Dimension Calculator (ratio: 16:9)
How to use
- 1 Enter the width and height of your original media (image, video frame, container).
- 2 The tool calculates and displays the simplified aspect ratio (e.g. 16:9).
- 3 Enter a new width to get the proportional height, or enter a new height to get the width.
- 4 Use it for responsive design, video thumbnails, or maintaining proportions when scaling.
Key features
- Calculates the simplified aspect ratio from any width and height
- Computes the missing dimension when width or height changes
- Supports any dimension: pixels, cm, inches, or abstract units
- Useful for responsive images, video embeds, and container sizing
What is Aspect Ratio?
An aspect ratio describes the proportional relationship between width and height, expressed as width:height. A 1920×1080 video and a 640×360 thumbnail both share the 16:9 ratio: scale either dimension by the ratio and the shape stays identical.
Getting aspect ratio right matters whenever you resize, crop, or embed media: a 4:3 image squeezed into a 16:9 frame looks stretched; a 16:9 video in a square container gets letterboxed or clipped. It also shows up in responsive CSS, where the aspect-ratio property (supported in all modern browsers since 2021) lets you define boxes that scale freely.
Common Use Cases
Video resizing and cropping
Calculate the exact new dimensions to resize a video without distortion before exporting.
Responsive image containers
Determine the height needed for a given width (or vice versa) to maintain the same ratio in CSS layouts.
Design asset preparation
Scale a design asset to multiple resolutions (1x, 2x, 3x) while preserving the original proportions.
Social media formats
Convert an image to the exact required ratio for LinkedIn (1.91:1), Instagram Reels (9:16), or Twitter (16:9).
Iframe and embed sizing
Calculate the correct height for a YouTube or Vimeo embed given a known container width.
Game and canvas rendering
Maintain a fixed game canvas ratio when scaling to different screen sizes in HTML5 or CSS.
Common Aspect Ratios
Reference for the formats you'll encounter across video, photo, and UI design.
| Ratio | Example sizes | Typical use |
|---|---|---|
| 16:9 | 1920×1080, 1280×720 | HD video, YouTube, TV, presentations |
| 4:3 | 1024×768, 800×600 | Legacy monitors, old TV, photography |
| 1:1 | 1080×1080, 512×512 | Instagram posts, avatars, thumbnails |
| 3:2 | 1500×1000, 3000×2000 | DSLR photography (standard sensor) |
| 9:16 | 1080×1920, 720×1280 | Reels, TikTok, Shorts, Stories |
| 21:9 | 2560×1080, 3440×1440 | Ultrawide monitors, cinematic video |
Paper Sizes and Their Ratios
If you are fitting an image to a page, the paper's ratio is the one that matters.
Every sheet in the ISO 216 series — A4, A3, A5 and the rest — shares the same ratio, 1 : √2 (≈ 1 : 1.414). That ratio is not arbitrary: it is the only one where cutting a sheet in half across its long side produces two sheets of exactly the same shape. An A3 halved is two A4s; an A4 halved is two A5s, and all of them keep the proportion.
US paper sizes are not part of that system and have no shared ratio: Letter is 8.5 × 11 in (8.5:11 ≈ 1:1.294) and Legal is 8.5 × 14 in (8.5:14 ≈ 1:1.647). Fitting a 4:3 photo on Letter paper, for instance, always leaves a margin on two sides, because 1.333 and 1.294 are close but not equal.
| Paper | Size (portrait) | Ratio (W : H) | Notes |
|---|---|---|---|
| A4 | 210 × 297 mm | 1 : 1.414 | The international standard sheet |
| A3 | 297 × 420 mm | 1 : 1.414 | Two A4 sheets |
| A5 | 148 × 210 mm | 1 : 1.414 | Half an A4 sheet |
| Letter | 8.5 × 11 in | 8.5 : 11 | US and Canada |
| Legal | 8.5 × 14 in | 8.5 : 14 | US legal documents |
The ISO sizes are defined in whole millimetres, so the measured ratio of any individual sheet is only approximately √2 — an A5 is 1 : 1.419, not 1 : 1.414. Use the preset for planning; for exact print output, work from the millimetre dimensions. The Print preset group gives you A4/A3/A5, Letter and Legal directly.
Frequently Asked Questions
How do I calculate aspect ratio from width and height?
Divide both the width and the height by their greatest common divisor (GCD). For example, 1920×1080: the GCD of 1920 and 1080 is 120. 1920 ÷ 120 = 16, 1080 ÷ 120 = 9 → 16:9. This tool does that automatically: just enter your width and height and it returns the simplified ratio.
What's the difference between aspect ratio and resolution?
Resolution is the absolute pixel count (width × height), e.g. 1920×1080. Aspect ratio is the proportional relationship between them, e.g. 16:9. Two different resolutions can share the same aspect ratio. For example, 1920×1080 and 640×360 are both 16:9. Knowing this helps when scaling content across devices without distortion.
What are the most common aspect ratios for video?
16:9 is the standard for HD video, YouTube, and TV. 9:16 for vertical formats (TikTok, Reels, Shorts). 4:3 for older TV content and classic photography. 21:9 for ultrawide cinematic video. 1:1 for square social media posts. Each ratio serves a different medium; this calculator makes it easy to convert between them.
How do I calculate height from a known width and aspect ratio?
Divide the width by the first number of the ratio, then multiply by the second. For a width of 800 and a 16:9 ratio: 800 ÷ 16 = 50, then 50 × 9 = 450. The same formula works backwards to find width from height. This tool does both calculations instantly.
What aspect ratio should I use for Instagram, LinkedIn, or Twitter?
Instagram feed: 1:1 (square), 4:5 (portrait), 1.91:1 (landscape).
Instagram Reels / Stories: 9:16.
LinkedIn: 1.91:1 for link previews, 1:1 for profile images, 4:3 for document posts.
Twitter / X: 16:9 for images and cards, 1:1 for profile photos.
YouTube: 16:9 for videos, 1:1 for thumbnails recommended.
Type the ratio you need into the Ratio field (for example 1.91:1) and the Dimension Calculator gives you the exact size for any width.
What is the aspect ratio of A4 paper?
A4 is 1 : √2, about 1 : 1.414 — 210 × 297 mm in portrait. Every size in the ISO 216 series shares it (A3, A4, A5 …) because it is the ratio that keeps its shape when the sheet is halved: an A3 cut in two gives two A4s, an A4 cut in two gives two A5s. The Print preset group also covers US Letter (8.5:11) and Legal (8.5:14), which belong to a different system and do not follow √2.