# What is my screen’s PPI?

Computes a screen’s pixel density in pixels per inch (PPI) from its resolution and diagonal size, with the dot pitch, the screen’s width and height, and its aspect ratio.

- Page: https://www.acalculator.org/logic/ppi-calculator
- JSON spec: https://www.acalculator.org/logic/ppi-calculator.json
- Version: b251cfbefc4c

## Default answer

Example with the default inputs (Horizontal pixels 1,920, Vertical pixels 1,080, Diagonal size 24 in): A 24 in screen at 1,920 × 1,080 pixels has 91.79 pixels per inch.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| w | Horizontal pixels | The screen’s width in pixels, for example 1920. |
| h | Vertical pixels | The screen’s height in pixels, for example 1080. |
| d | Diagonal size | The screen size, measured corner to corner across the visible area. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| ppi | Pixel density | √(width² + height²) ÷ diagonal in inches. |
| pitch | Dot pitch | The distance between pixel centres: 25.4 mm ÷ PPI. |
| diagonalPixels | Diagonal in pixels | √(width² + height²). |
| screenWidth | Screen width | Diagonal × width ÷ diagonal in pixels. |
| screenHeight | Screen height | Diagonal × height ÷ diagonal in pixels. |
| aspect | Aspect ratio | Width : height in lowest terms. |
| megapixels | Total pixels | Width × height, in millions. |

## Method

PPI = √(width² + height²) ÷ diagonal (inches); dot pitch = 25.4 ÷ PPI mm; screen width = diagonal × width ÷ √(width² + height²).

## Assumptions

- Pixels are square, so the same density holds across and down the screen.
- The diagonal is the visible area, corner to corner; screens with rounded corners are measured as the full rectangle.

## Worked examples

1. w = 1,920, h = 1,080, d = 0.6096 gives ppi = 91.787799, pitch = 0.276725, diagonalPixels = 2,202.90717, aspect = 16:9, megapixels = 2.0736. Source: NIST SP 811, Appendix B (1 in = 25.4 mm exactly), https://www.nist.gov/pml/special-publication-811.
2. w = 2,556, h = 1,179, d = 0.155448 gives ppi = 460. Source: Apple, iPhone 15 Technical Specifications (6.1-inch display, 6.12 in as a rectangle; 2556 × 1179 pixels at 460 ppi), https://support.apple.com/en-us/111831 (460 ppi, rounded to a whole ppi, so within 0.5 ÷ 460).
3. w = 3,840, h = 2,160, d = 0.6858 gives ppi = 163.178309, screenWidth = 0.597727. Source: NIST SP 811, https://www.nist.gov/pml/special-publication-811.

## FAQ

### How do I calculate PPI?

Find the diagonal in pixels with Pythagoras, √(width² + height²), and divide by the diagonal in inches. A 24 in 1920 × 1080 monitor has √(1920² + 1080²) = 2,202.9 pixels across its diagonal, so 2,202.9 ÷ 24 = 91.79 PPI.

### What is a good PPI?

It depends on how far away you sit. Desktop monitors at arm’s length are often 90 to 165 PPI (24 in full HD is 92, 27 in 4K is 163); phones held closer are often 400 to 500 PPI. Higher PPI makes text look sharper at the same distance.

### Is PPI the same as DPI?

Not quite. PPI counts the pixels a screen shows per inch; DPI counts the ink dots a printer lays down per inch. People often say DPI for both. To size an image for print, use the DPI calculator.

### Why does my phone say 460 ppi when I get 459.9?

Makers round the density and quote a rounded diagonal. Apple quotes the iPhone 15 as a 6.1 in screen, but 6.12 in when measured as a full rectangle; 2556 × 1179 pixels over 6.12 in gives 459.9, which Apple rounds to 460 ppi.

### What is dot pitch?

The distance between the centres of two neighbouring pixels: 25.4 mm ÷ PPI. A 91.79 PPI monitor has a dot pitch of 0.2767 mm. A smaller pitch means a finer screen.

### How do I find a screen’s width and height in inches?

Scale the diagonal by each side’s share of the pixel diagonal: width = diagonal × horizontal pixels ÷ diagonal pixels. A 27 in 3840 × 2160 screen is 27 × 3,840 ÷ 4,405.8 = 23.53 in wide.

## Sources

- Apple, iPhone 15 Technical Specifications: 6.1-inch (diagonal) display, 6.12 inches as a standard rectangle; 2556-by-1179-pixel resolution at 460 ppi. https://support.apple.com/en-us/111831 (retrieved 2026-10-01)
- NIST Special Publication 811, Appendix B: 1 in = 2.54 cm = 25.4 mm exactly. https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-01)