Estimating bitmap file size

A bitmap is pixels, and every pixel costs the same number of bits. So the file size is just how many pixels × bits per pixel.

Once you see that, it's a two-line calculation, and you'll know exactly why photos gobble your phone's storage.

$\text{file size (bits)} = \text{image width} \times \text{image height} \times \text{colour depth}$

Width and height are in pixels; colour depth is in bits. Then convert:

It's an estimate because the real file also holds the header. That's tiny next to the pixel data, so ignore it unless the question gives you its size.