Exactly two A4 sheets. It is also the size at which an ordinary photograph stops having enough pixels for 300 DPI, which makes this the one print size where the source matters more than the settings.
A3 at 300 DPI needs 3508 × 4961 pixels — about 17.4 megapixels. A 12-megapixel phone photograph does not have them. Neither does a 16-megapixel camera once you have cropped it.
That sounds like a problem and mostly is not, because DPI requirements fall with viewing distance. A3 is a poster size, looked at from a metre or more, and 150 to 200 DPI is entirely convincing at that range. A 12-megapixel photograph printed at A3 lands around 250 DPI, which is fine.
The number to be careful about is what happens after cropping. Crop half of a 12-megapixel photograph away and print the rest at A3 and you are at roughly 175 DPI — still acceptable on a wall, visibly soft in a portfolio someone is holding.
Two A4s, exactly
A3 is 297 × 420 mm, which is precisely two A4 sheets placed side by side along their long edge. The √2 proportion is what makes that work without any change of shape.
So an A4 design enlarges to A3 with nothing reflowed and nothing cropped — everything just gets bigger by a factor of the square root of two in each dimension. That is a rare property and worth exploiting: design once, print at whichever size the occasion needs.
It also means an A3 print folds neatly to A4 for posting, which is why so many maps and event programmes are A3.
Upscaling, honestly
If the source genuinely cannot reach a usable density, interpolation will not save it — bicubic and Lanczos average existing pixels and add no information, however sharp the result looks afterwards.
A super-resolution model generates plausible detail rather than averaging, and on photographic texture at A3 viewing distance that works well. It works badly on anything with text or fine geometry, where it invents confident and wrong detail. Know which you have before you enlarge it.
Yes. Resizing JPG, PNG, and WebP images is completely free with no sign-up, no watermark, and no file limit. Resizing runs entirely in your browser, so there is nothing for us to charge for. Pro plans add the AI tools, but everyday resizing stays free forever.
Are my images uploaded to a server?
No. OpusImg resizes images on your own device using WebAssembly and your browser's own canvas. Your files never leave your computer or phone — there is no upload, no cloud processing, and nothing stored on our side. That makes it safe for confidential documents, client work, and personal photos.
How do I resize an image to exact pixel dimensions?
Choose the "By dimensions" mode and type the width and height you need in pixels. Leave the aspect-ratio lock on to keep the image's proportions — change one field and the other follows automatically. Turn the lock off when you need an exact, possibly non-proportional canvas, such as a specific banner size.
What is the lock aspect ratio toggle for?
When the lock is on, the width and height stay proportional: setting the width to 1000px updates the height to match the original shape, so the picture is never squashed or stretched. When the lock is off, you can set width and height independently, which is useful for fitting an exact canvas even if it distorts the image slightly.
Which resampling algorithm should I pick?
Use Lanczos for photographs — it produces the sharpest, most natural result and is the default. Bicubic is a fast, smooth alternative handled by your browser. Choose Nearest neighbour only for pixel art, sprites, or icons where you want hard edges and no blending between pixels when scaling up.
What does the DPI setting do?
DPI (dots per inch) is metadata that tells print software how large to print the image. It does not change the pixel dimensions or how the image looks on screen — 72 DPI is standard for the web, while print shops usually want 150 or 300 DPI. We embed the value you choose into the file (PNG and JPG) so a print workflow reads the right density.
Can I resize for Instagram, YouTube, or other platforms?
Yes. The "By preset" mode includes 18 ready-made sizes — Instagram square, portrait, and story; Facebook and LinkedIn covers; X/Twitter headers; YouTube thumbnails and channel art; Pinterest pins; plus HD, QHD, and 4K. Pick a preset and your image is resized to the exact canvas that platform expects.
Will resizing reduce image quality?
Making an image smaller (downscaling) keeps it looking crisp — Lanczos and bicubic both average pixels cleanly. Making it larger (upscaling) can only invent so much detail, so very large enlargements look soft; for big upscales the AI upscaler in Pro reconstructs detail far better. We encode the result at high quality so the resize itself adds no visible compression artifacts.
Can I resize many images at once?
Yes. Drop a whole folder and apply one target to the batch. In percentage mode every image is scaled by the same percentage, so each keeps its own aspect ratio; in dimensions or preset mode every image is fitted to the same canvas. Results are bundled into a single ZIP for download, all processed on-device.
What formats can I resize?
You can drop in JPG, PNG, WebP, AVIF, GIF, BMP, TIFF, and HEIC. Each file is decoded in your browser, resized, and re-saved — JPG and HEIC come out as JPG, PNG/BMP/TIFF as PNG, and WebP/AVIF/GIF as WebP, so the result always opens everywhere. To change format deliberately, use the converter tool linked below.