Converting Between Units — Data Storage and Compression | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 4% of your exam marks.

stable

Rare

Stable4%

File size calculations and lossless vs lossy compression are regular 3 to 4 mark questions.

The standard rules are simple once you remember the two factors:

Bits ↔ bytes: multiply or divide by 8 (because 1 byte = 8 bits).
Bytes ↔ kB ↔ MB ↔ GB: multiply or divide by 1000 between adjacent units (or 1024 if the question uses the binary convention).

Going up a unit (smaller → larger) means dividing. Going down a unit (larger → smaller) means multiplying.

Example — A music album consists of 12 tracks, each 5 MB in size. Calculate the total album size in kilobytes (using 1 MB = 1000 kB).

Step 1: Total size in MB = 12 × 5 = 60 MB.
Step 2: Convert to kB: 60 × 1000 = 60 000 kB.

Example — A photograph has a file size of 3 MB. Express this size in bits (using 1 MB = 1 000 000 bytes).

Step 1: 3 MB × 1 000 000 = 3 000 000 bytes.
Step 2: 3 000 000 × 8 = 24 000 000 bits.

Example — Convert 2 TiB to MiB.

2 TiB = 2 × 1024 = 2048 GiB
2048 GiB = 2048 × 1024 = 2 097 152 MiB

Each step up through the binary units multiplies by 1024, so going up two steps multiplies by 1024² (≈ 10⁶). The same pattern continues for higher units: each extra binary prefix adds another factor of 1024.