GCSE File Size Calculator
Calculate file sizes for bitmap images and digital sound files with instant step-by-step working aligned to the AQA GCSE Computer Science (8525) specification. Free to use, no account required.
Calculated File Size
Step-by-Step Exam Working (Image File Size)
Formula: Image File Size = Width × Height × Colour Depth
2. Calculate size in bits: 480,000 × 24 = 11,520,000 bits
3. Convert to bytes (÷ 8): 11,520,000 ÷ 8 = 1,440,000 bytes
4. Convert to kilobytes (÷ 1,000): 1,440,000 ÷ 1,000 = 1,440 kB
5. Convert to megabytes (÷ 1,000): 1,440 ÷ 1,000 = 1.44 MB
Tip: In GCSE mark schemes, method marks are given for showing the multiplication and division steps even if you make an arithmetic slip in the final answer.
How GCSE Exam Questions Test File Size
File size calculation questions frequently appear on Paper 2. You will be asked to calculate the minimum file size needed to store an image or sound recording, convert units, and explain the trade-off between quality and file size.
Bitmap Image Formula
- Width & Height: The dimensions of the image in pixels.
- Colour Depth: The number of bits used to store the colour of each individual pixel. An image with n bits per pixel can represent 2ⁿ different colours.
- To convert to Bytes: Divide the total bits by 8.
Digital Sound Formula
- Sampling Rate (Hz): How many samples are taken per second. One hertz is one sample per second.
- Sample Resolution / Bit Depth: The number of bits used to record the amplitude of each sample.
- Duration: Length in seconds. If the question gives minutes, multiply by 60 first.
Top 4 Mistakes Students Make on File Size Questions
The basic formula gives the answer in bits. If the question asks for bytes, kB or MB, divide by 8 first, then by 1,000 for kilobytes and by 1,000 again for megabytes.
Sample rate is measured in Hertz (samples per second). If audio duration is given as “3 minutes”, you must use 3 × 60 = 180 seconds.
In the exam, 1 kilobyte (kB) is 1,000 bytes and 1 megabyte (MB) is 1,000 kilobytes. Divide by 1,000 at each step, not 1,024.
In real files, actual file size is slightly larger than the raw calculation because the file must also store metadata (dimensions, color space, audio bitrate header).
Check Your Whole Syllabus
Tick off every point in the GCSE specification with our free printable revision checklist.