DPI explained: what resolution to scan, print and export at

Dpi causes more confusion than any other number in imaging, and the reason is simple: most people think it is a property of a picture, and it is not. It is a ratio between a picture and a physical thing, and until the physical thing exists, the number is a note somebody wrote in the margin.

Once that clicks, every dpi question you will ever have becomes arithmetic you can do in your head.

What dpi actually is

Dots per inch is pixels divided by inches. That is the whole definition.

A picture that is 3,000 pixels wide has no dpi. Print it 10 inches wide and it is 300 dpi. Print it 30 inches wide and it is 100 dpi. Put it on a screen and dpi is meaningless, because the picture has no physical size until something displays it, and that depends on the display.

Every image format can store a dpi number, and in a JPEG it lives in the JFIF segment or the EXIF, and in a PNG in a chunk called pHYs. That stored number changes nothing about the pixels. It is a suggestion: “if you print me, this is the size I had in mind.”

This is why the answer to “how do I make my image 300 dpi” is almost always “you already have, or you cannot”. If the pixel count is enough for the size you want, it is 300 dpi at that size. If it is not, changing the label does nothing, and resampling invents pixels that were never photographed.

The one calculation

Work backwards from the finished physical size.

Pixels needed = inches × dpi

• A4 at 300 dpi: 8.27 × 300 = 2,480 wide, 11.69 × 300 = 3,508 tall. That is 8.7 megapixels.
• US Letter at 300 dpi: 2,550 by 3,300.
• A 6 by 4 inch print at 300 dpi: 1,800 by 1,200. That is 2.2 megapixels, which is why a ten year old camera prints fine at postcard size.
• A business card, 3.5 by 2 inches at 300 dpi: 1,050 by 600.

And the other direction, to find out what you can do with what you have:

Inches available = pixels ÷ dpi

A 12 megapixel phone photo is about 4,000 by 3,000. At 300 dpi that is 13.3 by 10 inches, comfortably bigger than A4. At 150 dpi it is 26 by 20 inches, which is a decent poster, because nobody reads a poster from six inches away.

That last point is the one professionals use and amateurs miss: the right dpi depends on viewing distance. 300 is the standard for something held in the hand. A poster on a wall is fine at 150. A billboard is often 20.

What to use, by job

Printing a photo or a document: 300 dpi. This is the number that exists because it is roughly where the eye stops resolving detail at reading distance. Going higher on paper is almost never visible.

Scanning text you want to read and keep: 300 dpi. A page of ordinary type at 300 dpi is legible, searchable after OCR, and about 2,480 by 3,508 pixels a page.

Scanning for OCR, small type, or anything you will enlarge: 400 to 600 dpi. OCR works better with more pixels per letter, and a 600 dpi scan of a small original is what lets you crop into it later. The file is four times the size of a 300 dpi one, which is usually worth it once.

Scanning photographs and film: 600 dpi for prints, much more for negatives. A 6 by 4 print at 600 dpi gives 3,600 by 2,400, which is enough to reprint at the original size with room to crop. Film is tiny, so it needs 2,400 dpi or more to be worth scanning at all.

A PDF page that will be read on a screen: 150 dpi. An A4 page becomes 1,240 by 1,753 pixels, which is sharp on any laptop and about a quarter of the file size of 300.

Screens and the web: forget dpi, count pixels. Decide how many pixels the picture will occupy and supply about twice that for high-density displays. A 600 pixel wide column wants a 1,200 pixel image. Nothing about dpi enters into it.

Why PDFs make this confusing

A PDF page is measured in points, seventy-two of them to the inch, and its text and vector shapes have no resolution at all: they are instructions, drawn as finely as whatever is displaying them can manage. That is why you can zoom into a PDF of a letter for ever and the type stays crisp.

Two places dpi does come into a PDF.

The images inside it. A photo placed on an A4 page at full width needs about 2,480 pixels across to be 300 dpi there. Fewer, and the printer enlarges it. This is the usual reason a document looks fine on screen and disappointing on paper.

Turning pages back into pictures. Because a page is instructions, converting a PDF to images means running those instructions at a resolution you choose, which is exactly the calculation above: A4 at 150 dpi is 1,240 by 1,753, at 300 dpi it is 2,480 by 3,508. Any tool that does not ask you has chosen for you. Does converting to PDF lose quality covers the other half of that question.

Going the other way, a good image-to-PDF tool reads the dpi stored in your file when you ask for a page the size of the picture. A 300 dpi scan then becomes a page the size of the thing that was scanned, instead of a page three times too big, which is what happens when a tool assumes 96 for everything.

Scanners, printers and the numbers they quote

Two more places the word appears, both worth a sentence so the specifications stop being mysterious.

