A screen size calculator converts the one number manufacturers advertise — the diagonal — into the two numbers you actually need, the width and the height. Those are what determine whether a television fits the alcove, whether a monitor fits the desk, and whether one screen is genuinely bigger than another. A 34-inch ultrawide and a 34-inch 16:9 monitor share a name and are not remotely the same object.
Arb Digital publishes this as part of a free tools library. The boundary with the two closest live tools is worth stating up front. The aspect ratio calculator works entirely in ratios and pixel dimensions and never produces a physical measurement. The DPI and PPI calculator gives pixel density from a resolution and a diagonal, but does not tell you how wide or tall the panel is. This page is the one that turns a diagonal into physical inches and centimetres, and then compares two screens by area.
What This Screen Size Calculator Does
Enter a diagonal and an aspect ratio and you get width, height and viewing area, in both imperial and metric. Add a resolution and you get pixel density in pixels per inch. Add a second diagonal and ratio and you get a direct area comparison, expressed as a percentage rather than as an unhelpful difference in inches.
The area comparison is the part that changes decisions. Screens are marketed on a linear measurement and experienced as an area, and those scale differently. The bars make the point immediately: a modest-sounding jump in diagonal produces a much larger jump in the bar.
Custom ratios are supported because the standard list does not cover everything. Cinema formats, older industrial panels, digital signage and phone screens all use ratios outside the common set, and entering the two numbers directly handles any of them.
How to Use It
- Enter the advertised diagonal. That is the number in the product name — 55 inch, 27 inch, 34 inch — measured corner to corner across the visible panel.
- Choose the aspect ratio. If you are not certain, divide the resolution width by the height: 3840 by 2160 is 16:9, 3440 by 1440 is 21:9.
- Add the resolution for pixel density. Pixel density decides how close you can sit before individual pixels become visible.
- Set the comparison screen. Use the size you currently own to see how much of a change you are actually making.
- Measure the space before you buy. Add the bezel and the stand footprint to the width and height this tool gives you — those are panel dimensions, not the outside of the product.
The Formula / How It's Calculated
The diagonal, width and height of a rectangle form a right triangle, so Pythagoras gives everything. If the aspect ratio is w:h, then the width and height are proportional to w and h, and the diagonal is proportional to √(w² + h²). Dividing gives the scaling factor:
Width = D × w ÷ √(w² + h²) and Height = D × h ÷ √(w² + h²), with Area = Width × Height.
Worked example, checked by hand. A 55-inch 16:9 screen. √(16² + 9²) = √337 = 18.3576. Width = 55 × 16 ÷ 18.3576 = 47.94 inches. Height = 55 × 9 ÷ 18.3576 = 26.96 inches. Area = 47.94 × 26.96 = 1,292.5 square inches, which is 8.98 square feet. In metric the same panel is 121.8 cm by 68.5 cm.
Pixel density follows from the pixel diagonal. At 3840 × 2160, the diagonal in pixels is √(3840² + 2160²) = 4,405.8, so the density is 4,405.8 ÷ 55 = 80.1 pixels per inch. Compare that with the 65-inch model in the comparison field: width 56.66 inches, height 31.87 inches, area 1,805.9 square inches. That is 39.7% more viewing area than the 55-inch, from a diagonal only 18.2% larger.
Why Diagonal Inches Mislead
Area scales with the square of the diagonal when the aspect ratio is held constant, so the relationship between the advertised number and what you see is quadratic. Going from 55 to 65 inches is an 18% increase in diagonal and a 40% increase in area. Going from 24 to 27 inches on a monitor is a 12.5% increase in diagonal and a 27% increase in area. The number on the box always understates the change.
Across different aspect ratios the diagonal becomes actively misleading rather than merely conservative. A 34-inch 21:9 ultrawide has an area of about 419 square inches; a 34-inch 16:9 monitor has about 494. The ultrawide is wider, shorter and smaller overall despite carrying the same number in its name. The same trap applies to 4:3 projectors quoted against 16:9 screens: for a fixed diagonal, the closer the ratio is to square, the larger the area.
This is also why "screen inches" cannot be added up. Two 27-inch monitors side by side do not equal a 54-inch display; they have roughly the same combined area as a 38-inch 16:9 panel. If you are working out how much desk or wall a setup needs, use the width figure from this tool, not the diagonal.
Aspect Ratios and Where They Came From
16:9 is not a natural constant; it is a standards decision. It was adopted as the aspect ratio for high-definition television and is codified in ITU-R Recommendation BT.709, the international specification for HDTV production and programme exchange. Once broadcast standardised on it, panels, cameras and content followed, which is why almost every consumer display made in the last fifteen years shares the shape.
The ratios that survive alongside it do so for real reasons. 16:10 gives more vertical space for documents and code, which is why it persists on laptops and professional monitors. 3:2 goes further in the same direction. 21:9 approximates the 2.35:1 cinema format and suits side-by-side windows. 4:3 remains standard for many projectors and for older archival content, and 1:1 exists for signage and specialist instrumentation.
