Count the pixels in the source
The dimensions 6000 × 4000 describe a grid of 24 million pixels. They do not, by themselves, describe a twenty-inch print or any other physical size. A smaller copy sent through another application may have a different grid, even if it shows the same scene. Read the file you will actually use.
For a rotated photograph, use the dimensions in its displayed orientation. A portrait image may be stored with an orientation instruction that changes which edge is presented as width. The important pair is the one that corresponds to the image you see and intend to print.
Divide pixels by PPI
Physical inches equal pixels divided by pixels per inch. At 300 PPI, 6000 ÷ 300 gives a width of 20 inches, and 4000 ÷ 300 gives a height of 13⅓ inches. Multiplying by 2.54 converts these values to 50.8 × 33.8666… cm. Display rounding does not change the underlying dimensions.
At 240 PPI, the same source corresponds to 25 × 16⅔ inches. At 150 PPI, it corresponds to 40 × 26⅔ inches. These are different distributions of the same image data. None of the calculations adds detail to the file.
| Example PPI | Size of a 6000 × 4000 image |
|---|---|
| 300 | 20 × 13⅓ inches |
| 240 | 25 × 16⅔ inches |
| 150 | 40 × 26⅔ inches |
Work backward from a frame
If the frame size is already fixed, divide the retained pixels by the physical dimensions instead. A 3000 × 2000 image at 6 × 4 inches gives 500 PPI in both directions. The equal values confirm that the matching 3:2 image has been scaled uniformly.
When the ratios differ, choose a crop or borders before reading the final PPI. Dividing the full original width by one frame edge and the full original height by the other can produce two different values. That is a sign that the original does not fit the frame by one uniform scale, not permission to stretch it.
Judge the image as well as the number
PPI is useful evidence, but it is not a quality certificate. A file can have many pixels and still show motion blur, missed focus, compression damage, or an earlier poor enlargement. Paper, printing method, processing, and viewing distance also affect the result.
Compare a representative part of the image at the intended physical scale when the print matters. Include fine detail and a smooth area in the test, because they reveal different weaknesses. Decide whether the result serves the actual use instead of treating one PPI threshold as a universal rule.
Use the result to make one decision at a time
First settle the composition, then the physical photo size, then the paper layout. Save the original and export a separate result. If the chosen size looks too demanding for the source, try a smaller print or a different composition before assuming that a new resolution label will solve the problem.