How to use this calculator
Choose your calculation first. Screen pixel density uses the native resolution and visible screen diagonal. Pixels to print size uses existing image dimensions and a target PPI. Print size to pixels finds the dimensions needed for a chosen physical size and PPI; photo, A4, A3 and US Letter presets fill the size for you. PPI at a print size checks an existing image at your intended dimensions. Choose inches, centimeters or millimeters; changing units converts the physical measurements. No image upload is required.
The math behind it
Screen PPI = √(pixel width² + pixel height²) ÷ diagonal in inches Print dimension (in) = pixels ÷ PPI Required pixels = round up(print dimension in inches × target PPI) Horizontal PPI = pixel width ÷ print width in inches Vertical PPI = pixel height ÷ print height in inches Megapixels = pixel width × pixel height ÷ 1,000,000 1 in = 2.54 cm = 25.4 mm
A worked example
A 1920 × 1080 screen with a 24-inch diagonal has about 91.79 PPI. A 3000 × 2400 image contains 7.2 megapixels and prints at 10 × 8 inches (25.4 × 20.32 cm) at 300 PPI. A4 at 300 PPI needs 2481 × 3508 pixels when both dimensions round up to meet the target; some software rounds to the nearest pixel and gives 2480 pixels for the width.
Assumptions & precision
Screen geometry assumes a flat display and square pixels. All physical sizes and PPI values must be positive. Pixel dimensions must be whole numbers up to 1,000,000,000, with total pixels and calculated magnitudes limited to 10¹⁵. Required pixel dimensions round up, allowing only floating-point noise at integer boundaries. Image and print proportions differing by more than 0.1% trigger a crop/border/stretch notice; horizontal and vertical densities are shown separately. Print presets exclude bleed, margins and printer scaling. Changing units preserves physical lengths to 15 significant digits; changing modes retains inputs. Density displays up to 2 decimals and physical dimensions and megapixels up to 3. The preview shows image proportions, not actual print size. No resampling, image enhancement or compressed file-size estimate is performed.
Common questions
Does changing image DPI create detail?
No. Changing a resolution label changes the intended print size; it does not add original pixels or recover missing detail.
What is the difference between DPI and PPI?
PPI counts pixels per inch in an image or display. DPI describes a printer’s ink-dot density. Printers can use multiple dots to reproduce one image pixel, so a printer’s advertised DPI is not a target image PPI and there is no universal direct conversion.
How many pixels do I need for a 300 DPI print?
For image preparation, the value usually meant is 300 PPI. Multiply each dimension in inches by 300. A 6 × 4 inch print needs 1800 × 1200 pixels. Choose Print size to pixels to calculate other dimensions.
Is 300 PPI always necessary?
300 PPI is a common starting point for close-viewed prints. Large prints viewed farther away may use a lower density. Source detail, paper and printing method affect the outcome, so follow your print provider’s requirements rather than treating one number as a quality guarantee.
Why do horizontal and vertical PPI differ?
Your image and intended print have different proportions. Stretching to fill changes the image shape. Cropping can fill the print while preserving shapes, or fitting the whole image leaves borders. The calculator reports each axis without choosing a crop for you.