Bitmap image file size calculations

Overview

The file size formula for bitmaps, worked calculations in bits, bytes, kilobytes and megabytes, working backwards to a width or colour depth, and how resizing changes storage.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   State the formula for the file size of a bitmap image in bits.

   Model answer

   The size in bits is width × height × colour depth. Width and height are in pixels and colour depth is in bits.

2. 2

   How do you find the file size of a bitmap image in bytes?

   Model answer

   Work out width × height × colour depth to get the size in bits. Divide that total by 8 to give bytes.

3. 3

   Calculate the file size in bits of an 800 × 600 image with a colour depth of 1 bit.

   Model answer

   The file size is 480,000 bits. Width × height × depth = 800 × 600 × 1 = 480,000.

4. 4

   Calculate the file size in bytes of a 100 × 50 image with a colour depth of 8 bits.

   Model answer

   The file size is 5,000 bytes. The image needs 100 × 50 × 8 = 40,000 bits, and 40,000 ÷ 8 = 5,000.

5. 5

   Calculate the file size in kilobytes of a 640 × 480 image with a colour depth of 4 bits.

   Model answer

   The file size is 153.6 kB. The image needs 640 × 480 × 4 = 1,228,800 bits, which is 153,600 bytes.

6. 6

   Calculate the file size in megabytes of a 1,920 × 1,080 image with a colour depth of 24 bits.

   Model answer

   The file size is about 6.22 MB. The image needs 1,920 × 1,080 × 24 = 49,766,400 bits, which is 6,220,800 bytes.

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