Pixels, aspect ratio and DPI in plain terms
Resizing changes how many pixels an image has. Most of the problems people run into, such as a blurry upload, a stretched face or a print that comes out the wrong size, come from mixing up three separate things: pixel dimensions, aspect ratio and print resolution. This guide explains each one with worked numbers, then shows how to get the exact result you need.
Pixel dimensions are the only thing that decides how much detail an image holds. A photo from a typical 12 megapixel phone camera is 4032 × 3024 pixels. A screen, a social network or an upload form only ever looks at those two numbers.
Aspect ratio is the shape of the image: width divided by height. 4032 × 3024 is 4:3, and 1080 × 1080 is 1:1. When the shape you need differs from the shape you have, something has to give. Either part of the picture is cropped, empty space is added around it, or the picture is distorted.
DPI (dots per inch) only matters on paper. It is a number stored in the file that tells a printer how many pixels to place in each inch. Changing it does not add or remove a single pixel. A 600 × 600 pixel image tagged 300 DPI prints at 2 × 2 inches. Tagged 150 DPI, the same pixels print at 4 × 4 inches.
Worked example: a phone photo for Instagram
The numbers below are illustrative, but they match what many phones produce. The source is a 4032 × 3024 JPEG with a 4:3 aspect ratio.
- Keep the whole picture at 1080 pixels wide: type 1080 as the width, leave the height on Auto and keep Fit inside. The height is worked out as 1080 × 3024 ÷ 4032 = 810, so the result is 1080 × 810.
- Make a square post: type 1080 × 1080 and choose Fill & crop. The photo is scaled until its short side is 1080 pixels (1440 × 1080), then 180 pixels are trimmed from the left and right, keeping the centre.
- Make a 4:5 portrait post at 1080 × 1350: Fill & crop scales the photo to 1800 × 1350 and trims 360 pixels from each side. If the subject is not in the middle, frame it yourself first with the crop button on the file card.
Shrinking like this never costs visible sharpness at the new size. It also makes the file much lighter: a 1080 × 810 copy holds about 7 percent of the original pixels.
Choosing a fit mode
Fit modes only matter when you type both a width and a height and the shape does not match the source. The default is Fit inside, which never cuts anything off.
| Mode | What happens | Output size | Good for |
|---|---|---|---|
| Fit inside | The whole image is scaled to fit within the box, keeping its shape | Can be shorter than the box on one side | Email, web pages, anything where nothing may be cut |
| Fill & crop | The image is scaled to cover the box and the overflow is trimmed from the centre | Exactly the box | Social posts, avatars, thumbnails, form photos |
| Stretch | The image is forced into the box and its shape is ignored | Exactly the box | Only when the shapes already match, or for patterns and textures |
| Pad | The image is scaled to fit and the empty area is filled with the background colour | Exactly the box | Product shots on white, slides, forms that want exact pixels with nothing cropped |
Common pixel sizes by platform
These are the sizes the platform presets use. Every one of them can also be typed by hand.
| Use | Pixels | Aspect ratio |
|---|---|---|
| Instagram square post | 1080 × 1080 | 1:1 |
| Instagram Story, TikTok | 1080 × 1920 | 9:16 |
| YouTube thumbnail | 1280 × 720 | 16:9 |
| YouTube banner | 2560 × 1440 | 16:9 |
| Facebook cover | 820 × 312 | about 2.6:1 |
| Facebook and WhatsApp profile | 500 × 500 | 1:1 |
| LinkedIn banner | 1584 × 396 | 4:1 |
| LinkedIn and X profile | 400 × 400 | 1:1 |
| X (Twitter) header | 1500 × 500 | 3:1 |
| Pinterest pin | 1000 × 1500 | 2:3 |
| Zoom background | 1920 × 1080 | 16:9 |
Resizing for print in centimetres, millimetres and inches
Choose cm, mm or in and a DPI box appears next to the units. The tool turns your measurement into pixels with one formula: pixels = inches × DPI, where inches = centimetres ÷ 2.54 or millimetres ÷ 25.4. The result is rounded to the nearest whole pixel, and the DPI is written into JPG and PNG files so the printer knows the intended size.
