Let’s cut straight to the chase: a 3.4 inch round TFT LCD 800x800 has an active area diameter of roughly 86.36 mm (since 3.4 inches converted to millimeters is 86.36 mm, given 1 inch = 25.4 mm). The overall module dimensions, including the bezel and FPC (flexible printed circuit) tail, typically measure about 92.0 mm x 92.0 mm in outline, with a thickness ranging from 1.2 mm to 2.5 mm depending on whether you’re looking at a bare glass panel or a fully assembled unit with backlight and touch overlay. The display resolution is 800x800 pixels, which gives a square pixel array within a circular aperture, so the pixel pitch works out to approximately 0.1079 mm per pixel (calculated as 86.36 mm / 800 pixels). This specific panel, the 3.4 inch round tft lcd 800x800, uses a MIPI DSI interface with 4 lanes, and its active area is circular, not square—meaning the 800x800 resolution is mapped to a round display, so the actual viewable area is a circle inscribed within that square pixel grid. The diagonal of the square pixel matrix is 86.36 mm, but the circle’s diameter is exactly that same value, so the effective display area is about 58.6 cm² (area of a circle: π x (43.18 mm)²).
Now, let’s dig deeper into the physical dimensions because “3.4 inch” is a nominal diagonal measurement, but for round displays, it’s the diameter. Industry standards for round TFTs often list the diameter as the “size,” so a 3.4-inch round display means the glass circle’s outer diameter is 86.36 mm, but the actual viewable image circle might be slightly smaller due to a black matrix border or sealant ring. For this specific model, the viewable diameter is typically 85.0 mm to 86.0 mm, with a tolerance of ±0.2 mm. The module outline, which includes the PCB or glass edge, can be as large as 95.0 mm x 95.0 mm if you factor in a metal frame or mounting tabs. The thickness varies: the glass-only version is about 1.2 mm (0.8 mm TFT glass + 0.4 mm polarizer), while a full assembly with white LED backlight (typically 6 to 9 LEDs in series) adds another 1.0 mm to 1.5 mm, bringing total thickness to 2.2 mm to 2.7 mm. If you add a capacitive touch panel (CTP), the thickness jumps to 3.5 mm to 4.0 mm.
The pixel density is another critical dimension-related metric. With 800 pixels across an 86.36 mm diameter, the PPI (pixels per inch) is about 235 PPI (800 / 3.4 inches). That’s sharp enough for most smartwatch, smart home, or instrument cluster applications. The sub-pixel layout is typically RGB stripe, with each sub-pixel measuring roughly 0.036 mm x 0.1079 mm (height x width). The aperture ratio (the percentage of each pixel that actually emits light) is around 55% to 65% for a standard a-Si TFT, which affects brightness—typically 300 to 500 cd/m² for indoor use, or up to 800 cd/m² with a high-brightness backlight option.
Let’s break down the key dimensional specs in a table for clarity:
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Nominal size (diameter) | 3.4 | inches | 86.36 mm |
| Active area diameter | 86.36 | mm | ±0.2 mm tolerance |
| Module outline (w/o FPC) | 92.0 x 92.0 | mm | Includes bezel |
| Module thickness (glass only) | 1.2 | mm | 0.8 mm glass + 0.4 mm polarizer |
| Module thickness (with backlight) | 2.2 – 2.7 | mm | Depends on LED count |
| Module thickness (with CTP) | 3.5 – 4.0 | mm | Glass + touch sensor |
| Pixel pitch | 0.1079 | mm | 800 pixels across 86.36 mm |
| PPI | 235 | pixels per inch | Sharp for smartwatch use |
| Active area | 58.6 | cm² | Circular area |
| FPC length | 20 – 30 | mm | Typically 0.3 mm pitch |
| FPC connector | 0.5 mm pitch, 40-pin | — | MIPI DSI 4-lane |
The FPC (flexible printed circuit) dimensions are just as important for integration. The tail length is usually 20 mm to 30 mm, with a width of about 10 mm to 15 mm, depending on the number of I/O lines. The connector is a 0.5 mm pitch, 40-pin ZIF type, which is standard for MIPI DSI interfaces. The FPC also carries the backlight LED connections (typically 2 pins for anode and cathode), plus touch controller lines if a CTP is integrated. The bend radius of the FPC is about 3 mm, so you can route it around tight spaces in wearables or smart home devices.
