lesson
Count pixels and bits per pixel
1 · Learn
For a simple uncompressed bitmap model, image data size is width × height × bits per pixel. Divide by eight to convert bits to bytes. Headers, metadata and compression can make an actual file size different.
Increasing dimensions in both directions multiplies the pixel count twice. Colour depth controls how many encoded colour values are available, not the physical size of the display.
2 · Worked example
A 10 by 10 image at 8 bits per pixel uses 800 bits, or 100 bytes, for its pixel data in this model.
3 · Your turn
Find the pixel-data size of a 20 by 10 image at 8 bits per pixel.
Check your answer
1,600 bits = 200 bytes, excluding overhead and compression.
4 · Apply your learning
Compare doubling width alone with doubling both width and height, keeping colour depth fixed.
Adult guidance and safety
Use an approved sandbox with invented data. Never ask for real passwords, personal records or access to another person's system. These text examples need executable projects and assessed evidence in two languages; reading code alone is not practical mastery.
Open your writing and drawing pad
Write, draw or show your working. Use a mouse, pen or finger. This pad is not a submitted answer.
Not saved to your account. Download your drawing as PNG and your typed notes as TXT before leaving or refreshing. They are separate files. Do not enter private information.
Open a picture saved on this device to draw on it again. Opening replaces the drawing, but Undo can restore it. PNG only, up to 4 MB and 4096 × 4096 pixels. The picture stays in this browser; it is not uploaded. Paper lines in a saved picture become part of that picture.
Clear the drawing? Your typed notes will stay. You can undo this action.
Your pad is empty.
Open a UTF-8 TXT file from this device, up to 40 KB and 10,000 characters. It stays in this browser. Opening can be undone until you type again. Downloads keep a new copy; they do not change the file you opened.
Replace the notes on this page with the opened file? Download the current notes first if you want to keep them.
Typed notes are not submitted answers.
Check a calculation · Year 5 onward
Use this only when your teacher or adult allows a calculator for the task. Show your thinking first. It does not replace mental or written arithmetic and does not mark your lesson answer.
Powers, roots and trigonometry · Year 7 onward
Use only when the task allows a calculator. Choose one operation at a time and record your working. Trigonometry here always uses degrees (DEG), not radians. Inverse sine, cosine and tangent return an angle, not a reciprocal. Keep exact fractions and π in answers when requested.
Approximate numerical results use up to 12 significant digits. Inverse sine returns −90° to 90°, inverse cosine 0° to 180°, and inverse tangent −90° to 90°; these are principal values, not every possible solution of a trigonometric equation.
Practice this lesson
Lesson resources
Browse the available practice activities and worksheets for this lesson.
Your progress