PPI Calculator
Enter the horizontal and vertical pixel resolution along with the diagonal screen size to compute the PPI (pixels per inch), total pixel count, and aspect ratio. Useful for comparing monitors, phones, and tablets.
Pixel density (PPI, pixels per inch) is the single number that tells you how sharp a display will look at a given viewing distance. A 27-inch 4K monitor (163 PPI) shows the same image with about twice the detail of a 27-inch 1440p monitor (109 PPI), and you'll see the difference immediately on text and fine UI elements. But once PPI crosses a perceptual threshold for your viewing distance — about 220 PPI at 18 inches for normal vision — adding more pixels stops being visible and starts being expensive.
This calculator takes width, height, and diagonal screen size and computes PPI using the Pythagorean theorem. It also reports the total pixel count and the aspect ratio. Use it to compare displays before buying, evaluate whether a phone qualifies as "Retina" by Apple's threshold, or check what diagonal a custom display would need to hit a target density.
The math behind "Retina" is straightforward: at the typical viewing distance for that device class, individual pixels should be smaller than the eye can resolve (about 1 arc-minute). For phones held 10-12 inches away, that's roughly 320 PPI. For tablets at 15 inches, ~265 PPI. For laptops at 20 inches, ~220 PPI. For monitors at 24 inches, ~180 PPI. For TVs at 8 feet, ~50 PPI is enough — which is why 4K TVs at sensible sizes look great even though their PPI looks low on paper.
Inputs
Results
Pixels Per Inch
91.8
Total Pixels
2,073,600
Aspect Ratio
16:9
Dot Pitch
0.277 mm
Display Details
| Metric | Value |
|---|---|
| Resolution | 1920 x 1080 |
| Aspect Ratio | 16:9 |
| PPI | 91.8 |
| Total Pixels | 2,073,600 |
| Megapixels | 2.07 MP |
| Screen Width | 20.9 in |
| Screen Height | 11.8 in |
| Dot Pitch | 0.277 mm |
Formula
How to use this calculator
- Enter the resolution (e.g., 1920 × 1080) and the diagonal in inches (e.g., 24).
- Read the PPI, total pixel count, and aspect ratio.
- Compare against the "Retina threshold" for your viewing distance — anything above is wasted pixels for that distance.
- For laptops/monitors, also consider DPI scaling. Windows and macOS upscale UI elements on high-PPI displays so text doesn't become microscopic.
- For video work or photo editing, color accuracy and gamut matter more than PPI alone.
- For e-ink readers (Kindle, ReMarkable), PPI is the dominant quality factor — paper is ~300 PPI equivalent.
Worked examples
Comparing a 27-inch 1440p vs 4K monitor
**Scenario:** You're choosing between a 27" 2560×1440 ($300) and a 27" 3840×2160 ($550) monitor. **Calculation:** 1440p: √(2560² + 1440²) / 27 = 2,938 / 27 = 109 PPI. 4K: √(3840² + 2160²) / 27 = 4,406 / 27 = 163 PPI. At a 24-inch viewing distance (typical desk), Retina threshold is ~143 PPI. **Result:** The 4K monitor exceeds the Retina threshold at desk distance; the 1440p falls below it. For text-heavy work (coding, writing), 4K is visibly sharper. For gaming, 1440p is easier to drive and the difference matters less. The price-to-perceived-quality ratio depends heavily on use case.
Why phone PPI seems so high
**Scenario:** Your phone has 460 PPI but your 4K TV only has 68 PPI. Is the phone really 7× sharper? **Calculation:** Hold phone at 10" → Retina threshold ~344 PPI. The phone (460 PPI) comfortably exceeds it. Sit 8 feet from the TV (96") → Retina threshold ~36 PPI. The TV (68 PPI) also comfortably exceeds it. **Result:** Both displays look "Retina" at their normal viewing distances. Raw PPI is misleading without distance context. A 50" 1080p TV at 10 feet looks as sharp as your phone at arm's length — pixels-per-degree-of-visual-arc is the real metric.
Choosing an external 4K monitor for a MacBook
**Scenario:** Your MacBook has a 224 PPI Retina display. You want an external 4K monitor that doesn't look "soft" by comparison. What size should you pick? **Calculation:** Target 224 PPI on a 3840×2160 display. Diagonal pixels = √(3840² + 2160²) = 4,406. Required diagonal = 4,406 / 224 ≈ 19.7 inches. **Result:** A 4K display matches MacBook Retina density only at ~20 inches diagonal — which doesn't really exist for monitors. The 5K (5120×2880) Studio Display at 27" hits 218 PPI, very close to laptop Retina. For 4K monitors at 27-32", PPI is 138-163 — sharp enough for most users but not visually matching a Retina laptop side-by-side.
When to use this calculator
**Use PPI calculation when you need to:**
- **Compare displays before buying**: spec sheets show resolution and size separately, leaving you to compute density. - **Verify "Retina" claims**: many displays marketed as "Retina-class" are well below the actual threshold for their viewing distance. - **Plan a multi-monitor setup**: mixing high-PPI and low-PPI monitors causes scaling headaches in Windows; matching PPI is more important than matching resolution. - **Choose an e-reader or tablet for reading**: 250+ PPI starts feeling paper-like; below 220, text aliasing is visible. - **Set DPI scaling correctly**: in Windows and macOS, the OS uses PPI to decide whether to scale UI 100%, 125%, 150%, or 200%. - **Spec a custom industrial / signage display**: viewing distance dictates required PPI, which dictates resolution at a given size.
**Reality checks:**
- **Anti-aliasing closes the gap**: subpixel rendering (Mac CoreText, ClearType) makes 100 PPI text feel sharper than the math suggests. - **HDR and color gamut matter more for video/photo**: a 90 PPI panel with great color outperforms a 200 PPI panel with poor color for creative work. - **Refresh rate and response time matter for gaming**: 240 Hz at 1080p feels smoother than 60 Hz at 4K for competitive play. - **Pen/touch displays have additional density specs**: digitizer LPI (lines per inch), separate from display PPI, affects drawing precision.
Common mistakes to avoid
- Computing PPI as resolution / diagonal directly. The diagonal is in *physical* inches, but the pixel count needs the diagonal in pixels (Pythagorean theorem) first.
- Comparing PPI across viewing distances. A 460 PPI phone and a 50 PPI TV both qualify as "sharp" because viewing distances differ by ~10x.
- Assuming higher PPI always means a better display. Color accuracy, contrast, refresh rate, and panel type often matter more.
- Buying a 4K laptop expecting it to look 4× sharper than 1080p. The OS scales UI elements, so what you actually get is sharper text and graphics at the same logical size, not 4× the screen real estate.
- Ignoring DPI scaling issues. Mixing a 4K monitor with an older 1080p one creates inconsistent UI sizes across screens.
- Confusing PPI with refresh rate (Hz) or response time (ms). PPI is about spatial detail, not temporal smoothness.
- Comparing CRT and LCD PPI. CRT phosphor dots and LCD subpixels behave differently; raw PPI isn't apples-to-apples.
Frequently Asked Questions
Sources & further reading
- VESA Display Stream Compression (DSC) specifications — Video Electronics Standards Association
- Apple Developer — Designing for High-Resolution Displays
- ISO 9241-303 — Visual display requirements — International Organization for Standardization