What are the dimensions of a 3.18 inch 128x64 COG LCD?

If you’re sizing up a 3.18 inch 128x64 COG LCD for your next project, the first thing you need is the exact dimensions. The module typically measures 84.0 mm (width) x 44.0 mm (height) x 8.0 mm (thickness), with a viewing area of 73.4 mm x 38.8 mm. The active area, where the pixels actually light up, is slightly smaller at 70.7 mm x 38.8 mm. These numbers aren’t pulled from thin air—they’re based on the standard mechanical specs of the common 3.18 inch 128x64 cog lcd display used in industrial and embedded systems. The COG (Chip-on-Glass) construction means the driver IC is bonded directly to the glass, which shaves off a few millimeters compared to older COB (Chip-on-Board) designs. This makes the module a solid fit for tight enclosures, but you’ll want to double-check the mounting holes—they’re usually spaced at 79.0 mm x 39.0 mm center-to-center, with a diameter of 3.2 mm for M3 screws. The overall thickness includes the PCB backplane, which is typically 1.6 mm thick FR4 material, plus the glass stack-up of about 2.8 mm and the backlight layer at 3.6 mm. That 8.0 mm total can vary by ±0.2 mm depending on the manufacturer, so always grab a datasheet before committing to a bezel design.

Let’s break down the pixel-level details because that’s where the real engineering lives. The 128x64 resolution gives you 8,192 individual pixels, each with a dot pitch of 0.52 mm x 0.52 mm and a dot size of 0.48 mm x 0.48 mm. The gap between pixels is just 0.04 mm, which is tight enough to make text and graphics look crisp without noticeable bleed. The contrast ratio hits about 2000:1 in transmissive mode with the backlight on, but if you’re using it in reflective mode (like a calculator), that drops to around 10:1 under good ambient light. The viewing angle is rated at 6 o’clock direction, meaning the optimal viewing angle is from below the panel, but you’ll get usable visibility up to 60 degrees left, right, and above, and 40 degrees below. The response time is around 200 ms at room temperature, which is typical for STN (Super Twisted Nematic) LCDs—don’t expect video-grade refresh rates, but it’s fine for menu systems and data displays. The operating temperature range is -20°C to +70°C, and storage can go from -30°C to +80°C. If you’re pushing into colder environments, the response time will slow down to about 500 ms at -20°C, so plan for that.

Now, COG vs. COB—why does it matter for dimensions? The COG process eliminates the need for a separate driver IC board, which reduces the module’s footprint by roughly 15-20% compared to a comparable COB display. For this 3.18 inch unit, the driver IC is the ST7565R or NT7534, both of which are mounted directly on the glass using anisotropic conductive film (ACF). This bonding method adds only about 0.2 mm to the glass thickness, but it also means the flex tail (the flat flex cable) is soldered directly to the glass. The tail is typically 20 mm wide and 30 mm long, with a 0.5 mm pitch for the 20-pin interface. That tail adds no extra width to the module but does require clearance in your enclosure—usually about 5 mm beyond the edge of the glass for the tail bend radius. The PCB mounting holes are on the backplane, not the glass, so the glass itself is slightly smaller than the PCB. The glass dimensions are roughly 75.0 mm x 40.0 mm, leaving a 4.5 mm border on each side for the PCB tabs that hold the mounting screws. If you’re designing a custom bezel, you’ll need to account for that 4.5 mm overhang.

Let’s talk backlight options because they affect both dimensions and performance. The standard backlight is a white LED edge-lit design with 4 LEDs in series, each rated at 3.0V to 3.2V forward voltage and 20 mA current. The LED strip is mounted on the bottom edge of the light guide plate, which adds 3.6 mm to the overall thickness. You can also get yellow-green or blue backlights, but the white version gives you the best luminance at about 250 cd/m² (nits) when driven at full current. The power consumption for the backlight alone is 0.24W (4 LEDs x 3.0V x 20 mA), and the LCD logic draws about 0.1W at 3.3V, so total system power is around 0.34W. That’s efficient enough for battery-powered devices if you use a PWM dimming scheme to drop the backlight to 50% duty cycle. The light guide plate is made of PMMA (acrylic) and is about 1.0 mm thick, which is why the backlight layer adds that 3.6 mm—it’s the sum of the LED strip, the LGP, and the diffuser film stack. If you need a thinner module, you can opt for a transflective version that doesn’t require a backlight, but that drops the active area to 70.7 mm x 38.8 mm and the overall thickness to 4.4 mm. The trade-off is that transflective displays need ambient light to be readable, so they’re not ideal for dark environments.

For interface and pinout, the 3.18 inch COG LCD uses a 20-pin FPC with a 0.5 mm pitch. The pin functions are standard: VDD (3.3V), VSS (GND), CS (chip select), A0 (data/command), RST (reset), SCK (serial clock), SDA (serial data), and LED+ (backlight anode), LED- (backlight cathode). The remaining pins are for contrast adjustment (V0) and optional negative voltage (VOUT). The SPI interface runs at up to 10 MHz, which gives you a full frame refresh rate of about 12 Hz if you’re updating all 8,192 pixels. That’s fine for static or slow-changing data, but for animations, you’ll want to use page mode writes to update only the changed areas. The driver IC supports both 68-series and 80-series parallel interfaces, but the SPI mode is the most common because it only uses 4 wires (CS, A0, SCK, SDA) plus power. The contrast voltage is generated internally via a charge pump, so you only need to feed it a 3.3V supply and a 10kΩ potentiometer on the V0 pin to adjust the bias. The typical V0 voltage is around 10V to 12V, which is why the module has that extra thickness—the charge pump capacitors need space on the PCB.