Content shot in one ratio and shown on another is where the black bars come from. A 2.39:1 film on a 16:9 television is letterboxed top and bottom, so the image you actually see is smaller than the panel you paid for. The aspect ratio calculator works out the letterboxed dimensions when you need to know how much of the panel a given format will actually use.
Pixel Density, Viewing Distance and When It Stops Mattering
Pixel density only matters relative to how far away you sit. A phone at 400 pixels per inch is held at 30 centimetres; a 55-inch television at 80 pixels per inch is watched from three metres. Both can look equally sharp, because what the eye resolves is angular size, not absolute size. Human visual acuity is conventionally taken as being able to resolve about one arc minute, which is roughly one sixtieth of a degree.
The practical consequence is that upgrading resolution past the point where you can resolve the pixels buys nothing at your seating position. A 4K panel at 80 pixels per inch stops being distinguishable from a 1080p one at typical living-room distances beyond a few metres, which is why the same content can look identical on both. Sitting closer, or choosing a larger screen at the same distance, is what makes the extra resolution visible.
For desk work the calculation runs the other way. A 27-inch 4K monitor at 163 pixels per inch is sharp at arm's length but renders interface text small enough to need scaling, which throws away some of the benefit. The DPI and PPI calculator handles the density side in more detail, including the print-resolution case where the same arithmetic applies to dots rather than pixels.
Measuring for a Space That Already Exists
The dimensions this calculator returns are the visible panel, not the product. Televisions add a bezel that is usually small on modern sets but not zero, and they add a stand that is often wider than the panel or splits into feet near the corners — which is the detail that catches people fitting a television onto a media unit. Monitors add a stand depth that can be substantial.
Wall mounting has its own constraint. The VESA mounting pattern on the back is a fixed hole spacing that has to match the bracket, and it is unrelated to the screen size. Check it before buying either component. Measure the alcove or wall space, subtract a few centimetres of clearance on each side for cables and airflow, and compare that against the width figure here plus the bezel.
If you need to work between the two unit systems while measuring, the inch is defined as exactly 0.0254 metres by international agreement, a definition reflected in NIST's guidance on SI units. The cm to inches converter applies it, the rectangle area calculator handles the area arithmetic for irregular spaces, and the Pythagorean theorem calculator is the general form of the diagonal relationship used here.
Arb Digital's free tools cover display, image and layout maths, and our team is happy to help with anything the tools do not answer.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Comparing diagonals across different aspect ratios — a 34-inch ultrawide has less area than a 34-inch 16:9 monitor despite the identical name.
- Assuming the panel dimensions are the product dimensions — bezels, stands and feet all add width, height or depth.
- Treating a 20% bigger diagonal as 20% more screen — area scales with the square, so it is closer to 44% more.
- Buying resolution without considering viewing distance — past the point where the eye resolves individual pixels, extra pixels are invisible.
- Forgetting that content ratio and panel ratio differ — a widescreen film on a 16:9 panel is letterboxed, so the visible image is smaller than the panel.
Related Free Tools From Arb Digital
Use the aspect ratio calculator for ratio and pixel-dimension work with no physical measurement involved, the DPI and PPI calculator for pixel and print density, the cm to inches converter when the measurements arrive in the wrong system, the rectangle area calculator for the space the screen has to fit into, and the Pythagorean theorem calculator for the underlying diagonal relationship. Everything else is in the free online tools hub.
Frequently Asked Questions
The panel is 47.94 inches wide and 26.96 inches tall, or about 121.8 cm by 68.5 cm. That is the visible screen only; the product will be wider once the bezel and any stand or feet are included.
Divide the diagonal by the square root of the sum of the squares of the two ratio numbers, then multiply that factor by each ratio number. For 16:9 the divisor is the square root of 337, which is 18.3576.
No, it is smaller in area. At a fixed diagonal, the closer the aspect ratio is to square the larger the area, so a 34-inch 21:9 panel has roughly 15 per cent less viewing area than a 34-inch 16:9 one.
About 40 per cent more viewing area, from a diagonal that is only 18 per cent larger. Area scales with the square of the diagonal when the aspect ratio stays the same.
It depends entirely on viewing distance, because the eye resolves angular size rather than absolute size. A phone needs several hundred pixels per inch at arm's length; a television watched from three metres looks sharp at around eighty.
It was chosen as the aspect ratio for high-definition television and codified in the international HDTV specification, and once broadcast standardised on it, cameras, panels and content followed.
No. Screen sizes are quoted for the visible panel measured corner to corner, so the physical product is always larger than the figures here by the width of the bezel and any stand.
These figures describe the visible panel only. Always confirm the manufacturer's stated product dimensions, stand footprint and mounting pattern before buying a screen for a specific space.
