What is aspect ratio, in one minute
Write width, a colon, then height. 16:9 means sixteen steps across for nine steps down. The numbers can be pixels, inches or abstract units. The shape is what matters.
People also write 1.78:1 for 16:9. That decimal is W÷H, rounded. This site keeps the colon form because 16:9 is not translated.
Aspect ratio is not resolution
1920×1080 and 1280×720 share the 16:9 aspect ratio and do not share a pixel count. Saying “1080p” names a height, not a ratio. The dedicated comparison guide expands this.
Orientation (landscape, portrait, square) is a property of the ratio, not of megapixels.
Where you meet aspect ratio
TVs and YouTube default to 16:9. Stories default to 9:16. Cameras default to 3:2 or 4:3. None of those choices change the GCD formula.
If a player shows black bars, the file ratio and the screen ratio disagree. That is a convert/pad situation, not a broken file.
Worked pictures
1920×1080 → 16:9. 1000×1000 → 1:1 square. 1080×1350 → 4:5. 3440×1440 → 43:18, often sold as 21:9. All four are fixtures.
A definition you can reuse
If a colleague asks for the shape, answer with two integers and a colon. Only then mention pixels. That order stops people from calling 4K a ratio. This page exists so the homepage can stay a calculator and this URL can stay the definition.
Orientation follows the two integers. 16:9 is landscape, 9:16 is portrait, 1:1 is square. Megapixels never flip a frame from landscape to portrait.
Frequently asked questions
What does the colon mean?
Width compared with height. 16:9 is sixteen units wide for every nine tall.
Is 16:9 an aspect ratio?
Yes. It is the HDTV landscape standard in ITU-R BT.709.
Does aspect ratio include pixels?
No. Pixels are resolution.
Why do I see black bars?
The file and the screen use different ratios. That is letterboxing or pillarboxing.
What is a square aspect ratio?
1:1, the same width and height.