How to use the 21:9 Aspect Ratio calculator
Choose a mode
Resize by Aspect Ratio keeps 21:9. Calculate Aspect Ratio reads the ratio from two pixel sides.
Enter the numbers
The form opens at 21:9. Change the width or the height, or type both pixel sides.
Read the result
A width of 2520 is 1080 tall at 21:9.
Summarize with
| Size | MP | Name | |
|---|---|---|---|
| 1920×823 | 1.5802 | 1920×823 | |
| 1280×549 | 0.7027 | 1280×549 | |
| 3840×1646 | 6.3206 | 3840×1646 | |
| 2560×1097 | 2.8083 | 2560×1097 | |
| 1366×585 | 0.7991 | 1366×585 | |
| 1600×686 | 1.0976 | 1600×686 | |
| 1440×617 | 0.8885 | 1440×617 | |
| 1920×823 | 1.5802 | 1920×823 |
The label on the box versus the pixels
21÷9 = 2.3333:1. 2560×1080 simplifies to 64:27 ≈ 2.3704:1 (about 1.59% wider). 3440×1440 simplifies to 43:18 ≈ 2.3889:1 (about 2.38% wider). Theatrical “scope” is commonly 2.39:1 (239:100), wider again, and it is not a computer-monitor standard.
Makers print “21:9” because it is easier to say than 64:27. This site shows the exact simplified ratio first, then a nearest-named 21:9 badge when the relative error is inside the 3% tolerance on that row in ratios.json.
Panel class here is general knowledge from widely sold UWFHD and UWQHD timings. 2.39:1 is the conventional scope notation used with DCI containers (2048×1080 and 4096×2160), which themselves are 256:135 ≈ 1.896:1 before a scope soft matte.
Relative-error tolerance on the 21:9 dataset row is 3%, not 1%, so 64:27 and 43:18 can still show a nearest-named 21:9 badge. The exact fraction always wins in the big result.
Who the 21:9 name is for
21:9 is a marketing name for ultrawide computer monitors and some cinema-adjacent frames. Exact panels are often 64:27 (2560×1080) or 43:18 (3440×1440), not 21:9 (2.333…:1). Theatrical “scope” is typically 2.39:1 (239:100), which is wider than 21:9.
If you need the exact panel ratio, use 64:27 or 43:18 in the custom tool. If you need theatrical letterboxing, use 2.39. The calculator still offers a 21:9 chip because that is what people type.
Buying a “21:9” monitor? Read the pixel grid on the spec sheet. 2560×1080 and 3440×1440 are the common UWFHD and UWQHD timings. 5120×1440 is a dual-QHD-class strip (32:9 after GCD). None of those three are 21:9 exactly.
Cinema posters and the monitor aisle
Consumer monitors borrowed “21:9” from cinema posters. 21÷9 = 2.333… while 2560÷1080 = 64/27 ≈ 2.370 and 3440÷1440 = 43/18 ≈ 2.389. SMPTE/DCI scope is commonly written 2.39:1. This page must say those are related but not equal.
Editors who letterbox 2.39 inside a 16:9 file are not making 21:9. They are putting a wider soft matte inside HD. The calculator can size that matte; it will not rename the container.
Sizing a 16:9 timeline for an ultrawide frame
If you truly want 21:9 from 1920×1080, the trim is 1920×822 and the padded canvas is 2520×1080. If the spec sheet says 2560×1080 or 3440×1440, open that size page or the custom calculator instead of trusting the box nickname. Letterboxing 2.39 inside a 16:9 file is a matte, not a 21:9 aspect ratio.
Buy from the pixel grid, not the nickname on the box. A 21:9 aspect ratio means 21 units wide for every 9 tall, about 2.33:1. A 3440×1440 panel is wider than that. Use this page when the brief says 21:9. Open the matching size page when the monitor already lists a pixel pair.
Frequently asked questions
Is my 3440×1440 monitor really 21:9?
No. It simplifies to 43:18, about 2.39% wider than 21÷9. The box says 21:9 because the nickname sells. The exact fraction is what the pixels are.
Is 21:9 the same as cinematic 2.39:1?
No. 2.39:1 (239:100) is wider. Editors who matte scope inside HD are not creating a 21:9 aspect ratio; they are masking a 16:9 container.
What about 5120×1440?
That strip simplifies to 32:9, a dual-QHD class. It is not 21:9. Use the custom tool if you need that fraction exactly.
When should I still pick the 21:9 chip?
When the brief literally says 21:9, or when you want the nearest-named badge explained. For a purchase, read the pixel grid.