Blur an image — all of it, or just the parts you choose
Blurring softens detail. It can turn a busy photo into a calm background for text, give a portrait a dreamy glow, or hide a name, number plate or address before you share a screenshot. The blur image tool applies a true Gaussian blur with a radius of up to 50 pixels, either to the whole picture or only to areas you paint with a brush or mark with a rectangle.
The blur is calculated with a separable kernel in a background worker on your device, at the image’s full resolution, and transparent PNGs keep clean edges. A batch of images can share the same settings and be downloaded as one ZIP. Nothing is uploaded.
How to use the Blur Image
- Drop one or more images onto the upload area, click it to browse, or paste a screenshot with Ctrl+V.
- Set the Blur radius. Small values (2–5 px) soften; 20 px and more turns detail into washes of colour.
- To blur only part of the image, choose Brush or Rectangle under Blur where and drag over the preview. Erase takes areas back and Edge feather softens the transition.
- Tick Blur everything except the marked areas to keep a subject sharp and soften its surroundings.
- Check the split preview, pick a format and press Download blurred image, or Download all as ZIP for a batch.
Blur Image features
True Gaussian blur
A normalised Gaussian kernel up to 101 taps wide, run along the rows and then the columns — not a box-blur shortcut.
Brush and rectangle selections
Blur only faces, text or backgrounds; erase mistakes, invert the selection and feather its edge from 0 to 50 px.
Clean transparent edges
Colours are weighted by their opacity while blurring, so transparent areas don’t leak a dark halo into logos or cut-outs.
Full-resolution output
The preview uses a lighter copy with the radius scaled to match; the download is processed from your original pixels.
Batch blur with ZIP
Apply the same blur — and the same selection, scaled to each picture — to many files at once.
Live histogram
Watch the spread of tones narrow as fine detail averages out.
How Gaussian blur works
Every output pixel is a weighted average of its neighbours. The weights follow the bell-shaped Gaussian curve G(x) = e^(−x² ÷ 2σ²), normalised so they add up to 1; flat areas therefore keep their exact colour, while edges and texture are smoothed. Here the radius is how far the kernel reaches and the standard deviation is σ = radius ÷ 3. Within three standard deviations the curve holds 99.7% of its weight, so stopping there changes nothing visible.
A two-dimensional Gaussian is the product of two one-dimensional ones, so the image is blurred along each row and then down each column. At a 50 px radius that is 2 × 101 = 202 multiplications per pixel and channel instead of 101 × 101 = 10,201 — about fifty times less work for the same result. Beyond the border, the edge pixels are repeated, so the frame doesn’t darken.
Radius guide
Blur strength is relative to image size: 10 px is heavy on a 600-pixel thumbnail but subtle on a 4,000-pixel phone photo. The preview scales the radius to match the downloaded result.
| Radius | σ | Kernel taps | Typical use |
|---|---|---|---|
| 1–3 px | 0.33–1 | 3–7 | Soften skin, noise or JPEG artefacts |
| 4–10 px | 1.33–3.33 | 9–21 | Dreamy glow, background behind small text |
| 12–25 px | 4–8.33 | 25–51 | Frosted-glass backgrounds, hiding fine print |
| 30–50 px | 10–16.7 | 61–101 | Abstract colour wash, wallpapers |
Blurring to hide information
Blur is a quick way to hide a face, a number plate or an email address, but a light blur over short text such as a card number can sometimes be guessed back. Use a radius at least as large as the text height, or replace the area completely.
For sensitive data, Pixelate Image has a solid-box mode, and Face Blur finds faces for you. To keep a person sharp against a soft background automatically, use Blur Background; here you can do the same by hand with the brush and the “blur everything except” option.
Tips and common mistakes
- Feather brush edges by 10–20 px when blurring around people; a hard edge looks cut out.
- Blur after enlarging an image, not before, if you want to hide visible pixels.
- Heavily blurred images compress very well — JPG at 80% quality is usually plenty.
- The blur cannot be undone in the downloaded file, so keep your original.
- To sharpen instead, use Sharpen Image — its unsharp mask is built on the same Gaussian.
Blur Image FAQ
How do I blur part of an image?
Choose Brush or Rectangle under “Blur where” and drag over the preview to mark the area. Adjust the radius and edge feather, check the split view and download. Tick “Blur everything except the marked areas” to do the opposite.
How do I blur the background of a photo?
Paint over your subject with the brush, tick “Blur everything except the marked areas” and set a feather of 15–25 px. For automatic subject detection, use the Blur Background tool.
What blur radius should I use?
Around 2–5 px softens, 8–15 px gives a clear blur on a phone photo and 30 px or more removes almost all detail. It depends on the image size, so judge it in the preview.
Is blurred text safe to share?
Not always. A weak blur on short text such as an account number can sometimes be recovered or guessed. Use a large radius or cover the text with a solid box.
Does the blur image tool upload my pictures?
No. The blur is calculated in your browser, in a background worker, and the result is saved straight to your device.
Why don’t transparent PNG edges get a dark outline?
Colours are multiplied by their opacity before blurring and divided by the blurred opacity afterwards (premultiplied alpha), so fully transparent pixels add no colour to their neighbours.