When you see a computer monitor advertised as "27 inches," that measurement refers to the diagonal distance from one corner of the screen to the opposite corner. However, the diagonal alone doesn't tell you how wide or tall the screen actually is. To find the specific dimensions, you must also know the Aspect Ratio.
The Math Behind the Screen
Calculating screen dimensions uses the Pythagorean theorem (a² + b² = c²). If we know the diagonal (c) and the ratio of width (W) to height (H), we can solve for the exact measurements:
Width Calculation: W = Diagonal × cos(atan(HeightRatio / WidthRatio))
Height Calculation: H = Diagonal × sin(atan(HeightRatio / WidthRatio))
Total Area: Width × Height
Common Aspect Ratios Explained
Modern displays come in various shapes tailored for specific tasks:
16:9: The standard for high-definition television and most computer monitors. Ideal for gaming and movies.
16:10: Slightly taller than 16:9, often found on professional laptops and monitors to provide more vertical space for documents.
21:9 (Ultrawide): Provides a cinematic experience and replaces dual-monitor setups for improved multitasking.
3:2: Popularized by the Microsoft Surface line, this ratio is excellent for web browsing and reading as it mimics the feel of a physical sheet of paper.
Why Pixel Density (PPI) Matters
While screen size tells you the physical footprint, Pixel Density (Pixels Per Inch) tells you the sharpness. A 27-inch monitor with 1080p resolution will look significantly "grainier" than a 27-inch monitor with 4K resolution because the pixels are physically larger and more spread out. For most desktop users, a PPI between 90 and 110 is considered the "sweet spot" for standard viewing distances.
function toggleCustomRatio() {
var ratioSelect = document.getElementById("aspectRatio");
var customW = document.getElementById("customRatioWGroup");
var customH = document.getElementById("customRatioHGroup");
if (ratioSelect.value === "custom") {
customW.style.display = "flex";
customH.style.display = "flex";
} else {
customW.style.display = "none";
customH.style.display = "none";
}
}
function calculateScreen() {
var d = parseFloat(document.getElementById("diagonalSize").value);
var ratioValue = document.getElementById("aspectRatio").value;
var rw, rh;
if (ratioValue === "custom") {
rw = parseFloat(document.getElementById("customRatioW").value);
rh = parseFloat(document.getElementById("customRatioH").value);
} else {
// Mapping back from decimal to parts
if (ratioValue == "1.7777") { rw = 16; rh = 9; }
else if (ratioValue == "1.6") { rw = 16; rh = 10; }
else if (ratioValue == "2.3333") { rw = 21; rh = 9; }
else if (ratioValue == "3.5555") { rw = 32; rh = 9; }
else if (ratioValue == "1.3333") { rw = 4; rh = 3; }
else if (ratioValue == "1.5") { rw = 3; rh = 2; }
}
if (isNaN(d) || isNaN(rw) || isNaN(rh) || d <= 0 || rw <= 0 || rh 0 && resH > 0) {
var diagonalPixels = Math.sqrt(Math.pow(resW, 2) + Math.pow(resH, 2));
var ppi = diagonalPixels / d;
document.getElementById("resultPPI").innerHTML = Math.round(ppi) + " PPI";
ppiRow.style.display = "flex";
} else {
ppiRow.style.display = "none";
}
document.getElementById("screenResults").style.display = "block";
}