Photo size and DPI calculator for print and digital. Convert pixels to centimeters, calculate DPI, and check aspect ratios.
Four tabs: standard photo sizes (passport, 2×3, 3×4, 4×6, ID-card), pixel ↔ cm conversion with DPI, DPI calculator (print-quality rating), and aspect-ratio tool (common ratios + resize).
Calculator information
📋 How to use this calculator
- Choose the type of calculation: standard photo sizes (wallet, 4x6, 5x7, 8x10, passport), pixel-to-inch/cm conversion, DPI calculator, or aspect ratio.
- For standard photo sizes, select the photo type needed (e.g., 2x2 inch for US passport, 4x6 inch for standard prints).
- For pixel-to-inch conversion, enter pixel dimensions (px) and print DPI (commonly 300 DPI for photos, 72 DPI for web).
- For the DPI calculator, enter the image resolution (pixels) and desired print size (in/cm) to see the quality rating: print quality (300+), good (200-299), acceptable (150-199), or low (<150).
- For aspect ratio, enter width and height to obtain the ratio (16:9, 4:3, 1:1, etc.) and resize suggestions.
- Tip: For high-quality photo prints, use a minimum of 300 DPI; for official US documents (passport), use 2x2 inch JPEG with a plain white or off-white background per State Department requirements.
🧮 Pixel, DPI, and Aspect Ratio Conversion
inches = pixel / DPI | cm = (pixel / DPI) x 2.54 | pixel = inches x DPI | DPI = pixel / inch | Aspect Ratio = width / height (simplified using GCD) | Megapixel = (width x height) / 1,000,000 | File_size_MB = (width x height x bit_depth) / (8 x 1,048,576)
- pixel = number of image dots (px)
- DPI = Dots Per Inch (print resolution)
- 1 inch = 2.54 cm
- GCD = Greatest Common Divisor for simplifying the ratio
- bit_depth = 24 for standard RGB, 32 for RGBA
- Megapixel = total pixels in millions
US standard photo sizes: 2x2 inch (passport, per US Department of State), 4x6 inch (standard print), 5x7 inch (portrait), 8x10 inch (large print). Print resolution 300 DPI = print quality, 150 DPI = acceptable, <100 DPI = pixelated.
💡 Worked example: Convert 1800x1200 px photo to a 4x6 inch print at the optimal DPI
Given:- Photo resolution: 1800 x 1200 pixels (2.16 megapixels)
- Target print size: 4 x 6 inch (standard print)
- Want to know the resulting DPI and aspect ratio
Steps:- Print DPI: 1200 / 4 = 300 DPI and 1800 / 6 = 300 DPI
- Quality: 300 DPI meets the print-quality standard
- Aspect ratio 1800:1200 = 1800/1200 = 1.5, or 3:2 (GCD 600)
- Target aspect ratio 4:6 = 2:3 (GCD 2), the inverse of 3:2 - requires rotation or crop
- Megapixel: (1800 x 1200) / 1,000,000 = 2.16 MP
- Maximum print size at 300 DPI: 1800/300 x 1200/300 = 6 x 4 inch
Result: DPI 300 (print quality), aspect ratio 3:2 (matches 4x6 print after rotation). A 2.16 MP file is sufficient for prints up to 6x4 inch @ 300 DPI.
❓ Frequently asked questions
What is DPI and why does it matter?
DPI (Dots Per Inch) is the number of ink dots or pixels per inch on a print. The higher the DPI, the sharper and more detailed the print. Industry standards: 300 DPI for high-quality photo prints/books, 150 DPI for newspapers/magazines, 72 DPI for web/screen display. DPI differs from PPI (Pixels Per Inch), which refers to pixel density on a digital screen, though the terms are often used interchangeably.
What are standard photo print sizes in the US?
Common US print sizes: wallet (2.5x3.5 inch), 4x6 inch (standard, 3:2 ratio matching most cameras), 5x7 inch (portrait, slightly cropped), 8x10 inch (large frame, 5:4 ratio requires cropping from 3:2), and 11x14 inch (poster). For official documents: US passport requires 2x2 inch with a plain white/off-white background; the photo must be taken within the last 6 months.
How do I determine the maximum print size from my photo?
Use the formula: max_size_inch = pixel_resolution / 300 (for print quality). Example: a 3000x2000 pixel photo prints up to 3000/300 = 10 inch x 2000/300 = 6.67 inch at 300 DPI. For posters viewed from a distance, 150 DPI is sufficient, so the print can be 2x larger. A 12 MP smartphone photo (4032x3024) can be printed up to letter size (8.5x11 inch) with good quality.
What is the difference between resolution, DPI, and PPI?
Resolution is the total number of pixels (e.g., 1920x1080) and does not specify a physical size. PPI (Pixels Per Inch) is the pixel density on a digital screen and determines visual sharpness. DPI (Dots Per Inch) is the number of ink dots per inch on a physical print. A 3000x2000 pixel photo can look sharp on a screen (3000 pixels is still many) but blurry if printed too large (low DPI because pixels are spread out).
What is aspect ratio and why does it matter?
Aspect ratio is the proportion of width to height of an image, written as width:height (e.g., 16:9 for HD video, 4:3 for older monitors, 1:1 for Instagram square, 3:2 for standard DSLR cameras, 9:16 for vertical TikTok/Reels). Aspect ratio matters when printing or displaying images so they are not distorted (stretched) or cropped. Match the image ratio to the output medium for the best result.
📚 Sources & references
Last updated: May 11, 2026