The Complete Aspect Ratio & Resolution Guide
Updated 2026-07-11 · A practical guide for developers, video makers, and social creators
1. What an aspect ratio is
An aspect ratio is the proportion between the width and height of an image, video, or screen. It is usually written as width:height (e.g. 16:9) or as a single decimal (e.g. 1.78:1). A ratio describes shape independently of pixel size: 1280×720 and 1920×1080 have different pixel counts but are the same shape — both 16:9.
Ratios matter because when a source and target ratio differ, the image gets cropped, stretched, or letterboxed. Each platform expects a different ratio, so the same content has to be recalculated to fit.
2. Reducing a ratio with GCD
To turn any resolution into a standard ratio, divide width and height by their greatest common divisor (GCD). For 1920×1080:
- GCD(1920, 1080) = 120
- 1920 ÷ 120 = 16, 1080 ÷ 120 = 9 → 16:9
This tool uses the Euclidean algorithm to reduce your pixel size to lowest terms instantly: 1080×1350 reduces (GCD 270) to 4:5, and 1080×1920 (GCD 120) to 9:16.
3. Solving the missing dimension
Lock a ratio (W:H) and one known side, and the other follows from a simple proportion:
height = width × ratioHeight ÷ ratioWidth
width = height × ratioWidth ÷ ratioHeight
Example: at 16:9 a width of 1280 gives height = 1280 × 9 ÷ 16 = 720. A width of 1000 gives 562.5, which becomes 563 (integer) or 562 (codec-safe even). Lock the ratio and type one value to solve it.
4. Standard aspect ratio table
| Ratio | Decimal | Common use |
|---|---|---|
| 16:9 | 1.78:1 | YouTube, monitors, TV, widescreen video |
| 9:16 | 0.56:1 | Reels, Shorts, TikTok, Stories (vertical) |
| 4:5 | 0.80:1 | Instagram feed portrait |
| 1:1 | 1:1 | Square profile, feed, thumbnail |
| 4:3 | 1.33:1 | Old TV, tablets, classic photo |
| 3:2 | 1.5:1 | DSLR / mirrorless default photo |
| 21:9 | 2.33:1 | Ultrawide monitors, cinematic |
| 1.85:1 | 1.85:1 | Widescreen theatrical film |
| 2.39:1 | 2.39:1 | Anamorphic Cinemascope |
5. Even dimensions and codecs
Most video codecs — H.264 (AVC), H.265 (HEVC) — use 4:2:0 chroma subsampling, which processes color in 2×2 pixel blocks and therefore requires even width and height to encode without errors. When a derived value is odd (e.g. 563), enable the "Even (codec)" rounding mode to snap it to the nearest even number (562). Web images (PNG/JPEG) accept odd sizes, so use "Integer" rounding there.
6. Recommended sizes per platform
- Instagram feed: square 1:1 1080×1080, portrait 4:5 1080×1350
- Instagram Story/Reels, YouTube Shorts, TikTok: 9:16 1080×1920
- YouTube video: 16:9 1920×1080, thumbnail 1280×720
- X (Twitter): image 16:9 1600×900, header 3:1 1500×500
- Facebook / LinkedIn feed: ~1.91:1 1200×630
- Naver Blog thumbnail: square 600×600
※ Platform specs change over time and are reference values. Use the tool's platform preset buttons to fill them in.
7. Common mistakes
- Resizing without matching the ratio → distortion. Always check source vs target ratio first.
- Encoding video at odd dimensions → fails in some codecs. Force even for video.
- Confusing portrait and landscape → 9:16 ≠ 16:9. Reels and Shorts are vertical (9:16).