How to Stretch an Image for Printing — Inches, cm, mm & DPI Guide

Print shops ask for a 4×6 inch print. Forms ask for a photo five centimeters wide. Your file only knows pixels. Stretching to a physical size means picking a unit, setting DPI so those ruler numbers become a pixel canvas, then exporting a bitmap that still measures in pixels on disk. DPI is the conversion layer — leave it wrong and the download either looks soft at print size or balloons into a huge pixel box for the same inches on the ruler.

What the unit pills actually change

In ImageStretcher, px writes straight into the output width and height. Switch to in, cm, or mm and the same fields become physical measurements; the stretcher multiplies by DPI (dots per inch) to get the pixel box it paints. The mental formula for inches is simple: width in inches × DPI = width in pixels. A four-inch side at 300 DPI becomes 1200 px; at 72 DPI the same four inches collapse to 288 px. Centimeters and millimeters take one extra step through inches — ten centimeters is about 3.94 inches, so at 300 DPI you land near 1181 px on that edge. Change the unit pill and the field labels rewrite to match; the canvas underneath stays a pixel grid. Export is always PNG, JPEG, or WebP. Physical units are a control language for the brief, not a different file format.

Worked sizes people actually type

Photo labs still speak inches. A classic 4×6 at 300 DPI is 1200×1800 px — common enough that shops expect that density when they say “four by six” without further notes. European print orders lean on centimeters: a 10×15 cm frame at the same 300 DPI is roughly 1181×1772 px, close to the same print feel with metric paperwork. ID photos, packaging stickers, and product labels often arrive in millimeters because a one-millimeter miss shows on a small trim; dialing width and height in mm keeps the brief’s precision instead of rounding through inches first. In each case you stretch so the bitmap fills that physical box; you are not inventing a new paper template, only matching the numbers the shop or form already wrote down.

Picking a DPI that matches the brief

The tool defaults to 300, the density most print briefs assume when they name inches or centimeters and stop there. Screen-first layouts — banners that only live on a monitor, mockups that never hit a press — often sit nearer 72 or 96, where a given physical width needs fewer pixels and the file stays light. 150 is the middle ground when you want something denser than screen defaults without committing to a full print master. Custom DPI shows up when a shop, passport office, or print PDF names an exact value that is not on the preset row. Wrong DPI does not change how wide the ruler reads in the UI; it changes how many pixels that ruler distance becomes, which is what the download actually contains. Set 72 when the brief meant 300 and the print softens; set 600 by accident and the same four inches explode into a canvas you did not budget for.

Steps in ImageStretcher

1. Open the free stretcher on the home page
2. Drop in a PNG or JPEG — processing stays in the browser
3. In the size controls, choose a unit pill: px, in, cm, or mm
4. When the unit is physical, set DPI with a preset (72, 96, 150, 300) or type a custom value
5. Enter width and height in that unit; unlock aspect ratio when the axes need to move independently
6. Preview on canvas until the fill looks intentional, then download PNG, JPEG, or WebP

Landings that open the right unit

Jump straight into stretch in inches, stretch in cm, or stretch in mm when the brief already names a physical unit. Prefer stretch in pixels when the target is a pure pixel box and you do not want DPI in the way; stretch with DPI opens the same tool with print-oriented density in focus so you can tune the conversion first.

The templates gallery lists every unit entry alongside platform and paper seeds. When the brief is a named sheet such as A4 or Letter rather than a freehand physical box, start from stretch for printing or the longer walkthrough in how to stretch for printing.

Freehand physical size vs a Print template

Typing 5.2 cm × 7.8 cm — or any odd millimeter box from a form — is what the unit pills are for: freehand width and height in the unit the brief uses, with DPI filling in the pixel math. Named paper sizes (A4, Letter, tabloid) already have finished pixel seeds under Print templates, so grabbing those is faster than reinventing the sheet in inches when the brief already names the paper. The printing guide covers that paper path without repeating the unit math above.

Privacy note

Files never leave your device on ImageStretcher. Client proofs sized in centimeters, inch prints for a lab, or millimeter ID crops stay on this machine — no account upload of the original.

Open the home page tool when you have a physical size to hit and want the pixel math done for you.