Screen size, PPI, refresh rate and frame rate — the four most commonly confused display parameters, explained with definitions, formulas, worked examples and typical values. Plus a "ZJWY view" on using them as acceptance criteria and fault-layering tools on the repair bench.
A display has four core parameters: screen size, PPI, refresh rate and frame rate. They are the four most commonly confused figures — and the four most commonly dressed up in marketing language. This article clears them all up, and sets the groundwork for the deeper topics that follow.
Common misconception: that "inches" refers to the screen's length or width.
Correct: screen size = the diagonal length of the display area, in inches (1 inch = 2.54 cm), excluding the bezel.
d = √(L² + W²) ÷ 2.54 (L and W are the physical length and width of the display area in cm; d is in inches)
Example: a phone screen measuring 15.5 cm by 7.2 cm →
d = √(15.5² + 7.2²) = √292.09 = 17.09 cm → 17.09 ÷ 2.54 = 6.73 inches, i.e. the familiar "6.7-inch screen".
💡 At the same diagonal size, different aspect ratios (16:9 vs 21:9) give different actual display areas.
Definition: PPI = Pixels Per Inch. The higher the number, the finer the image and the crisper the text.
Key distinction: PPI ≠ resolution. Resolution is the total pixel count (e.g. 1920×1080); PPI describes how densely those pixels are packed.
PPI = √(W² + H²) ÷ d (W and H are the resolution width and height in px; d is the screen diagonal in inches)
Example: 6.1 inches at 2532×1170 (iPhone 15) → √(2532² + 1170²) = 2789 → 2789 ÷ 6.1 ≈ 457 PPI.
At a normal viewing distance the human eye can barely detect pixel structure once PPI exceeds 300.
| Device | Typical PPI | Notes |
|---|---|---|
| Phone | 300 – 500 | Flagships generally above 400 |
| Computer monitor | 100 – 150 | A 27-inch 4K panel is about 163 PPI |
| TV | 80 – 120 | Viewing distance is long, so high PPI is unnecessary |
Definition: how many times per second the screen updates, in Hz.
Effect: determines how smooth scrolling, gaming and video feel.
Common values: 60Hz (baseline), 90 / 120Hz (high refresh), 144Hz (gaming).
⚠️ Refresh rate is a hardware capability — the highest update frequency the panel physically supports, independent of content. Even when playing 30 fps video, a 120Hz screen still refreshes 120 times per second (repeating frames).
Experience difference: 60Hz is fine day to day, but fast scrolling shows clear smearing; 120Hz feels smooth and noticeably improves touch response in games.
Definition: how many frames per second the video or game generates, in fps.
Common values: 24 fps (cinema), 30 fps (ordinary video), 60 / 120 fps (high-frame video and games).
| Parameter | Subject | Determined by | Can it exceed the other? |
|---|---|---|---|
| Refresh rate | Screen hardware | Panel physical capability | Frame rate can be higher, but the screen cannot show the extra frames |
| Frame rate | Content / software | Camera, game engine performance | Can exceed the refresh rate (needs VRR to match) |
Three combinations:
VRR (variable refresh rate, e.g. FreeSync / G-Sync): dynamically matches screen refresh to frame rate, eliminating tearing and stutter at the source.
| Parameter | Meaning | Unit | Higher is better? | Typical values |
|---|---|---|---|---|
| Size | Diagonal length of the display area | inch | Choose as needed | Phone 6 – 7″, TV 55 – 85″ |
| PPI | Pixel density (sharpness) | PPI | Yes, with diminishing returns | Phone > 300, monitor > 100 |
| Refresh rate | Screen updates per second | Hz | Yes, affects smoothness | 60 / 90 / 120 / 144Hz |
| Frame rate | Content frames per second | fps | Yes, but must match refresh rate | 24 / 30 / 60 / 120 fps |
Remember: parameters are just tools — real-world experience is the final standard.
For ordinary users these four parameters guide a purchase. For screen repair technicians they are a basis for acceptance and a basis for fault layering. The most common misjudgements on the bench come precisely from confusing them with one another.
1. Verification order after cover-glass, flex or full-assembly replacement. First check resolution — a blurry image or fuzzy font edges usually points to a resolution or timing mismatch, not to a "bad panel". Then check whether refresh rate reaches the original machine's tiers — an unusable high-refresh mode or flicker at high refresh is most often tied to flex pinout definitions, the DDIC, or panel specification mismatch. PPI is rarely an acceptance item on its own, but it explains a customer's "why does it look soft".
2. Resolution and PPI set the magnification your inspection system needs. Take 500 PPI: pixel pitch is roughly 25.4 ÷ 500 ≈ 51 µm, and a single sub-pixel is only about 17 µm — at that scale neither the naked eye nor an ordinary magnifier can resolve structure. Microscopic imaging is required to determine the true cause of bright dots, dark dots or line defects. This is exactly why sub-pixel-level judgement needs metallurgical or electronic microscopes, and it is the physical justification for "look carefully before you change equipment".
3. Refresh rate feeds directly into rework yield. A high refresh rate compresses per-frame charging time, and insufficient TFT charging amplifies process variation. Bonding pressure, temperature profile shape and platform flatness can all end up surfacing as "fine at low refresh, flickering at high refresh" or local Mura. That is why blade leveling, temperature-curve setup and pressure-sensitive paper measurement are not optional extras — they are fundamentals for reworking high-refresh models.
4. When a customer says "the screen lags", do not open the device yet. First separate a frame-rate problem (content, software, game settings) from a refresh-rate problem (hardware capability, system refresh setting, power-saving limits). Confusing the two is the most common reason for a wasted teardown.
5. Specification mismatch creates phantom faults. Fitting an assembly of the wrong specification onto a mismatched mainboard can produce flicker, a black screen, or loss of high-refresh tiers. That is not a broken panel but a resolution / refresh-rate / flex pinout mismatch — checking model and pinout before dispatch is far cheaper than reworking afterwards.
In short: size decides whether it fits, PPI decides how crisp it looks, refresh rate decides how smooth it feels, and frame rate decides whether the content keeps up. Get those four separate and half the diagnostic work is already done.
← Back to Support