Optical dimensions also matter. The viewing angle for this panel is typically 80° / 80° / 80° / 80° (left/right/up/down) for IPS (in-plane switching) technology, which is common in round TFTs. The contrast ratio is around 800:1 to 1000:1, and the response time is 25 ms (typical) for gray-to-gray transitions. The color depth is 16.7 million colors (8-bit per channel), achieved via dithering or native 8-bit drivers. The backlight consumes about 150 mA to 200 mA at 3.3 V (typical), which translates to 0.5 W to 0.66 W power draw. The LED forward voltage is 3.0 V to 3.2 V per LED, with 6 LEDs in series, so the total backlight voltage is around 18 V to 19.2 V (boosted from 3.3 V input).
Mechanical mounting considerations: The module has four mounting holes (typically 2.0 mm diameter) at the corners of the square outline, spaced 80.0 mm apart (center to center). The bezel width is about 2.8 mm on each side (92.0 mm – 86.36 mm = 5.64 mm total, divided by 2). The glass thickness is 0.8 mm ± 0.1 mm, with a surface hardness of 6H (for the top polarizer) if it’s a standard version, or 7H if it’s an anti-glare coating. The weight of the module is approximately 15 g to 20 g for the glass-only version, and 25 g to 30 g with backlight and touch.
Let’s talk about the interface dimensions. The MIPI DSI interface uses 4 data lanes plus a clock lane, all differential pairs, so the FPC must route these with controlled impedance (typically 100 ohms differential). The physical layer voltage is 1.2 V for the MIPI signals, and the logic I/O voltage is 1.8 V or 3.3 V (configurable). The display driver IC (e.g., ILI9881C or similar) is usually COG (chip-on-glass) bonded, with a die size of about 8 mm x 4 mm, located at the bottom edge of the glass. The driver IC’s package adds about 0.3 mm to the thickness in that area, but the overall module thickness remains uniform because the backlight is designed to accommodate it.
For thermal management, the backlight LEDs generate heat, and the module’s thermal resistance is about 10°C/W (junction to ambient). The maximum operating temperature is -20°C to +70°C, and storage temperature is -30°C to +80°C. The glass itself has a coefficient of thermal expansion (CTE) of about 3.2 ppm/°C, so mounting in a plastic or metal bezel needs to account for that to avoid stress cracking.
Here’s a comparison table of the 3.4-inch round TFT versus other common round sizes:
| Size (inches) | Diameter (mm) | Resolution | PPI | Active Area (cm²) | Typical Use |
|---|---|---|---|---|---|
| 1.28 | 32.5 | 240x240 | 261 | 8.3 | Smartwatch |
| 1.54 | 39.1 | 240x240 | 218 | 12.0 | Wearable |
| 2.1 | 53.3 | 480x480 | 323 | 22.3 | Smart home |
| 3.4 | 86.4 | 800x800 | 235 | 58.6 | Instrument cluster |
| 4.0 | 101.6 | 720x720 | 179 | 81.1 | Dashboard |
The 3.4-inch round TFT is a sweet spot for applications that need a readable display without being too bulky. The 800x800 resolution gives a crisp image at typical viewing distances of 30 cm to 50 cm, and the 235 PPI is above the threshold for “retina” quality at that distance (about 200 PPI for 30 cm). The circular shape means you lose about 21.5% of the pixel area compared to a square 800x800 display (since the circle’s area is π/4 times the square’s area), but the round format is often preferred for analog gauges, speedometers, or smartwatch faces where a circular bezel is more aesthetic.
One more dimension to consider: the FPC’s exit point. On most round TFTs, the FPC exits from the bottom edge (at the 6 o’clock position), but some designs allow for side exit (3 o’clock or 9 o’clock) to accommodate different PCB layouts. The FPC’s bend radius is 3 mm, so you need at least that much clearance behind the display. The connector on the host PCB should be placed within 15 mm to 25 mm of the FPC exit point to avoid excessive strain. The FPC’s copper thickness is typically 0.5 oz (18 μm), with a total thickness of 0.2 mm including coverlay.
If you’re integrating this display into a product, you’ll also need to account for the gap between the glass and the bezel. The recommended mechanical tolerance is ±0.3 mm for the cutout in the housing, and the display should be mounted with a silicone gasket or foam tape to absorb vibration and provide dust sealing. The viewing angle of 80° in all directions means the display is readable from almost any angle, but the contrast ratio drops to about 10:1 at 80° off-axis, so it’s still usable for side viewing.
Finally, the electrical interface dimensions: The MIPI DSI signals run at up to 500 Mbps per lane (total 2 Gbps for 4 lanes), which is enough for 800x800 at 60 fps with 24-bit color. The pixel clock is about 38.4 MHz (800 x 800 x 60 Hz = 38.4 million pixels per second). The backlight driver IC (if separate) is typically a boost converter with a 2.5 mm x 2.5 mm QFN package, and the touch controller (if used) is a 3 mm x 3 mm QFN. The total PCB area needed for the display interface is about 30 mm x 20 mm on the host board.