Image measurement method
Why image pixels and DPI are different
Pixels describe the image data that exists. PPI or DPI describes how software may map those pixels to a physical size. Changing density metadata does not create missing detail.
On this page
Pixels are the finite image grid
A 1200 × 800 image contains 960,000 pixels regardless of the density label attached to it. That grid determines the maximum source detail available for a screen display or a printed reproduction.
Resampling changes the pixel grid by adding or removing samples. It can make an image fit a required canvas, but it cannot reconstruct visual information that was not captured in the original.
Try it with a safe sample
Change print density, keep the same pixels
The sample starts at 96 PPI. Updating its density to 300 PPI changes its printed size, not its detail.
- Download the 96 PPI sample.
- Open the print-density tool, enter 300 PPI, and save a copy.
- Reopen it in the inspector. The pixel dimensions should stay the same.
| Pixel dimensions, before and after | 480 × 300 px |
|---|---|
| Source print density | 96 PPI |
| Output print density | 300 PPI |
| Print size at 300 PPI | 1.6 × 1 in |
This changes the JPEG density fields without resampling. It cannot make a small image sharper.
How we checked this
Synthetic samples. No personal files. Results can vary with browser encoders.
Tested: · Chromium 149.0.7827.55
Download measurements and file hashes · Testing method
Sample files are free to reuse under CC0.
Density metadata maps pixels to a physical size
At 300 PPI, 1200 pixels correspond to four inches. At 150 PPI, the same 1200 pixels correspond to eight inches. The pixels did not change; the physical interpretation changed.
Some files have conflicting or missing density fields. A reliable workflow should display the source reading separately from a newly selected output target.
| Pixel width | Density label | Calculated print width |
|---|---|---|
| 1200 px | 300 PPI | 4 inches |
| 1200 px | 150 PPI | 8 inches |
| 1200 px | No reliable density | Cannot be calculated without an assumed target |
Choose the right action for the actual problem
Use a pixel-resize workflow when a portal or screen requires a specific canvas. Use a density update when a print workflow already has enough pixels but needs a stated physical interpretation. Use neither as a substitute for checking whether the source contains sufficient visual detail.
A printed result also depends on printer, paper, ink, scaling settings, and viewer behaviour. The calculated size is a useful planning value, not a guarantee of physical output quality.
Avoid the common “300 DPI” misunderstanding
Labeling a 600-pixel-wide image as 300 PPI makes its nominal print width two inches; it does not turn it into a high-detail large print. A viewer can still enlarge it, but the enlarged pixels may look soft.
Aftermath distinguishes the metadata setting from a true resample so the selected action is visible in the result.
Desktop tools and commands
Use a local tool