A scanner quotes an optical resolution and an interpolated one. The optical figure, often 1,200 or 4,800 dpi, is what the sensor actually measures. The interpolated figure, sometimes 19,200, is the software inventing pixels between them, and it is marketing. Only the optical number is real, and for paper documents you will never approach it.

A printer quotes something much larger, such as 4,800 by 1,200 dpi. That is ink droplets, not pixels. An inkjet builds each pixel out of many tiny dots of a few fixed colours, so it needs far more dots than the picture has pixels. This is the one place where the distinction between dpi and ppi is genuinely useful: the printer wants roughly 300 pixels per inch from you, and lays down thousands of its own dots to make them.

A screen quotes its own pixels per inch, and this is why a picture that looked crisp on an older monitor looks soft on a new laptop: the screen now packs twice as many pixels into the same space, so the same image occupies half the physical width unless it is scaled up, and scaling up is where the softness appears. The formats behind your photos has more on how this plays out when choosing what to save in.

The mistakes worth knowing

Thinking dpi is quality. A 72 dpi label on a 6,000 pixel image does not make it low quality. Clients and print shops ask for “300 dpi files” when they mean “enough pixels for the size”, and the honest answer is the pixel count.

Resampling up to reach a number. Setting a 1,000 pixel image to 300 dpi at A4 makes the software invent 2,480 pixels from 1,000. It is sharper than nothing and it is not detail. The detail was never captured.

Scanning at 1,200 dpi because more is better. For an ordinary printed page, above about 600 dpi you are recording the texture of the paper and the halftone dots of the printing, not the content, and the file is enormous.

Forgetting the crop. If you will use a quarter of the frame, you need four times the resolution over the whole picture. Work out the dpi of the part you will keep, not of the original.

Assuming screens are 72 dpi. They have not been for twenty years. A modern laptop is around 200 and a phone can be over 400, which is why images that looked fine in 2010 look soft today.

The short version

Dpi is pixels divided by inches, so it only means something once you know the finished size. Work out the size, multiply by 300 for print or 150 for a screen, and compare that with the pixels you have. Scan at 300 for documents, 600 for OCR or anything you will enlarge, and stop worrying about the number stored in the file: it is a note, not a measurement.

The steps, in short

1. Decide the finished physical size. How big will it actually be, in inches or centimetres? A4 is 8.27 by 11.69 inches. A 6 by 4 print is 6 by 4.
2. Multiply by the dpi you need. 300 dpi for print, 150 for a screen document. A4 at 300 dpi is 8.27 x 300 by 11.69 x 300, which is 2,480 by 3,508 pixels.
3. Check what you have. Compare that with the pixel dimensions of your picture. If yours is smaller, it will be enlarged and will look soft; if it is much larger, you are only wasting file size.
4. Scan or export at that number. Set the scanner or the export dialogue to the dpi you worked out, at the size of the original. Scanning a small original that will be enlarged needs proportionally more.

Questions

What does dpi mean?

Dots per inch: how many pixels are packed into each inch of the finished physical thing. It is a ratio, so it only means something once a physical size exists. On a screen, where nothing has a physical size until it is displayed, what matters is the pixel count.

Is dpi the same as ppi?

In everyday use, yes. Strictly, ppi is pixels per inch in an image and dpi is ink dots per inch on a printer, and a printer lays several dots down per pixel. Nobody outside a print shop keeps them apart, and every dialogue that says dpi means ppi.

Does changing the dpi of an image change its quality?

No, not on its own. Changing the number stored in the file without resampling changes nothing about the pixels; it only changes how large the picture claims it wants to be printed. Quality only changes if the tool resamples, which is a different checkbox.

What dpi should I scan a document at?

300 dpi for normal text you want to read and keep. 400 to 600 if you are going to run OCR on it, if the type is very small, or if the original is a photo you may want to enlarge. Above 600 for ordinary paper mostly captures the texture of the paper.

Why is my 300 dpi photo blurry when printed?

Because dpi is relative to size. A 600 by 400 pixel image labelled 300 dpi is a 2 by 1.3 inch print. Ask for it at A4 and it has to be enlarged six times, which is where the blur comes from. The pixel count is what limits you, not the label.

What dpi should a PDF page be?

The page itself has no dpi: it is measured in points, 72 to the inch, and text and shapes are drawn as sharply as the display allows. Dpi only matters for the images inside it, and for the resolution you pick when turning pages back into pictures.

Is 72 dpi still the standard for the web?

No, and it never really was. Screens now range from about 96 to over 400 pixels per inch. For the web, work in pixels and ignore dpi entirely; for a phone, plan on supplying roughly twice the pixel dimensions of the space the picture occupies.