How to Measure Screen Size: Diagonals, Aspect Ratio & PPI

Consumer displays — TVs, monitors, laptops, tablets, and smartphones — are universally marketed by their diagonal measurement from one inner corner of the active display area to the opposite corner. But what does a “27-inch monitor” actually mean in terms of width and height? This guide explains exactly how screen sizes work and how to calculate the physical dimensions you need.

How Screen Size Is Measured

Screen size refers to the diagonal distance across the visible display panel, measured from the inner top-left corner to the inner bottom-right corner (or vice versa). This measurement:

Includes only the active illuminated area (the part that displays content)
Excludes the bezel (the plastic or metal frame surrounding the screen)
Is measured corner-to-corner, not edge-to-edge horizontally

Why Diagonal?

The diagonal measurement became the industry standard because it provides a single number that represents overall screen area regardless of aspect ratio. A 27-inch 16:9 monitor and a 27-inch 16:10 monitor have different widths and heights, but their diagonal measurement provides an easy comparison metric.

How to Physically Measure Your Screen

If you need to verify your screen’s diagonal size:

Turn the display on so you can see where the active area begins (some screens have dark borders inside the bezel)
Place a tape measure at the inner top-left corner of the illuminated area
Stretch diagonally to the inner bottom-right corner
Read the measurement — this is your screen’s true diagonal size

Common surprises: Many monitors are slightly smaller than advertised. A “24-inch” monitor may measure 23.8 inches diagonally. This is within normal marketing tolerances but worth knowing for precise PPI calculations.

Understanding Aspect Ratios

The aspect ratio describes the proportional relationship between a screen’s width and height. Common aspect ratios include:

Aspect Ratio	Description	Common Uses
16:9	Widescreen standard	Most monitors, TVs, laptops
16:10	Slightly taller	MacBooks, some productivity monitors
3:2	Even taller	Surface devices, some Chromebooks
4:3	Classic square-ish	Older monitors, some tablets (iPad)
21:9	Ultrawide	Gaming monitors, video editing
32:9	Super ultrawide	Dual-monitor replacement displays

The aspect ratio directly affects how much horizontal and vertical space you get from a given diagonal size.

Calculating Width & Height from Diagonal

For any screen with diagonal $D$ and aspect ratio $W:H$, the physical width and height can be calculated using trigonometry:

Width  = D × W / √(W² + H²)
Height = D × H / √(W² + H²)

Pre-Calculated Multipliers by Aspect Ratio

Aspect Ratio	Width Multiplier	Height Multiplier
16:9	0.8716	0.4903
16:10	0.8480	0.5300
3:2	0.8321	0.5547
4:3	0.8000	0.6000
21:9	0.9138	0.3916

Worked Examples

Example 1: 15.6-inch Laptop (16:9)

Width  = 15.6 × 0.8716 = 13.60 inches (345.4 mm)
Height = 15.6 × 0.4903 =  7.65 inches (194.3 mm)

Example 2: 27-inch Monitor (16:9)

Width  = 27 × 0.8716 = 23.53 inches (597.7 mm)
Height = 27 × 0.4903 = 13.24 inches (336.3 mm)

Example 3: 13.6-inch MacBook Air (16:10)

Width  = 13.6 × 0.8480 = 11.53 inches (293.0 mm)
Height = 13.6 × 0.5300 =  7.21 inches (183.1 mm)

Example 4: 65-inch TV (16:9)

Width  = 65 × 0.8716 = 56.65 inches (1438.9 mm) = 4.72 feet
Height = 65 × 0.4903 = 31.87 inches (809.5 mm)  = 2.66 feet

Common Screen Sizes: Complete Reference Table

Laptops

Diagonal	Aspect Ratio	Width (in)	Height (in)	Width (cm)	Height (cm)
13.3”	16:9	11.59	6.52	29.4	16.6
13.6”	16:10	11.53	7.21	29.3	18.3
14.0”	16:9	12.20	6.86	31.0	17.4
14.2”	16:10	12.04	7.52	30.6	19.1
15.6”	16:9	13.60	7.65	34.5	19.4
16.0”	16:10	13.57	8.48	34.5	21.5
16.2”	16:10	13.74	8.59	34.9	21.8
17.3”	16:9	15.08	8.48	38.3	21.5

Desktop Monitors

Diagonal	Aspect Ratio	Width (in)	Height (in)	Width (cm)	Height (cm)
24.0”	16:9	20.92	11.77	53.1	29.9
27.0”	16:9	23.53	13.24	59.8	33.6
27.0”	16:10	22.90	14.31	58.2	36.4
32.0”	16:9	27.89	15.69	70.8	39.9
34.0”	21:9	31.07	13.31	78.9	33.8
38.0”	21:9	34.72	14.88	88.2	37.8
49.0”	32:9	47.17	13.27	119.8	33.7

TVs

Diagonal	Width (in)	Height (in)	Width (ft)	Height (ft)
43”	37.48	21.08	3.12	1.76
50”	43.58	24.51	3.63	2.04
55”	47.94	26.96	4.00	2.25
65”	56.65	31.87	4.72	2.66
75”	65.37	36.77	5.45	3.06
85”	74.08	41.67	6.17	3.47

Calculating PPI from Screen Size and Resolution

Once you know the diagonal size and pixel resolution, you can calculate the exact PPI:

PPI = √(width_px² + height_px²) / diagonal_inches

Screen	Resolution	Diagonal	PPI
24” FHD Monitor	1920 × 1080	24.0”	92
27” 4K Monitor	3840 × 2160	27.0”	163
27” QHD Monitor	2560 × 1440	27.0”	109
13.6” MacBook Air	2560 × 1664	13.6”	224
15.6” FHD Laptop	1920 × 1080	15.6”	141

Why This Matters for Screen Rulers

Knowing your screen’s exact PPI is essential for digital ruler accuracy. A ruler that assumes 96 PPI on a 163 PPI monitor will show measurements that are 41% too small. On a 224 PPI MacBook, the error grows to 57% too small.

RulerInches automatically detects your device’s PPI from our hardware database, or you can calibrate manually using a credit card. Either way, accurate screen measurements start with understanding your display’s physical pixel density.