Print Size & DPI Calculator

About the Print Size & DPI Calculator

An image has pixels; a print has inches or centimetres; the resolution — pixels per inch, PPI, which printers usually call DPI — links them: inches = pixels ÷ PPI. Give any two and this calculator works out the third: how big an image prints (a 6000 × 4000 photo is 50.8 × 33.87 cm at 300 PPI), how many pixels a print needs (A4 at 300 PPI is 2480 × 3508), or what resolution an image gets on a print you have in mind, filling it or printed whole. You can also open an image to read its pixel size and the DPI written in the file.

Every answer comes with a quality rating, from photo quality at 300 PPI to poster use at 100, a table of how the image prints on A6 to A0, Letter, photo sizes and posters (each turned to match the image, with how much would be cut to fill it), the largest print at each quality level, bleed for print shops, and the resolution beyond which a viewer at your distance cannot see single pixels. Without a pass you see a free preview: the quality rating and the first rows of each table, with the answer and every other figure hidden.

Common uses

• Checking whether a photo is sharp enough for a canvas, a poster or a photo book page.
• Setting the pixel size of artwork for A4, Letter or a poster before you start.
• Preparing files with bleed for a print shop.
• Choosing the resolution for a sign or banner that is seen from across a room.

The formulas

One inch is 25.4 mm, and the resolution is the number of pixels in one inch, so:

• inches = pixels ÷ PPI — 6000 px at 300 PPI = 20 in
• pixels = inches × PPI — 8.27 in × 300 = 2480 px
• PPI = pixels ÷ inches — 1800 px over 6 in = 300 PPI

When the image and the print have different shapes, the resolution across and down differ. Printed whole, the image is scaled until both sides fit, leaving a blank band; filling the print, it is scaled until both sides are covered and the rest is cut off. The page shows both, and the share that would be cut.

DPI, PPI and the number in the file

PPI counts an image’s pixels per inch; DPI strictly counts a printer’s ink dots, but people and print shops use DPI for both when they size images. A JPG or PNG may carry a DPI value, such as 72 or 300: it is only a note about the intended size and does not change a single pixel, so two copies marked 72 and 300 DPI print the same at the same size. What decides sharpness is the pixels ÷ the print size. To change the pixels, use the Image Resizer.

How much resolution a print needs

300 PPI is the usual benchmark for photos and books: Lulu asks for images of 300 PPI (PDF creation settings) and Adobe calls 300 the benchmark for high-quality prints, with at least 100 for posters depending on size and distance (Adobe). The page rates 300 PPI and more as excellent, 150–299 as good, 100–149 as fair (posters seen from a distance) and less as low. Viewing distance decides the rest: normal 20/20 vision resolves about one minute of arc, so pixels cannot be seen above 1 ÷ (distance × tan 1′) PPI — about 291 PPI at 30 cm, 175 at 50 cm, 87 at 1 m and 29 at 3 m.

Bleed

Printers trim a sheet after printing, so a picture that should reach the edge has to carry on a little past it — the bleed, often 3 mm on each side. With bleed, Pixels for a print works out the pixels for the trimmed size plus the bleed on all four sides.

Limitations

• Pixel counts are rounded to the nearest pixel; round up by one if your printer must not get less than the resolution asked for.
• The quality rating is a guide: a soft, noisy or heavily compressed photo looks worse than its pixel count suggests, and a large print seen from far away needs fewer pixels.
• JPG and PNG files are measured from their own header; other images are opened with your browser’s decoder, so formats it cannot open (HEIC in many browsers, RAW) need the size typed in.
• In the free preview you see the quality rating and the first rows of each table; the answer, the other figures and copying unlock with a pass.

Privacy

Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.