Why is my upscaled image still blurry?

Nine times out of ten because the source was blurred before you started, and enlarging a blur gives you a larger blur. The quick test is to look at the original at 1:1 rather than fitted to the window: if it was already soft at its own size, no factor and no method will change that, and what you need is sharpening or a better original rather than more pixels.

I sharpened it and now it has white outlines. What went wrong?

You went past the point where sharpening adds contrast at an edge and into the point where it adds a bright band beside the edge. That band is a halo, it cannot be removed afterwards, and the fix is to sharpen less and to do it after the enlargement rather than before.

Should I sharpen before or after enlarging?

After, every time. Sharpening first creates edge contrast that the enlargement then spreads across four or sixteen times as many pixels, which is how a picture ends up both soft and crunchy. Enlarge, look at 1:1, then sharpen if it still needs it.

Enlarged and still soft

You made it four times bigger and it looks worse than before. That is a diagnosis, not a failure — and there are only six things it can be.

First, look at the original properly

Before blaming the enlargement, open the source on its own and view it at 1:1 rather than fitted to a window. Almost every browser and photo viewer shrinks a picture to fit, and shrinking hides softness beautifully — which is why so many pictures look fine until the moment somebody makes them bigger.

If the source is already soft at its own size, the enlargement did exactly what it was asked to do. Everything below assumes you have checked this, because six causes out of six are easier to identify once you know what you started with.

The six causes

1. The source was out of focus, or the camera moved

Motion blur has a direction; missed focus does not. Zoom to a point highlight — a lamp, a reflection, a catchlight in an eye — and see whether it is a smeared streak or an even disc. Neither is recoverable by enlargement, and a streak is not recoverable by sharpening either.

2. The wrong method for the material

Mitchell on a photograph that needed Lanczos reads as a very slight focus problem across the whole frame. Lanczos on flat artwork reads as a glow beside every boundary. Switch method while looking at the same spot at 1:1 and the difference takes a second to spot.

3. The clean-up pass was too strong

Set to Strong on a picture that did not need it, the pass removes fine variation along with the artefacts, and the enlargement then spreads what is left. The result is smooth in a way that looks like plastic rather than like a photograph. Drop to Light or Off and compare.

4. You are judging it at the wrong size

A 4× result viewed fitted to a laptop screen is being shrunk by the browser to roughly its original size, with a cheap filter. It will look soft because you are looking at a bad reduction of a good enlargement. Press 1:1, or print a test strip.

5. The file was re-encoded after the enlargement

Saving a freshly enlarged picture as a low-quality JPEG spends part of what you just waited for. PNG is the default here for that reason. If the file must be a JPEG, keep the quality high and save once rather than editing and re-saving in a chain.

6. It was never a size problem

This is the common one that nobody checks. A picture can have plenty of pixels and still look soft, because the lens, the sensor, the light or the encoder took the local contrast out of its edges. Making it bigger does nothing at all for that, and after four attempts at different factors the picture is exactly as soft as it was — only slower to load.

That case is a sharpening job rather than an enlargement one, and it is a proper craft of its own: unsharp masking, radius against amount, deconvolution when the blur has a known shape, and halo control so the fix does not announce itself. sharpenpicture.com is built around exactly that problem, and if the diagnosis above sounds like your picture, it is the tool you actually wanted.

An order of operations that works

Clean up first, at the lowest strength that removes the grid. Enlarge once, to the factor you actually need, rather than twice to get there. Judge at 1:1. Sharpen last, if at all, and stop before you can see a light band beside dark edges. Save as PNG unless the file size forces your hand.

Every step of that order exists because the opposite order is what produces the picture people bring to this page.

The questions that follow

Why is my upscaled image still blurry?: Nine times out of ten because the source was blurred before you started, and enlarging a blur gives you a larger blur. The quick test is to look at the original at 1:1 rather than fitted to the window: if it was already soft at its own size, no factor and no method will change that, and what you need is sharpening or a better original rather than more pixels.
I sharpened it and now it has white outlines. What went wrong?: You went past the point where sharpening adds contrast at an edge and into the point where it adds a bright band beside the edge. That band is a halo, it cannot be removed afterwards, and the fix is to sharpen less and to do it after the enlargement rather than before.
Should I sharpen before or after enlarging?: After, every time. Sharpening first creates edge contrast that the enlargement then spreads across four or sixteen times as many pixels, which is how a picture ends up both soft and crunchy. Enlarge, look at 1:1, then sharpen if it still needs it.