Understanding Image Resolution: DPI, PPI, and Megapixels

Resolution Jargon Explained

DPI, PPI, megapixels — these terms get thrown around constantly in photography and photo restoration, often incorrectly. Understanding what they actually mean helps you make better decisions when scanning old photos, preparing images for printing, and choosing upscale settings.

This guide cuts through the confusion and gives you practical knowledge you can use immediately.

Pixels: The Building Block

Every digital image is made of pixels — tiny squares of color arranged in a grid. A pixel is the smallest unit of a digital image. Zoom in far enough on any photo and you'll see the individual squares.

The total number of pixels determines the image's resolution. More pixels means more detail, larger possible prints, and bigger files.

Pixel Dimensions

When someone says an image is "4000 x 3000 pixels," they mean the image is 4000 pixels wide and 3000 pixels tall. Multiply these together to get the total pixel count: 12,000,000 pixels, or 12 megapixels.

PPI: Pixels Per Inch (Screen Resolution)

PPI (pixels per inch) describes how densely pixels are packed on a screen or in a digital image. Higher PPI means finer detail.

Screen PPI

Standard desktop monitor: 72-96 PPI
Retina/HiDPI display: 220-460 PPI
Smartphone screen: 300-500+ PPI

At 96 PPI, a 4000 x 3000 image fills about a 42 x 31 inch area on screen. On a 460 PPI Retina display, the same image fills a much smaller physical area but looks incredibly sharp.

What PPI Means for Your Photos

PPI is metadata — it doesn't change the actual pixel count. A 4000 x 3000 image is the same image whether labeled as 72 PPI or 300 PPI. What changes is how large the image is intended to be displayed or printed. At 72 PPI, that image is "meant" for a 55 x 42 inch display. At 300 PPI, it's meant for a 13 x 10 inch print.

The actual pixels are identical.

DPI: Dots Per Inch (Print Resolution)

DPI (dots per inch) describes how many dots of ink a printer places per inch of paper. It's a printer specification, not an image specification — though the terms DPI and PPI are often used interchangeably (incorrectly).

Print DPI Guidelines

72 DPI: Screen resolution. Looks terrible in print.
150 DPI: Minimum for acceptable print quality. Fine for large posters viewed from a distance.
300 DPI: The standard for photo printing. Sharp and detailed at normal viewing distance.
600 DPI: Professional print quality. Overkill for most photographs but valuable for fine art reproduction.

The Critical Calculation

To determine what size you can print a photo at 300 DPI:

Print width = pixel width / 300 Print height = pixel height / 300

Examples:

Pixel Dimensions	At 300 DPI	Max Print Size
1200 x 1800	4" x 6"	Standard snapshot
2400 x 3000	8" x 10"	Standard frame
3300 x 4200	11" x 14"	Large frame
4800 x 6000	16" x 20"	Poster

Megapixels: Total Pixel Count

Megapixels (MP) measure total pixel count in millions. A 12 MP camera produces images that are approximately 4000 x 3000 pixels.

Megapixels and Quality

Megapixels measure quantity (how many pixels) not quality (how good each pixel is). A 12 MP image from a modern smartphone may look better than a 20 MP image from an early DSLR because pixel quality depends on sensor size, lens quality, and image processing.

What You Need

2 MP: Enough for a 4x6 print at 300 DPI
5 MP: Enough for an 8x10 print
8 MP: Enough for an 11x14 print
12 MP: Enough for a 16x20 print
24+ MP: Large format printing and heavy cropping

How This Applies to Photo Restoration

Scanning Old Photos

When scanning old prints for restoration, DPI determines how many pixels your scanner captures per inch of the physical photo.

A 4x6 print scanned at 300 DPI produces a 1200 x 1800 image (2.2 MP). Good enough for a same-size reprint.
A 4x6 print scanned at 600 DPI produces a 2400 x 3600 image (8.6 MP). Good for enlarging to 8x12 at 300 DPI print quality.

Rule of thumb: Scan at the DPI that matches your largest intended print size. If you want to double the print size, scan at double the DPI (e.g., 600 DPI to enlarge a 4x6 to 8x12).

AI Upscaling and Resolution

When you scan at the highest DPI your scanner supports and still need a larger image, AI upscaling fills the gap. The upscale tool uses AI to add genuine detail when enlarging:

2x upscale: Doubles each dimension (4x total pixels). A 2400 x 3600 scan becomes 4800 x 7200.
4x upscale: Quadruples each dimension (16x total pixels). A 1200 x 1800 scan becomes 4800 x 7200.

AI upscaling is not the same as simply stretching pixels. The AI generates new detail based on what it infers from the existing image. The result looks dramatically better than traditional bicubic upscaling.

Restoration and Resolution

When you run a photo through the restore tool or face restore tool, the output resolution matches the input resolution. The AI improves quality (removes damage, sharpens detail, corrects color) but does not change the pixel dimensions. Upscale as a separate step after restoration if you need a larger image.

Practical Recommendations

For Scanning Family Photos

Scan at 300 DPI for same-size reprints. Scan at 600 DPI if you plan to enlarge. Scan small photos (wallet size, passport photos) at 1200 DPI because they need the most enlargement.

For Printing Restored Photos

Aim for 300 DPI at your target print size. If your image falls short, use the upscale tool to add resolution.

For Screen Display Only

Nearly any resolution works for screens. Even a 1000 x 750 image looks fine on most monitors. But if there's any chance you'll want to print later, scan and save at higher resolution now.

Check Your Photo's Resolution

Before printing or sharing, check your image dimensions:

Windows: Right-click the file, Properties, Details tab
Mac: Select the file, press Cmd+I, or open in Preview and check Tools > Show Inspector
Phone: Open the photo details/info in your camera app

If the resolution is too low for your intended use, upload to the upscale tool — it's free and takes seconds. For damaged photos, start with the restore tool first. See all AI tools.