Let’s look at mechanical tolerances because they matter in production. The PCB outline is usually specified with a tolerance of ±0.2 mm, and the mounting hole positions are ±0.1 mm. The glass thickness is 2.8 mm ±0.1 mm, and the backlight assembly adds 3.6 mm ±0.15 mm. If you’re stacking multiple modules or placing them in a tight enclosure, you’ll need to account for these variations. The flex tail has a minimum bend radius of 3 mm, so don’t fold it tighter than that or you’ll crack the ACF bond. The connector on the tail is a 0.5 mm pitch ZIF socket with a locking tab—common part numbers like FH12-20S-0.5SH from Hirose or XF2M-2015-1A from Omron. The mating connector on your PCB should have a 0.5 mm pitch and a height of 2.0 mm to align with the display’s mounting plane. The overall weight of the module is about 18 grams, which includes the glass, PCB, backlight, and flex tail. That’s light enough for handheld devices but heavy enough to need mechanical support in high-vibration environments.

Now, how does this compare to other common sizes? A 2.7 inch 128x64 COG LCD typically has a module size of 72.0 mm x 38.0 mm and a viewing area of 61.0 mm x 32.0 mm, while a 3.2 inch version jumps to 90.0 mm x 48.0 mm with a viewing area of 78.0 mm x 42.0 mm. The 3.18 inch sits right in the middle, offering a good balance between readability and footprint. The pixel density is about 45 DPI (dots per inch), which is lower than a smartphone screen but perfectly adequate for 8x8 or 8x16 character fonts. You can fit 16 characters x 8 lines of text using a 5x7 font with some spacing, or 21 characters x 8 lines if you use a 4x6 font. For graphics, the 128x64 resolution is enough for simple icons, bar graphs, and waveforms. The contrast adjustment is done via a potentiometer or a PWM signal on the V0 pin, and the temperature compensation is built into the driver IC, so the display won’t wash out as the temperature changes. The viewing mode is typically STN negative blue or STN positive yellow-green, with the negative version offering better contrast in dark environments when paired with a white backlight.

For real-world applications, this display is a workhorse in industrial control panels, medical devices, point-of-sale terminals, and test equipment. The 3.18 inch diagonal is large enough to show a menu with 4-5 options and a status bar, but small enough to fit in a 4-inch wide panel cutout. The COG construction makes it resistant to shock and vibration because there are no wire bonds to break—the IC is bonded directly to the glass with ACF, which has a pull strength of about 10 N/mm. The operating humidity range is 10% to 90% RH non-condensing, and the storage humidity is 5% to 95% RH. If you’re using it outdoors, you’ll need a polarizer with a UV coating, which is available as an option. The LED backlight lifetime is rated at 50,000 hours to half brightness, which is about 5.7 years of continuous use. That’s decent for most applications, but if you need longer life, you can undershoot the LED current to 15 mA, which extends the lifetime to about 70,000 hours at the cost of slightly lower brightness.

Let’s get into the electrical characteristics because they’re critical for integration. The logic supply voltage (VDD) is 2.7V to 3.6V, with a typical value of 3.3V. The LCD driver voltage (V0) is generated internally and can range from 8V to 15V depending on the contrast pot setting. The current consumption for the logic is 1.5 mA typical at 3.3V, and the backlight draws 80 mA typical at 3.0V. So total current is about 81.5 mA at full brightness, which drops to 1.5 mA if you turn off the backlight. The sleep mode current is 10 µA when the display is in power-down mode, making it suitable for battery-powered devices with a wake-up timer. The SPI bus is compatible with 3.3V logic levels, but if you’re using a 5V microcontroller, you’ll need level shifters on the CS, A0, SCK, and SDA lines. The reset pin is active low and should be held low for at least 1 µs after power-up to initialize the driver IC. The driver IC also has a built-in oscillator that runs at 2.5 kHz for the DC-DC converter, so you don’t need an external clock. The charge pump uses two external capacitors: a 1 µF capacitor between VOUT and VSS, and a 0.1 µF capacitor between VOUT and V0. These are typically included on the module’s PCB, so you don’t need to add them.

For mounting and assembly, the display has 4 mounting holes with a diameter of 3.2 mm for M3 screws. The recommended torque is 0.3 Nm to avoid cracking the glass. The PCB thickness is 1.6 mm, and the standoff height between the PCB and your enclosure should be at least 2.0 mm to allow for the flex tail bend. The glass surface is protected by a polarizer film that is 0.2 mm thick and has a hardness of 3H on the pencil scale. If you’re using a touch panel, you’ll need to leave a 0.5 mm gap between the glass and the touch sensor to avoid pressure artifacts. The viewing area is offset from the center of the module by about 2.0 mm toward the bottom, so the top border is wider. That’s because the driver IC is mounted on the glass at the bottom edge, which eats into the bezel. The active area is centered within the viewing area, so the left and right borders are equal at about 6.65 mm each, while the top border is 5.2 mm and the bottom border is 8.0 mm (including the driver IC area).

Finally, let’s talk about optical performance in different lighting conditions. In transmissive mode with the backlight on, the luminance is 250 cd/m² typical, and the contrast ratio is 2000:1. In reflective mode (backlight off, ambient light on), the reflectance is about 10%, and the contrast ratio drops to 10:1 under 500 lux ambient light.