What does 300 DPI mean?

300 DPI means 300 dots per inch: a statement about how densely the image is meant to be reproduced on paper. It is not a quality level, not a file format, and not a property that can exist without a physical size to go with it.

The whole idea rests on one relationship:

So “is this image 300 DPI?” is an incomplete question. The answerable version is “is this image 300 DPI at the size I need to print it?”

Strictly, PPI (pixels per inch) describes the image and DPI (dots per inch) describes the printer — and a printer lays down several ink dots per image pixel, so its native DPI figure is much higher, often 1200 or 2400.

In everyday use the two words have collapsed into one. When a client, a print shop or a stock agency asks for 300 DPI they mean 300 pixels per inch in the file. The distinction only matters when you are reading a printer's own specification sheet, where the numbers are not comparable.

This is the point that costs people the most time. Opening a 1000 × 800 image and setting its DPI tag to 300 does not make it printable at 300 DPI in any useful sense. The pixels are unchanged; you have only changed the note attached to them, which now says “print me at 3.3 × 2.7 inches”.

There are two genuinely different operations, and software often hides the difference behind one dialog:

The DPI converter keeps these separate on purpose: it writes the density tag by default, and resamples only if you also give it dimensions.

• A print shop or publisher asked for it. Their workflow may reject a file tagged 72, even when the pixel dimensions are ample. The tag is a five-second fix.
• Placing an image in a layout. InDesign, Word and Illustrator read the tag to decide the default placed size. A wrong tag means the image lands at the wrong scale and someone resizes it by eye.
• Submitting to a stock agency with print products. Requirements are usually stated in megapixels, but the tag is often checked too.

When it does not matter at all

On screen. Browsers, phones and social platforms ignore the density tag completely and lay images out by pixel dimensions. “Save at 72 DPI for web” is a habit from an era of fixed-density monitors, and it changes nothing today — the reasoning is in image resolution explained.

1. Work out the finished print size in inches, and the density asked for.
2. Multiply: that is the pixel count you need. 8 × 10 in at 300 DPI is 2400 × 3000.
3. Compare it with what you have. If you have enough pixels, set the tag and stop. If you have more than enough, you can resample down. If you have fewer, print smaller or accept a lower density — enlarging will not rescue it.
4. Deliver as JPG or PNG. WebP carries no density field, so a WebP file cannot be tagged at all.

The DPI converter writes real density metadata — a JFIF header for JPG, a pHYs chunk for PNG — so the value survives into software that reads it, and it can resample in the same pass when you do want the pixels to change.

Does lowering DPI reduce file size?

Not on its own. The tag is a handful of bytes. File size drops only if you also resample to fewer pixels — which is what the resizer and the KB/MB reducer do.

My camera files say 72 DPI. Is something wrong?

No. Many cameras write 72 as a default because they have no idea what size you will print. The pixel dimensions are what matter, and they are almost certainly fine.

Is 600 DPI better than 300?

For photographic print, rarely. Most presses and the human eye at normal reading distance stop benefiting somewhere around 300; 600 mainly doubles your file size. Line art and text-heavy scans are the exception, because hard edges do keep improving.