How to Make a 300 DPI PDF From an Image
A printer or a form has told you the PDF must be 300 DPI, and you are hunting for the setting. There is a resolution setting here, but it only decides how big the page comes out: image to PDF at 300 DPI is arithmetic, not a checkbox, and whether you hit the number depends entirely on how many pixels your image has and how big it gets printed.
Choose the sheet, the margin and how the image sits on it — and see the print resolution you get.
🔒 Private by design: your images are converted in this browser tab and never uploaded.
What 300 DPI actually means
DPI describes a relationship between two things: the pixels in your file, and the size it gets printed at. Print one unchanged image twice, once as a postcard and once as a poster, and you get two wildly different DPI figures from the same file. That is why there is no switch for it.
Most image formats can store a resolution tag — the number sitting in an editor's Image Size box. That tag changes no pixels, and no printer is bound by it. When a print shop asks for 300 DPI, they open your file, count the pixels, and divide by the size you want printed. The formula is the whole story: DPI = pixel width ÷ printed width in inches.
The arithmetic, in both directions
Working forwards, here is what a full-bleed page needs at 300 DPI:
| Page | Printed size | Pixels needed at 300 DPI |
|---|---|---|
| A6 | 105 × 148 mm | 1240 × 1748 |
| A5 | 148 × 210 mm | 1748 × 2480 |
| A4 | 210 × 297 mm | 2480 × 3508 |
| US Letter | 8.5 × 11 in | 2550 × 3300 |
| A3 | 297 × 420 mm | 3508 × 4961 |
Add a margin and the image covers less of the sheet, so the bar drops: A4 with a 13 mm margin only needs about 2180 × 3208.
Working backwards stings. A 1200 × 1600 photo on A4 with no margin is limited by its width, so it is drawn 8.27 inches across — 1200 ÷ 8.27 is about 145 DPI. The same photo on A6 is drawn 4.13 inches across and reaches about 290 DPI. Not one pixel changed between those two results. Only the page did.
Check what your file can actually do
- Find the pixel dimensions. On Windows, right-click the file, then Properties and Details. On a Mac, select it in Finder and press Cmd-I, then open the More Info section — Quick Look with the Space bar shows the picture but not its pixel size. On an iPhone, open it in Photos and swipe up.
- Divide both numbers by 300. That gives the largest print size, in inches, that still holds 300 DPI. A 3024 × 4032 phone photo gives 10.1 × 13.4 inches.
- Compare that with the page. A4 is 8.27 × 11.69 inches, so that phone photo clears it comfortably — dropped straight onto A4 it lands at about 366 DPI.
Check the pixel count, never the file size. A big file can still be short on pixels — screenshots saved as PNG and lightly compressed scans both inflate fast — and a crisp 300 DPI scan can be smaller than a blurry phone photo. Phone photos of 12 megapixels or more clear A4 at 300 DPI, so it is worth checking before assuming you need a new file.
How to convert an image to PDF at 300 DPI
The converter above runs in your browser, so nothing is uploaded. There are two routes, depending on which is fixed: the page size or the resolution.
Route one: the page size is fixed. Your printer wants A4, so set page size to A4, orientation to match the image, fit to fit inside the page, and margin to none if it should fill the sheet. The DPI you end up with is the pixel width divided by 8.27 inches, or the pixel height divided by 11.69 — whichever comes out higher, because the image is scaled to fit and one edge always lands short of the sheet. The converter also reports the DPI each page actually achieved, so you can check before sending it. If the image is at least 2480 × 3508, that page is 300 DPI or better.
Route two: the resolution is fixed. Set page size to Match the image and resolution to 300 dpi. The page then comes out at exactly the physical size those pixels support. A 2480 × 3508 image produces a 210 × 297 mm page — a genuine A4 at 300 DPI. A 1200 × 1600 image produces a 101.6 × 135.5 mm page, which really is 300 DPI, just smaller than A4. The resolution control only appears in Match the image mode: on a fixed page size, the DPI falls out of the geometry and there is nothing to choose.
Worth knowing before a paid run: JPG and PNG go into the PDF as their own bytes, with no decoding and no re-compression, so the printer gets the pixels you started with. A sideways photo is turned upright by a page transform, so its bytes still survive; only the rarer mirrored EXIF orientations force a decode and re-encode, and the tool says so when that happens. HEIC, TIFF, WebP, AVIF, GIF, BMP and SVG have no place in the PDF format, so each is decoded and re-encoded once — JPEG at high quality for photographs, PNG for graphics and transparency. SVG is rasterised at twice its declared size rather than staying vector.
Why upscaling to 300 DPI does not help
Typing 300 into an editor's resolution box does one of two things. Either it rewrites the metadata tag and changes nothing at all, or it resamples: invents new pixels by interpolating between the ones you have. Interpolation cannot recover detail that was never captured. A 1200-pixel-wide image resampled to 2480 gives you twice the pixels, the same real detail, and softer edges. It usually shows.
This converter never resamples upwards. It embeds your pixels and describes where they sit on the page, so a bigger page simply spreads the same pixels thinner — which is precisely what the DPI figure is reporting. If you genuinely need more pixels, the fix is upstream: rescan at a higher resolution, re-export from the camera's original file, or ask the sender for that instead of a messaging-app copy. Print scans often arrive as TIFF, including multi-page ones, and the multi-page TIFF converter handles those.
When 150 DPI is genuinely fine
300 DPI is the convention for things held at reading distance: photo books, brochures, anything with small type. Further away, the requirement relaxes fast, because your eye cannot resolve what the paper holds.
- 150 to 200 DPI is fine for posters and roll-up banners read from a metre or more.
- 100 to 150 DPI is fine for internal documents, drafts and home inkjet on plain paper — uncoated paper spreads ink, and the extra pixels vanish into the fibres.
- Large-format printers often specify 100 to 150 DPI at full size on purpose, because an A0 sheet at 300 DPI would be an unmanageable file.
If the 300 DPI requirement came from a web form rather than a person, it is often copied boilerplate. It costs nothing to ask what size it is being printed at.
Two last checks before you send it
A 300 DPI A4 page carries around 8.7 million pixels, so these PDFs get large. If the result is too big to email, there is a guide on what drives that size and which settings to change without dropping below your target resolution.
Then check the colour. A print that comes back duller or warmer than expected is a separate problem from resolution — why image colours look different in a PDF covers it. Print one page yourself at 100% scale before committing to a run. Camera RAW files (.dng, .cr2, .nef, .arw) are not supported here, so export them to JPEG or TIFF from your camera software first.
Doing this on an iPhone
The iOS app converts photos entirely on the device, with no connection needed, but it resizes images to 1200 pixels wide and re-encodes them as JPEG to keep the PDFs small. Spread across a full A4 page that works out at roughly 145 DPI. That is fine for receipts, forms and everyday admin, and not fine for anything a print shop will run. If the 300 DPI requirement is real, open this page in your phone's browser instead, because the browser converter uses the photo's full pixel count.
