions and 18 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 2 of 10 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Storage Units
How many bits make up one nibble?
Explain what is meant by colour depth and describe how increasing the colour depth affects both the image quality and the file size.