Screen Size and Viewing Distance Calculator
A screen's advertised size is the diagonal, so the actual picture width and height depend on the aspect ratio.
Enter Values
How to use this calculator
- Enter the screen diagonal in inches (the size manufacturers advertise, e.g. 55, 65, 75).
- Enter the aspect ratio - 16 and 9 for almost all modern TVs, or 21 and 9 for an ultrawide/cinema screen.
- Read the width, height, and recommended SMPTE and THX seating distances, then pick a spot inside the comfortable range.
How it works
A screen's advertised size is the diagonal, so the actual picture width and height depend on the aspect ratio. For an a:b ratio and diagonal D, width = D x a / sqrt(a^2 + b^2) and height = D x b / sqrt(a^2 + b^2). A 16:9 screen is therefore always about 87% as wide and 49% as tall as its diagonal.
Recommended viewing distance comes from the horizontal field of view the screen fills at your seat: distance = (width / 2) / tan(FOV / 2). This calculator uses the two widely cited targets - SMPTE's 30 degrees for a relaxed, movie-theatre-like view and THX's 36 degrees for a more immersive reference view. Any distance between the two figures is generally comfortable.
Worked example
How far to sit from a 65-inch 16:9 TV. For a 65 in diagonal 16:9 screen, the picture is 56.6 in (143.8 cm) wide and 31.9 in (80.9 cm) tall. The SMPTE relaxed-cinematic distance (30 deg field of view) is 8.24 ft (251 cm) and the THX immersive distance (36 deg) is 6.72 ft (205 cm), giving a comfortable seating range of about 6.72-8.24 ft.
Common mistakes
- Treating the diagonal as the width. A 65 in TV is only about 56.6 in wide, not 65 in - always convert with the aspect ratio before planning furniture placement.
- Using the wrong aspect ratio. Entering 4:3 or 21:9 for a normal HDTV changes the width and every distance; use 16:9 unless your screen is genuinely ultrawide.
- Assuming one 'correct' distance. Sharper 4K panels let you sit closer without seeing pixels, so treat the SMPTE-to-THX span as a range and choose by preference and room size.
Frequently asked questions
What aspect ratio should I use?
Use 16 and 9 for virtually all modern flat-panel TVs and most projectors. Use 21 and 9 (enter 21 and 9) for ultrawide 'cinemascope' displays, or 4 and 3 for older sets. The ratio only affects the width, height, and distances - not the diagonal you enter.
Why are two distances shown?
They are the two standard targets for how much of your vision the screen fills. SMPTE (30 degrees field of view) gives a relaxed, cinema-back-row feel; THX (36 degrees) is more immersive and closer to a reference setup. Sitting anywhere between them is comfortable - closer for a bigger picture, farther for a calmer one.
Does screen resolution change the recommended distance?
These figures are based on how large the picture appears (field of view), not resolution. A higher-resolution 4K screen simply lets you sit at or below the THX distance without seeing individual pixels, whereas on a 1080p screen you may prefer the farther SMPTE distance to keep the image looking sharp.
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