What a megapixel actually counts
A megapixel is one million pixels, and a pixel is one stored dot in a raster image. Multiply the width by the height and you have the pixel count, so a 4000 by 3000 photo holds 12 million pixels, which is 12 megapixels.
Resolution is therefore a property of size in pixels, not of how sharp the photo looks. A camera can produce a high-resolution file of a soft scene, and a modest file can look perfectly crisp at the size it is displayed.
Calculate the dimensions you want
Decide the target count and then pick dimensions that fit your aspect ratio. A 4 by 3 image at 1600 by 1200 is 1.92 million pixels, just under 2 megapixels, while 1920 by 1080 is about 2.07 megapixels in a 16 by 9 shape.
Doubling or halving each side changes the count much faster than it changes the look. Halving both dimensions leaves you with a quarter of the pixels, which is why resolution drops so quickly in percent mode.
- Multiply width by height to get the pixel count
- Divide by one million to read it in megapixels
- Halve both sides to drop to a quarter of the pixels
- Keep the same ratio so nothing is distorted
Enter the new size with the aspect lock on
Open the resize tool, choose the photo, and enter the target width and height in pixels. With the aspect ratio lock on, typing one dimension fills in the other, so the shape of the image survives the reduction.
Percent mode is the quick route when you only know that the image should be smaller. Enter 50 percent to halve each side, or 25 percent to keep a quarter of the original dimensions.
Resolution, file size, and DPI are three different things
File size is the amount of stored data, resolution is the width and height in pixels, and DPI describes how those pixels are spaced when the image is printed. Reducing resolution usually reduces file size, but compression decides the final number of bytes.
Changing DPI alone does not shrink a file, because the stored pixels stay exactly the same. Adjust it only when a print workflow asks for a specific value, and treat pixel dimensions as the control that matters on screen.
Confirm the reduction and go further if needed
Check the new dimensions once the job finishes, so you can verify that the megapixel count really dropped. Inputs are accepted up to 16,000 pixels per side and 48 megapixels, and larger files are refused with a clear message.
If the file still exceeds a size limit after reduction, compress it next. Fewer pixels give the compressor less work, so a reduced image reaches a modest target far more easily than the original does.