What Is DPI and Does It Matter for Screens
DPI, dots per inch, controls how large a fixed number of pixels prints on paper. It does not affect how an image looks on a screen at all, since a screen just displays the pixels it is given at whatever size the layout tells it to, completely ignoring any DPI value stored in the file. DPI only matters the moment ink meets paper: a print job, a photo lab order, or a passport photo application that checks the file's metadata before accepting it, all of which read that stored number directly.
Quick answer: DPI sets how large a fixed number of pixels prints on paper and has no effect on how sharp an image looks on a screen. Divide the pixel width by the DPI to get the print width in inches, so 1800 pixels at 300 DPI prints 6 inches wide.
A worked example
Take an image that is 1800 pixels wide. At 300 DPI, the standard for a sharp printed photo, that same 1800 pixels prints at 6 inches wide, since 1800 divided by 300 equals 6. Tag that identical file at 72 DPI instead, the old standard for screen resolution, and the pixel count has not changed at all, but the file now claims to print at 25 inches wide, since 1800 divided by 72 is 25. Same pixels, same on screen appearance, wildly different printed size, purely because of one metadata number.
How to check the DPI of an image
The most direct way to check an image's DPI is the Image DPI Converter. It reads the DPI stored in a JPG or PNG and shows it in the results table before you download anything, so you can tell whether the file already meets a form or a print shop's requirement. The file stays on your device while it is read.
- Open the Image DPI Converter and drop in your JPG or PNG.
- Read the first number in the DPI column of the results table. In a row that reads 150 DPI, then 300 DPI, the 150 is the value stored in your file and the 300 is the value the tool would write.
- If the first number is followed by the word assumed, the file stores no DPI the tool can use, and the 72 DPI convention is shown in its place.
- Leave the file alone if the stored number is already the one you need. Download a copy only when you want a new DPI written in.
Why a screen ignores DPI completely
A monitor or phone screen has a fixed grid of physical pixels, and a browser or image viewer maps an image's own pixels onto that grid directly, at whatever display size the page's layout specifies in its own units, not the units DPI is measured in. The DPI value sits in the file's metadata unused for this purpose; nothing about how a browser renders a JPG or PNG reads it. This is the single most common DPI misunderstanding: setting an image's DPI to 300 does nothing to make it look sharper or more detailed on a screen, because pixel count, not DPI, is what determines on screen sharpness.
When DPI actually matters
Print is the one place DPI genuinely controls something real. A print driver or a photo lab's own software reads the DPI value to decide how large to physically print a given number of pixels, which is exactly the calculation in the worked example above. Get the DPI wrong on a file headed to print, and the physical output size is wrong even though the pixel data itself is perfectly fine, which is why passport photo services, some print shops, and print-ready templates specifically ask for a file tagged at 300 DPI rather than just a pixel size.
300, 150 and 72: what the common numbers mean
| DPI | Typical use |
|---|---|
| 300 | Standard for sharp printed photos and documents, including passport photos |
| 150 | Acceptable for a larger print viewed from further away, like a poster |
| 72 or 96 | Historic screen resolution standard; irrelevant to how sharp an image looks on a modern screen |
72 and 96 DPI persist mostly out of habit, inherited from decades old screen hardware, rather than reflecting anything about current displays, which vary enormously in their own actual pixel density. A file's DPI tag being 72 does not mean it looks worse on screen than the same file tagged 300; the pixel data is identical either way, and only the print calculation changes.
Why so many images have no DPI set at all
A camera, a screenshot tool, or a browser saving an image often does not bother writing a DPI value into the file at all, since it has no bearing on how the image displays digitally. That is not a corrupted or broken file; it just has nothing in that particular metadata field, which only becomes a problem the moment something downstream, most commonly a print job or an application form, specifically checks for a DPI value and rejects a file that does not have one. Checking a file's current DPI first, before assuming it needs fixing, avoids doing unnecessary work on a file that was already fine.
DPI versus PPI, and why both terms get used
Strictly, DPI (dots per inch) describes a printer's output, how many ink dots it lays down per inch, while PPI (pixels per inch) describes an image file's own pixel density. In practice the two terms get used interchangeably almost everywhere, including inside most image editing software and most passport photo or print requirements, since a 300 PPI image printed on a 300 DPI printer is exactly the calculation this guide has been describing throughout. Do not worry about the terminology distinction for practical purposes; a rule asking for 300 DPI and a rule asking for 300 PPI mean the same thing here, and both describe the same pixels-to-inches relationship.
Working out print size or pixels needed ahead of time
The same relationship between pixels, physical size and DPI works in any direction, which is useful before a file even exists. Given a pixel size and a DPI, the calculator works out the physical print size that combination produces. Given a target physical size and a DPI, such as needing a 4 x 6 inch print at 300 DPI, it works out the exact pixel dimensions to scan, photograph or export at to hit that target cleanly, which is useful for planning a scan resolution or an export setting before running the job, rather than finding out afterward that the file came out the wrong physical size.
Setting DPI without losing pixels
Changing an image's DPI value does not have to change its pixel dimensions. The DPI Converter writes a new DPI into a JPG or PNG's own metadata directly, leaving the pixel data untouched by default, which is the right move when a form just wants to see 300 in the file's metadata and does not actually need a different resolution. Turning on the optional resample setting instead recalculates the pixel dimensions so the image keeps the same physical print size at the new DPI, which is a different operation for a different situation, typically when the printed size itself, not just the metadata, needs to change.
Pixel sizes for common prints
| Print size | Size at 300 DPI (pixels) | Size at 150 DPI (pixels) |
|---|---|---|
| 4 x 6 in | 1200 x 1800 | 600 x 900 |
| 5 x 7 in | 1500 x 2100 | 750 x 1050 |
| 8 x 10 in | 2400 x 3000 | 1200 x 1500 |
| A4 (210 x 297 mm) | 2480 x 3508 | 1240 x 1754 |
A 4000 x 3000 phone photo has enough pixels for an 8 x 10 inch print at 300 DPI once it is cropped to that shape, while a 1200 pixel wide image saved from a web page covers only 4 inches at the same density. The calculator labels anything under 150 DPI as low for print, 150 to 299 as OK, and 300 or more as print quality.
Where the number is kept inside the file
A JPG records its DPI in its JFIF header, and many camera photos carry a second copy in their EXIF data. The DPI Converter writes the JFIF value and also updates the EXIF resolution fields when the photo has them, so programs that read either place see the same number. A PNG keeps the figure in a small chunk called pHYs, counted in pixels per meter rather than per inch: 300 DPI is saved as 11,811 pixels per meter, which converts back to 299.9994 and reads as 300. The converter accepts JPG and PNG files only.
Resampling the worked example
Take the 1800 pixel file from the start of this guide, tagged 72 DPI, and change it to 300 with resampling on: to keep its 25 inch print width it becomes 7500 pixels wide, an enlargement of just over four times that adds pixels but no detail. The converter refuses a resample that would pass 100 megapixels or 16,384 pixels on a side, and leaves the pixels alone on a file with no saved DPI, since its print size was never defined.
Next: Passport Size Photo in Pixels, Inches and mm by Country