File size and capacity calculations

How big is a photo, a song or a text file, and how many fit on a memory card? Every calculation follows the same pattern: work out the bits, then convert the units.

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The units

8 bits = 1 byte. Then each binary unit is 1024 times the one before:
1 kibibyte (KiB) = 1024 bytes, 1 MiB = 1024 KiB, 1 GiB = 1024 MiB, 1 TiB = 1024 GiB.
To go down to a smaller unit, multiply. To go up to a bigger unit, divide.
International GCSE also uses the decimal units: 1 kilobyte (kB) = 1000 bytes, 1 MB = 1000 kB, and so on. Read the question carefully to see which it wants.

Three formulas

Divide bits by 8 to get bytes, then by 1024 for KiB, and again for MiB.
FileSize in bits
Bitmap imagewidth (pixels) × height (pixels) × colour depth (bits per pixel)
Sound (one channel)sample rate (samples per second) × bit depth × length (seconds)
Textnumber of characters × bits per character (usually 8, one byte each)

Constructing an expression

Paper 1 has no calculator. Questions often say construct an expression: write the calculation with the right numbers and operators, without working it out.
Example: a 1024 × 768 image with 24-bit colour, in KiB: 1024 * 768 * 24 / 8 / 1024. If you do work it out: 2304 KiB.
A 60-second mono sound sampled 44 100 times a second at 16 bits: 44100 * 16 * 60 / 8 = 5,292,000 bytes.

Capacity questions

To find how many files fit, divide the capacity by the size of one file, using the same units, and round down: you cannot store part of a file.
Example: how many 3 MiB photos fit on a 2 GiB card? 2 GiB = 2 × 1024 = 2048 MiB. 2048 ÷ 3 = 682.67, so 682 photos.
Worked example

An image is 400 pixels wide and 300 pixels high with a colour depth of 8 bits. Calculate its size in KiB.

  1. Bits = 400 × 300 × 8 = 960,000
  2. Bytes = 960,000 ÷ 8 = 120,000
  3. KiB = 120,000 ÷ 1024 = 117.19

Answer: 117.19 KiB (about 117 KiB)

Key idea

Image bits = width × height × colour depth. Sound bits = sample rate × bit depth × seconds. Text bits = characters × 8. Divide by 8 for bytes and by 1024 for each step up (KiB, MiB, GiB).

Key words
Colour depth
The number of bits used to store the colour of each pixel.
Sample rate
The number of sound samples taken per second, in hertz.
Bit
A single binary digit, 0 or 1.
Bit depth
The number of bits used to store each sound sample.
Byte
8 bits.
Binary
Base 2: the number system that uses only the digits 0 and 1.

All GCSE and International GCSE Computer Science key terms and definitions →

Quick check

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Extra practice

2 more questions

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  1. An image is 400 × 300 pixels with a colour depth of 2 bits. What is its size in bytes?

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    Answer: 30000 bytes

    400 × 300 × 2 = 240 000 bits. 240 000 ÷ 8 = 30 000 bytes.

  2. A sound is sampled 6000 times a second with a bit depth of 8 bits for 4 seconds (one channel). What is its size in bytes?

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    Answer: 24000 bytes

    6000 × 8 × 4 = 192 000 bits. 192 000 ÷ 8 = 24 000 bytes.

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Sources and references

Checked by the GCSE-ready editorial team, 16 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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