DPI, pixels and file size: stop confusing them

You exported a photo at 300 DPI because a form said it needed to be “high resolution,” and it still got rejected. Or someone told you to set your image to 72 DPI for the web, you did, and nothing changed. The confusion is normal. Three different things get jammed under one word, and once you separate them the rejections start making sense.

TL;DR: Screens care about pixels. DPI is a print instruction that browsers throw away. File size is a separate number you control with dimensions and compression. Fix the right one and forms stop complaining.

Pixels are the only thing a screen reads

An image is a grid of colored squares called pixels. A photo described as 1200x800 has 1200 columns and 800 rows, which is 960,000 pixels total. That grid is the actual image. Everything you see on a screen is the screen drawing that grid.

When a website, a phone, or an upload preview shows your image, it reads the width and height in pixels and draws exactly that many. Nothing else about the file changes what appears. So when a form says “minimum 600x600px” or “maximum 4000px wide,” it is talking about this grid, and it is the easiest requirement to check. Right-click the file and open its info, or look at the details panel in your photo app. The two numbers you want are width and height in pixels.

If your image is smaller than the minimum, you need a genuinely larger version. Stretching a 300x300 image up to 600x600 fills the grid but invents detail that was never captured, so it looks soft. If your image is larger than a maximum, you can safely shrink it, because throwing pixels away is clean and keeps things sharp.

DPI is a print note, and the web ignores it

DPI stands for dots per inch. PPI, pixels per inch, is the same idea for screens. Both answer one question: when this image gets printed, how many pixels should be packed into each physical inch of paper?

That question only has meaning on paper. A 1200x800 image printed at 300 DPI comes out 4 inches by about 2.7 inches. The same 1200x800 image printed at 150 DPI comes out twice as big and looks softer, because the same pixels are spread over more paper. The DPI number is just a ratio that tells a printer how big to make each pixel. It is stored as a tag inside the file, and it changes nothing about the pixels themselves.

Here is the part that trips everyone up. A screen has its own fixed pixel density, baked into the hardware. It does not read your image’s DPI tag at all. It puts your 1200 pixels across 1200 of its own pixels and moves on. So a 1200x800 image at 72 DPI and the very same image at 300 DPI display identically on every screen, every browser, every upload preview. The DPI tag is along for the ride and never consulted.

This is why “save it at 300 DPI” is meaningless advice for a web upload. You can set the tag to 1, 72, or 3000 and the picture on screen will not move a single pixel. If a website form ever checks resolution, it is checking pixel dimensions, not the DPI tag. The phrase “high resolution” almost always means “enough pixels,” not “high DPI.”

File size is a result, not a setting

File size is how many megabytes the file takes up on disk. It is a third, separate thing, and it is the one most forms actually limit with rules like “max 2MB.”

Two ingredients decide file size. The first is pixel dimensions: more pixels means more data to store, so a 4000x3000 photo is heavier than a 800x600 one. The second is compression: how aggressively the format squeezes that data. JPEG compression, for example, can be gentle (large file, near-perfect quality) or strong (small file, some softening or blocky patches). You can take one image and save it at many different megabyte sizes by changing the compression, without touching its dimensions at all.

Notice what is not on that list: DPI. Changing the DPI tag does not change the file size, because it does not add or remove a single pixel. People often “lower the DPI to make the file smaller” and are baffled when the megabytes do not budge. They were turning the wrong dial.

So when a form rejects your image for being over 2MB, you have two real levers. Shrink the pixel dimensions, or compress harder, or do a bit of both. A free in-browser tool at image.hivly.net lets you resize and compress without uploading the file anywhere, so you can hit a target size and re-upload in a minute.

What the form says vs what to actually change

Upload forms describe requirements in mixed language. Here is the translation.

The form says	What it actually means	What to change
”Max 2MB” or “under 500KB”	File size in megabytes is too high	Shrink dimensions, compress harder, or both
”Min 600x600px”	Pixel grid is too small	Use a larger original; enlarging won’t add real detail
”Max 4000px wide”	Pixel grid is too big	Resize down to or under the limit
”Must be 300 DPI” (print shop)	Pack 300 pixels per printed inch	Make sure pixel count covers the print size; ignore for screens
”High resolution” (web upload)	Enough pixels, usually a minimum dimension	Use a larger version, not a higher DPI tag
”Square / 1:1”	Width and height must match	Crop to equal width and height

Most everyday cases come down to two fixes. If the complaint is about megabytes, work on file size. If the complaint is about size in pixels, work on dimensions. DPI almost never enters the picture unless a printer is involved.

Quick mental check before you re-upload

When a form pushes back, sort the message into one of three buckets first.

If it mentions MB or KB, that is file size. Compress or resize down until the megabyte number fits, and remember you can usually drop a few hundred KB with barely visible quality loss.

If it mentions px or “resolution” or “dimensions,” that is the pixel grid. Check width and height and match the rule. Shrinking to fit a maximum is painless; growing to meet a minimum needs a real larger source.

If it mentions DPI and you are uploading to a website, you can almost certainly ignore it, because the screen will. If a print shop mentions DPI, multiply the inches you want by the DPI to find the pixel count you need. A 4x6 print at 300 DPI needs 1200x1800 pixels, and that is the number that decides whether it prints sharp.

Separate the three and each upload becomes a single, obvious adjustment instead of a guessing game.