Screen Size Calculator

What the Screen Size Calculator does

This calculator turns a screen's advertised diagonal and aspect ratio into its real width, height and area, works out the pixel density (PPI) for its resolution, and gives the seating distances that the film and home-cinema guidelines suggest. Give it your seating distance instead and it suggests the range of TV sizes that suit the seat. You can also compare two screens, such as a 55-inch and a 65-inch TV or a 27-inch and a 34-inch ultrawide monitor.

A TV's size is measured corner to corner, so screens with the same diagonal but different shapes are not the same size: a 34-inch ultrawide is wider but much less tall than a 34-inch 16:9 screen would be. Viewing distance is best judged by the angle the screen fills in your view. SMPTE's EG-18 guidance gives 30 degrees as a minimum, and THX recommends 36 to 40 degrees for a cinema-like seat. The calculator applies plain trigonometry to those angles; it runs entirely in your browser.

How to use it

1. Type the diagonal in inches or centimetres and choose the aspect ratio: 16:9 for almost every TV, 16:10 or 3:2 for many laptops, 21:9-class for ultrawide monitors. Choose "Other" for a projector screen such as 2.35:1.
2. Pick the resolution, such as 4K UHD 3840 × 2160, if you want the pixel density and the distance beyond which you can no longer see individual pixels.
3. To compare, open "Compare with a second screen" and enter its diagonal and shape.
4. Enter the distance from your eyes to the screen to see the size range that suits that seat. You can also leave the diagonal blank and use only the distance.

Reading the results

Width and height are the visible picture, not the TV's outside dimensions - add the bezel and stand from the maker's specification sheet when checking a TV unit or wall space.

The three seating distances run from farthest to closest: at the 30-degree distance the screen fills a comfortable part of your view (SMPTE's minimum), at 36 degrees it starts to feel like a cinema (THX's recommended minimum), and at 40 degrees it is immersive (THX's ideal). Sitting farther away than the 30-degree distance is fine; the screen simply feels smaller.

PPI is pixels per inch along the diagonal. The "pixels blur together beyond" distance uses the usual definition of 20/20 (6/6) vision - resolving detail one arcminute across. Beyond it, extra resolution adds little for that viewer; closer than it, a sharper screen is visible.

Worked example: a 55-inch 4K TV

Formulas and scoring rules

Width and height from diagonal d and aspect a:b

width = d × a ÷ √(a² + b²); height = d × b ÷ √(a² + b²); area = width × height

Pixel density

PPI = √(pixels wide² + pixels high²) ÷ diagonal in inches; pixel pitch = 25.4 ÷ PPI mm

Seating distance for a viewing angle θ

D = (width ÷ 2) ÷ tan(θ ÷ 2), with θ = 30° (SMPTE EG-18 minimum), 36° (THX recommended) or 40° (THX ideal)

Screen size for a seat

width = 2 × D × tan(θ ÷ 2); diagonal = width × √(a² + b²) ÷ a

Distance where pixels blur together (20/20 vision)

D = 1 ÷ (PPI × tan(1 arcminute)) ≈ 3,438 ÷ PPI inches

Units

1 in = 2.54 cm exactly; results rounded for display only

Choosing a TV size for your room

Measure from where your eyes will be to where the screen will be, not from the wall. For a 2.5 m (8.2 ft) seat and a 16:9 TV, the 30-degree guideline suggests about 61 inches, the 36-degree guideline about 73 inches and the 40-degree guideline about 82 inches. Many people choose something between the first two; going bigger makes sense if you mostly watch films and sport and the room is dark.

Picture quality matters as well as size. At a cinema-style distance a Full HD picture shows its pixels and a 4K one does not, so 4K is worth more the closer you sit, while from far away the difference is hard to see.

Monitors and ultrawides

"21:9" is a marketing name: 2560 × 1080 and 3440 × 1440 panels are really 64:27, about 2.37:1. A 34-inch ultrawide of that shape has about the same height as a 27-inch 16:9 monitor but is roughly a third wider. Use the comparison to see it in numbers before you buy, and the PPI to compare sharpness: 109 PPI for a 27-inch 2560 × 1440 screen is a common desk reference point.

Limitations: what the result does not prove

• The size is the visible picture from the advertised diagonal. Makers round diagonals (a "55-inch" TV is often 54.6 in), and the frame and stand add to the outside size.
• Viewing-angle guidance from SMPTE and THX is about immersion for film and video, not eye health. Comfort also depends on room lighting, seat height and what you watch.
• The 20/20 acuity distance is a guideline for typical sharp vision; people with better or worse vision, and content with fine text, see differently.
• Curved screens, projector throw distances and lens-shift calculations are not covered.

Privacy: where your data goes

Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.

Standards and sources

• SMPTE - Society of Motion Picture and Television Engineers (EG-18 viewing-angle guidance)
• THX - display and home theatre set-up guidance
• Sound & Vision - How do I arrange seating for a dual-screen setup? (SMPTE 30° and THX 40° explained)
• NIST Special Publication 811 - Guide for the Use of the International System of Units

Frequently asked questions

Last reviewed 20 October 2026 by the A2Z.Tools team against the sources listed above.