I work on OnePassApps, the team behind app.onepassapps.com. Every number below was measured in the live Screen & Cinema Planner, not copied from a spec sheet, and I re-checked the geometry outside the browser afterwards.
If you have ever compared two screens by diagonal size alone, you already know the trap: a bigger number is not automatically more picture. Our planner takes two diagonals, two aspect ratios and one seating distance and turns them into width, height, visible area and viewing angle, entirely in the tab.
You can try it here: Screen & Cinema Planner.
What the app opens with
The page title is Screen & Cinema Planner — Screen size calculator — OnePassApps, and the panel line reads: Compare two flat screens at your seating distance using real dimensions, area and field of view.
There are four stages: 01 Units, 02 Screen A, 03 Screen B, 04 Seating. The shipped example is Screen A at 55 in and 16:9, Screen B at 65 in and 16:9, with a seating distance of 2.5 m. Two pieces of the app's own wording matter here: Diagonals are measured corner to corner. and, for the distance field, From your seat to the centre of the screen.
The default measurement table:
| Measurement | Screen A | Screen B |
|---|---|---|
| Width (cm) | 121.8 | 143.9 |
| Height (cm) | 68.5 | 80.9 |
| Width (in) | 47.9 | 56.7 |
| Height (in) | 27.0 | 31.9 |
| Area (m²) | 0.834 | 1.165 |
| Horizontal view | 27.4° | 32.1° |
| Vertical view | 15.6° | 18.4° |
The summary line above the table says it in one sentence: Screen B has 39.7% more visible area than Screen A. From 2.5 m, A fills 27.4° of your view and B 32.1°.
I recomputed the whole table outside the browser
Before trusting those figures, I rebuilt them from first principles in Python. A 55-inch diagonal is 139.7 cm, so a 16:9 screen is 139.7 × 16 ÷ √(16² + 9²) = 121.8 cm wide and 68.5 cm high, which is 0.834 m² of visible area. The horizontal angle at 2.5 m is 2 × atan(1.218 ÷ (2 × 2.5)) = 27.4°. Every default figure in the app matched the value I calculated, to the decimal it displays. That is the same formula the product guide gives: for each dimension, the angle is 2 × atan(size ÷ (2 × distance)).
So the app is not showing you a marketing number, it is showing you the geometry.
Changing the diagonal
Setting Screen B to 75 in, keeping 16:9 and 2.5 m, moved every line at once: width 166.0 cm, height 93.4 cm, area 1.551 m², horizontal view 36.7°, vertical view 21.2°. The headline changed with it: Screen B has 86.0% more visible area than Screen A. From 2.5 m, A fills 27.4° of your view and B 36.7°.
My independent calculation for the same case lands on 166.0 cm, 93.4 cm, 1.551 m², 36.7° and 21.2° as well.
The aspect ratio is where people get it wrong
Here is the finding I did not expect. With Screen B held at 65 in, switching its aspect ratio from 16:9 to 21:9 made its visible area go down, from 1.165 m² to 0.987 m², while its horizontal view went up from 32.1° to 33.8° and its vertical view dropped from 18.4° to 14.8°. A wider screen made of the same diagonal is a shorter screen, and the planner shows the area you actually lose rather than hiding it behind a single big number.
The ratio picker offers 16:9, 4:3, 21:9, 2.39:1, 1:1 and Custom. Picking Custom opens two extra fields for the ratio width and height, which open at 16 and 9, so you can type something the presets do not cover.
Switching Screen A to 4:3 is the mirror image of the same effect: its width falls to 111.8 cm and its height rises to 83.8 cm, its area becomes 0.937 m², and the summary line changes to Screen B has 65.5% more visible area than Screen A. From 3.5 m, A fills 18.1° of your view and B 26.7°.
Seating distance moves the angles, not the area
Moving the seat from 2.5 m to 3.5 m left every measurement row exactly as it was — width 121.8 cm and 166.0 cm, area 0.834 m² and 1.551 m² — and changed only the angles: Screen A narrowed from 27.4° to 19.7° horizontally and Screen B from 36.7° to 26.7°. The area is a property of the screen, the angle is a property of where you sit, and the table keeps those apart.
Units
There are two unit pickers. Screen size offers Inches and Centimetres. Distance offers Metres, Centimetres and Inches. The guide states the rule plainly: Switching units preserves physical dimensions. In this session, switching the screen-size unit to Centimetres left the measurement table unchanged at 121.8 cm for Screen A and 151.8 cm for Screen B, with the same physical screens behind the numbers.
The SVG export is a real file
Download SVG writes screen-comparison.svg — 1,905 bytes in this session — straight from the button, with no account and no upload step. Reading the file off disk:
- The root element carries width 1000, height 660 and viewBox 0 0 1000 660, with role img and an accessible title and description.
- Screen A is drawn with a solid stroke, Screen B with a dash pattern, so the two outlines are distinguishable without relying on colour.
- Each rectangle carries its own title giving that screen's width and height in centimetres.
- The drawing carries the line A: solid outline · B: dashed outline · same scale, a row of figures per screen (diagonal, width × height, area in m², horizontal field of view in degrees), and a footer reading Seating distance: 2.50 m · FOV measured from the centre of a flat screen.
- The last line is the caveat the export repeats: Visible screen area only; frames, stands, mounting and room constraints are excluded.
The wire stays clean
With CDP network tracing on for the whole session, the page made 112 requests. Every one was a GET, and not one carried a request body. Nothing about your two screens was sent anywhere.
Storage matches: localStorage held only the clerk environment key, the recent-apps list, the theme preset and the tool store, and sessionStorage held one feature key. No screen sizes, no diagonals, no distances. The guide puts it as Inputs remain in tab memory only.
What the app will not claim for you
The panel is careful about this, and so is the guide. The comparison is A geometric comparison, not a recommended viewing distance. The area note says Visible area excludes frames, stands and mounting clearance. Other content ratios may add black bars. The guide adds the rest of the boundary: Dimensions describe visible flat-screen geometry and exclude frames, stands, mounting clearance, room layout, curved screens, resolution and content letterboxing, and Viewing angles assume a screen centred in front of the viewer. They are not comfort, health or safety recommendations.
For the money question, the guide answers it directly: Screen & Cinema Planner runs in your browser with no account or payment details, and it never uses OnePassApps credits.
Limits of this test
One build, one browser, one signed-out session, 6 November 2026. I measured the 16:9, 21:9 and 4:3 presets, the Custom ratio fields, both unit pickers and the SVG export. I did not test 2.39:1 or 1:1, I did not re-enter values after switching to centimetres, and I did not check what the Reset control does to the fields. The angles assume a flat screen centred in front of you, exactly as the guide says; a curved or off-centre panel would sit differently. All the figures above are for visible flat-screen geometry only.
The step-by-step version, with the formulas and the wording the app uses, is on the Screen & Cinema Planner guide.
I work on OnePassApps, so everything above is our own build measured on our own site. If it helps you pick between two panels, the app is free and it runs without an account.
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