For example, a 10 × 15 cm print at 300 DPI is 10 ÷ 2.54 × 300 = 1181 pixels wide and 15 ÷ 2.54 × 300 = 1772 pixels tall.
| Print size | At 300 DPI | At 600 DPI |
|---|---|---|
| 3.5 × 4.5 cm | 413 × 531 | 827 × 1063 |
| 5 × 5 cm | 591 × 591 | 1181 × 1181 |
| 10 × 15 cm | 1181 × 1772 | 2362 × 3543 |
| 13 × 18 cm | 1535 × 2126 | 3071 × 4252 |
| A5 (14.8 × 21 cm) | 1748 × 2480 | 3496 × 4961 |
| A4 (21 × 29.7 cm) | 2480 × 3508 | 4961 × 7016 |
| 2 × 2 in | 600 × 600 | 1200 × 1200 |
| 4 × 6 in | 1200 × 1800 | 2400 × 3600 |
| 5 × 7 in | 1500 × 2100 | 3000 × 4200 |
| 8 × 10 in | 2400 × 3000 | 4800 × 6000 |
300 DPI is the usual standard for photo prints and documents. 600 DPI doubles each side and quadruples the pixel count, which only helps if the source really has that much detail. A 4032 × 3024 photo has enough pixels for A4 at 300 DPI, but asking for A4 at 600 DPI would simply enlarge it. For screen-only use, work in pixels and ignore DPI altogether.
How to resize step by step
- Drop your images on the page, choose them with the file picker, or paste one from the clipboard. The free version takes up to 50 files per batch and up to 200 MB per file.
- Under Resize by, choose Size for exact dimensions, or Percentage to scale every image by the same factor, anywhere from 5 to 400 percent.
- Pick a unit: px, cm, mm or in. For a physical unit, type the DPI in the box beside it.
- Type a width, a height or both. Leave one of them on Auto and it is calculated from each image's own shape, which is the easiest way to batch photos of different shapes.
- If you typed both, choose a fit mode: Fit inside, Fill & crop, Stretch or Pad. Switch off Allow upscaling if small images should be left at their size rather than enlarged.
- Choose the output format (JPG, PNG, WebP or AVIF) and the quality. If a form also has a file-size cap, turn on Also limit file size and type it.
- Press Resize, check the before and after view, then download each file or the whole batch as a ZIP.
Troubleshooting
- The result looks soft or blurry. It was enlarged. Upscaling spreads the existing pixels over a larger area and cannot invent detail. Start from the original file rather than a copy saved from a chat app, or switch off Allow upscaling. When you must enlarge, the Upscale image tool uses multi-step resampling with gentle sharpening, which looks cleaner than a plain resize.
- Faces look stretched or squashed. Stretch was selected and the shapes did not match. Use Fill & crop to trim instead, or Pad to add a border.
- Something important was cut off. Fill & crop keeps the centre. Open the crop editor on that file, frame the subject yourself, apply, then resize.
- The output is smaller than the size I typed. Fit inside keeps the whole picture within the box, so one side comes out shorter. Use Pad if you need the exact dimensions without cropping.
- The print came out the wrong size. Print at 100 percent or actual size, not fit to page. Also make sure the file is JPG or PNG, because only those two carry the DPI value here.
- A form says the DPI is wrong although the pixels are right. In pixel mode the tool does not set a DPI value. Run the file through Change DPI, which rewrites only the header and leaves the pixels alone.
- Transparent areas turned white. JPG has no transparency, so they are filled with the background colour. Choose PNG or WebP to keep them.
Resizing in your browser compared with upload-based resizers
Most online resizers send your file to a server, process it there and send a copy back. That works, but it means your photo exists on someone else's computer for a while under their retention policy. Here, decoding, resampling and encoding all run in your browser, and the page shows how many bytes left your device, which should be zero.
The trade-off is that your own device does the work. A large batch on an older phone takes longer than on a laptop, and the free limits of 50 files and 200 MB per file keep the browser responsive. Pro raises them to 500 files and 1 GB per file, and the installed app keeps working offline.