CS02-05 Computer Science Coming soon

Working out the size of a text file

CS02-05

This lesson is coming soon.

Watch now instead: CS04-00 Chapter intro: Inside the computer

In this lesson

Calculate the size of a text file from the bits per character and the number of characters, convert the answer into sensible units, and work out the storage capacity required for a set of files.

What it covers

Text file size in bits = bits per character x number of characters (OCR prints this formula and it is NOT given in the exam)
Which bit count to use, per board - 7 for AQA and Edexcel, 8 in an OCR exam - taken as settled from CS02-02, not re-argued
Bits to bytes is DIVIDE BY 8, and it is a separate step from the prefix step - two divisions, never one
Converting the result into kilobytes or megabytes on AQA and OCR, and into kibibytes or mebibytes on Edexcel
Storage capacity for a SET of files: work out one, then multiply by how many
Showing working, because the marks sit in the steps and not only in the final number
One line on metadata: OCR states that an allowance for metadata in calculations may be used, so a real file is slightly bigger than the characters alone

Key words

file size text file characters bytes storage capacity

For: AQA GCSE 8525, Edexcel GCSE 1CP2, OCR GCSE J277

On the specification

Board	Spec	Statement
AQA GCSE 8525	3.3.3	State that a bit is the fundamental unit of information and a byte is 8 bits, use the decimal prefixes kilo, mega, giga and tera, and compare quantities of bytes.
Edexcel GCSE 1CP2	2.3.1	Data storage and compression
OCR GCSE J277	1.2.3	Units

For teachers

This GCSE Computer Science lesson teaches working out the size of a text file. By the end, students should be able to calculate the size of a text file from the bits per character and the number of characters, convert the answer into sensible units, and work out the storage capacity required for a set of files. It works through four worked examples and the mistakes examiners report.