Change Image DPI

About Change Image DPI

Someone has asked you for a 300 DPI picture. This sets it, properly — the number is written into the file, not just shown on screen.

It also works the other way round: give a print size in cm or inches and it tells you how many pixels you need, or resizes the picture to match.

Free, no account, and the picture is never uploaded — everything happens in your browser.

What DPI actually is

This confuses almost everybody, and understanding it takes one minute.

A digital picture is a grid of pixels. It has no physical size at all until something prints it. DPI — dots per inch — is a number stored inside the file that tells a printer or a design program how densely to lay those pixels down.

The whole thing is one small sum:

inches = pixels ÷ DPI and pixels = inches × DPI

So a 3000 × 2400 photo:

• At 300 DPI prints at 10 × 8 inches (25 × 20 cm)
• At 150 DPI prints at 20 × 16 inches — same file, bigger and coarser
• At 72 DPI prints at 41 × 33 inches — same file, poster-sized and blurry

Every one of those is the same picture. Not one pixel changed. Only the instruction to the printer changed.

The two things this tool does

Keep every pixel, change the number. This is what "make it 300 DPI" almost always means. Nothing is re-encoded, so nothing is lost — the file is identical apart from the density field. Use this when your picture is already big enough and a form or a print shop just wants the tag to say 300.

Keep the print size, change the pixels. Ask for 10 × 8 inches at 300 DPI and you need 3000 × 2400 pixels. If your photo is smaller than that, pixels have to be invented and it will look soft. The tool warns you when that is about to happen instead of quietly doing it.

Why other tools do not really change it

Worth knowing, because it explains a common frustration.

A browser can redraw a picture, but when it saves one it always writes 96 DPI. So a tool that simply redraws and re-saves your photo has changed nothing: the file still says 96, and the print shop still says no.

This tool goes into the file's bytes and writes the density itself — a pHYs block for PNG, and the JFIF density fields for JPG. That is why the value survives when you check it in Photoshop, GIMP or Windows file properties.

How many pixels do you actually need?

| Print size | At 300 DPI | At 150 DPI |

|---|---|---|

| 4 × 6 in (10 × 15 cm) | 1200 × 1800 | 600 × 900 |

| 5 × 7 in (13 × 18 cm) | 1500 × 2100 | 750 × 1050 |

| 8 × 10 in (20 × 25 cm) | 2400 × 3000 | 1200 × 1500 |

| A4 (21 × 29.7 cm) | 2480 × 3508 | 1240 × 1754 |

| A3 (29.7 × 42 cm) | 3508 × 4961 | 1754 × 2480 |

300 DPI is the normal standard for printing. 150 DPI is fine for a poster you look at from a distance. 72 or 96 DPI is for screens only.

What you can do with it

• Meet a "300 DPI required" rule for a print shop, a publisher or a form
• Work out the biggest size a photo can be printed at before it looks soft
• Set a photo to an exact print size in cm or inches
• Prepare artwork for a T-shirt, a poster or a business card
• Check what DPI a picture already has — it is shown as soon as you open it

Features

• Reads the DPI already in the file and shows it
• Writes real DPI into PNG (pHYs) and JPG (JFIF) — not a fake label
• Retag without touching a pixel, so nothing is re-encoded and no quality is lost
• Or resize to an exact print size in cm or inches
• Works in DPI or dots per cm
• One-tap values for 72, 96, 150, 300 and 600
• Warns before inventing pixels, instead of silently making a soft picture
• 100% in your browser — nothing uploaded, no account

Good to know

• DPI does not make a picture sharper. If it is 600 pixels wide, it is 600 pixels wide. Tagging it 300 DPI just means it prints small and crisp rather than large and soft.
• PNG and JPG only. WebP, GIF and BMP have no standard place to store a DPI value.
• DPI and PPI mean the same thing here. Printers say DPI, screen software says PPI, and the number in the file is the same.
• If a form wants a file size rather than a DPI, use Compress Image to a Size.

Related tools

• Image Resizer — change the pixel size directly
• Compress Image to a Size — hit a file-size limit in KB
• Image Metadata — see everything else stored in the file
• Passport Photo Maker — official photo sizes, already at print DPI

How to use it

• Drop a PNG or JPG in — the DPI it already has appears straight away
• Choose what to change — just the DPI, or the print size
• Type the DPI you need, or tap 300
• For a print size: switch to the second option and enter the width in cm or inches
• Watch for the warning — if it says pixels must be invented, use a bigger photo or a lower DPI
• Press Set the DPI, then download the picture