Image size vs dimensions vs resolution vs DPI: what each one means
Bytes, pixels, megapixels, aspect ratio and DPI explained with worked examples — including the print formula and why raising DPI adds no detail.
Published and last reviewed · Independent explainer — sources are linked, and nothing here is an endorsement.
One photo, five numbers, five different questions
A single image file has a byte count, a pixel grid, a megapixel figure, an aspect ratio and possibly a DPI field. People call all of them "the size", then wonder why a form rejected a file that looks fine.
Here is the mapping between what people say and what they actually mean — and which number each phrasing controls when a form, a platform or a print shop is the destination.
| What people say | What it means | What it controls |
|---|---|---|
| "The file is too big" | Bytes on disk | Upload limits, email attachments, storage |
| "The image is too small" | Pixel dimensions | Sharpness on screen and in print |
| "It has to be 300 DPI" | Print density field | How large the pixels are reproduced on paper |
| "It is not square" | Aspect ratio | Whether a platform crops or letterboxes the image |
| "It looks blurry" | Real detail (resolution) | Only the original capture and pixel count decide this |
Pixels, megapixels and what they promise
Pixel dimensions are the width and height of the grid: 2400 × 1600 means 3.84 million pixels, which is the same as 3.84 megapixels. Megapixels are simply pixels divided by a million, and camera marketing made them famous.
More pixels allow larger prints and cropping headroom, but they do not make a picture sharper by themselves. A 12-megapixel photo from a cheap lens can look worse than an 8-megapixel photo taken carefully — resolution is a ceiling on detail, not a guarantee of it.
DPI and PPI, without the mythology
DPI is a print instruction: how many dots of ink cover one inch. PPI is the image-side equivalent: how many pixels are reproduced per inch. In everyday use people say DPI for both, and the meaning is the same — a density value that tells print software how large to reproduce the pixels you have.
The arithmetic is the whole story. Print size in inches equals pixels divided by density: 3000 pixels at 300 DPI prints 10 inches wide; at 150 DPI it prints 20 inches. Working the other way, a 12-inch print at 300 DPI needs 3600 pixels, and if the file has fewer than that, no setting fixes it — the printer has to interpolate.
A worked example
A 2400 × 1600 photo is 3.84 megapixels. At 300 DPI it prints 8 × 5.3 inches — a normal photo print. At 150 DPI the same pixels print 16 × 10.7 inches, which looks fine on a wall poster and soft in a hand-held brochure.
Now the same photo submitted to a form that demands "under 50 KB": bytes are the constraint, not pixels. The sensible order is to reduce the pixel grid to what the form actually displays (often a few hundred pixels), then compress to the ceiling — which is exactly the handoff the size checker and the exact-size compressor are built around.
Why raising DPI cannot sharpen a photo
Changing the DPI field changes how large existing pixels are printed; it does not create new pixels. Tools that advertise "increase DPI to 300 for sharper prints" are either resampling — inventing pixels by interpolation, which softens real detail — or simply rewriting the field and letting the print shop discover the truth.
The honest options are two: mark the density you intend (a metadata write), or supply more real pixels (a better scan or a larger original). This site does the first and says so plainly.
Sources
- DPI definition and print context (Adobe)
- PNG pHYs chunk — physical pixel dimensions (W3C)
- JFIF density fields in JPEG files (ITU T.871)
Specifications and platform behaviour change. Values on this page were reviewed on ; if a source contradicts this page, the source wins — and the correction belongs in the issue tracker.
Frequently asked questions
Is file size the same as image size?
No. File size is bytes on disk and decides whether an upload limit is met. Image size is the pixel grid and decides how large the image can be displayed or printed. A file can be heavy and low-resolution (uncompressed) or light and high-resolution (efficiently encoded).
Does resizing change the DPI?
No. Resizing changes the pixel grid; the density field stays as it was, and most print software then recalculates the physical size. If a shop gave you a DPI requirement, set the density explicitly and check the pixel count separately.
What resolution do I need for printing?
Multiply the print size in inches by the density you are targeting: 8 × 10 inches at 300 DPI needs 2400 × 3000 pixels. Aim for 300 DPI for hand-held prints, 150 DPI for posters seen from a distance.
Why does the checker show two print sizes?
Because there is no single correct density. 300 DPI is the standard for hand-held print quality and 150 DPI is a common reference for large format; showing both saves you a second calculation when the answer depends on how the image will be used.