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Image Resolution Calculator
Calculate image resolution, megapixels, print size, and file size from pixel dimensions and DPI.
Calculator
Formula
W_px is the image width in pixels, H_px is the image height in pixels, DPI is dots per inch (print resolution), and B is the bit depth (e.g. 24 bits for standard RGB color). Megapixels is the total pixel count divided by one million. Print width and height are found by dividing pixel dimensions by DPI. Uncompressed file size is calculated by multiplying total pixels by bit depth, dividing by 8 to convert bits to bytes, then dividing by 1,048,576 to convert bytes to megabytes.
Source: ISO 12233 (Photography — Electronic still picture imaging — Resolution and spatial frequency responses); Adobe Systems DPI/PPI conventions.
How it works
Image resolution describes how much detail an image contains, and it can be expressed in several ways depending on context. The total pixel count — commonly called megapixels — is simply the width multiplied by the height in pixels, divided by one million. A 4000 × 3000 pixel image contains 12 megapixels regardless of how large it is displayed or printed. Megapixels matter most when comparing camera sensors or determining how large an image can be cropped while retaining detail.
DPI (dots per inch) — sometimes called PPI (pixels per inch) in digital contexts — governs print size. Dividing the pixel dimensions by the DPI yields the physical print dimensions. A 4000-pixel-wide image printed at 300 DPI produces a print 13.33 inches wide, while the same file printed at 150 DPI yields a 26.67-inch-wide print. Professional photo labs typically require 300 DPI, while large-format banners and billboards often use 72–150 DPI because they are viewed from farther away. Uncompressed file size is calculated by multiplying total pixels by bit depth (bits per pixel), then converting to megabytes. A 24-bit RGB image with 12 million pixels has an uncompressed size of approximately 34.3 MB. Formats like JPEG apply lossy compression to reduce this dramatically, while RAW files add metadata but compress losslessly or not at all.
Practical applications span many fields. Portrait photographers use resolution calculations to confirm a 20 × 30 inch canvas print will be sharp. Web designers verify that hero images will not degrade when scaled. Video production teams check that still frames from 4K footage have enough resolution for magazine covers. Archivists and medical imaging professionals use bit depth calculations to ensure sufficient tonal range for their data. Understanding how pixel count, DPI, and bit depth interact is fundamental to any image-based workflow.
Worked example
Suppose you have a photo from a 12 MP camera with dimensions of 4000 × 3000 pixels, and you want to print it at 300 DPI in standard 24-bit RGB color.
Step 1 — Megapixels: 4000 × 3000 = 12,000,000 pixels ÷ 1,000,000 = 12 MP.
Step 2 — Print dimensions (inches): Width = 4000 ÷ 300 = 13.33 inches. Height = 3000 ÷ 300 = 10.00 inches. This is slightly larger than a standard 13 × 10 inch print, so it is well within the 300 DPI threshold.
Step 3 — Print dimensions (cm): 13.33 in × 2.54 = 33.87 cm wide; 10.00 in × 2.54 = 25.40 cm tall.
Step 4 — Uncompressed file size: (4000 × 3000 × 24) ÷ (8 × 1,048,576) = 288,000,000 ÷ 8,388,608 ≈ 34.33 MB. A JPEG version of this image typically compresses to 3–6 MB depending on quality settings.
Step 5 — Aspect ratio: 4000 ÷ 3000 ≈ 1.333, confirming the classic 4:3 ratio common in many cameras.
Limitations & notes
This calculator computes uncompressed file sizes, which will be significantly larger than real-world JPEG, PNG (with compression), WebP, or camera RAW file sizes. JPEG images are typically 5–20× smaller due to lossy compression, and PNG uses lossless compression that varies with image content. The DPI value has no effect on digital display quality — it only affects print output size. Monitors display images based purely on pixel dimensions, and screen DPI (PPI) depends on the physical display. Additionally, this tool does not account for color profiles (sRGB vs. Adobe RGB), embedded metadata, or bit-packed formats that some cameras and scanners use. Very high-resolution images (over 500 MP) may require specialized software and tiling workflows that are beyond the scope of a single DPI-based print size calculation.
Frequently asked questions
What DPI should I use for high-quality photo printing?
300 DPI is the standard for professional-quality photo prints viewed at normal reading distance (roughly arm's length). Some photographers use 240 DPI for slightly larger prints with acceptable results. Large-format prints like posters viewed from a distance can look excellent at 100–150 DPI.
What is the difference between DPI and PPI?
PPI (pixels per inch) refers to the pixel density of a digital image or screen, while DPI (dots per inch) technically refers to the physical ink dots a printer lays down. In practice, the terms are used interchangeably when discussing image resolution for printing, and most software treats them as equivalent when calculating print dimensions.
How many megapixels do I need for a large print?
For a 300 DPI print at 16 × 20 inches, you need at least 4800 × 6000 pixels — about 28.8 MP. An 8 × 10 inch print at 300 DPI only requires 2400 × 3000 pixels (7.2 MP). Most modern smartphones with 12–50 MP cameras are sufficient for any standard print size up to about 24 × 30 inches.
Why is my actual image file much smaller than the calculated size?
The calculator shows the uncompressed (raw) size, which represents the maximum data the image could contain. JPEG compression removes redundant pixel data to reduce file size by 80–95%, and PNG uses lossless compression that is especially effective on images with large flat-color areas. RAW camera files include sensor metadata but use lossless or no compression, so they are closer to the uncompressed size.
Does changing DPI in photo editing software affect image quality?
Changing only the DPI metadata in software like Photoshop — without resampling — does not alter the actual pixel data or image quality at all. It simply changes the instructions sent to a printer about how large to print the image. Quality only changes if you resample (add or remove pixels), which should be done carefully using high-quality interpolation algorithms.
Last updated: 2025-01-15 · Formula verified against primary sources.