Quick answer
Divide both dimensions by their GCD to get the simplified ratio. 1920×1080: GCD = 120 → 16:9. 2560×1440: GCD = 80 → 16:9. To resize: multiply the known dimension by the opposite side's ratio component.
How to use this calculator
Enter the width and height of any rectangle — a screen, image, video frame, canvas, or print size. The calculator returns the simplified aspect ratio, its decimal equivalent, and the height as a percentage of width. Use the resize section to find the corresponding dimension at any new size.
What is an aspect ratio?
An aspect ratio is the proportional relationship between the width and height of a rectangle, expressed as W:H. It describes the shape of the rectangle without specifying its size. Two rectangles with different dimensions but the same aspect ratio are geometrically similar — one is a scaled version of the other.
The ratio is always simplified to the smallest whole numbers by dividing both dimensions by their greatest common divisor (GCD). 1920:1080 simplifies to 16:9 because GCD(1920, 1080) = 120.
Formula and calculation
The aspect ratio formula uses the GCD to reduce to lowest terms:
$$\text{Aspect Ratio} = \frac{W}{\gcd(W,H)} : \frac{H}{\gcd(W,H)}$$
The decimal ratio (W ÷ H) expresses how many units wide the rectangle is per unit of height:
$$\text{Decimal ratio} = \frac{W}{H}$$
To resize while preserving the aspect ratio, scale the new dimension by the ratio of the sides:
$$H_{\text{new}} = W_{\text{new}} \times \frac{H}{W}$$
Common aspect ratios reference
| Ratio | Decimal | Common use | Example resolution |
|---|---|---|---|
| 16:9 | 1.778 | HD/4K TV, YouTube, most monitors | 1920×1080, 3840×2160 |
| 16:10 | 1.600 | Widescreen laptops, older monitors | 1920×1200, 2560×1600 |
| 4:3 | 1.333 | Traditional TV, iPad, older screens | 1024×768, 2048×1536 |
| 1:1 | 1.000 | Instagram square posts, profile photos | 1080×1080 |
| 21:9 | 2.333 | Ultrawide monitors, cinema | 2560×1080, 3440×1440 |
| 3:2 | 1.500 | DSLR/mirrorless cameras, prints | 3000×2000, 6000×4000 |
| 5:4 | 1.250 | Medium format, some monitors | 1280×1024 |
| 9:16 | 0.563 | Mobile video, TikTok, Instagram Reels | 1080×1920 |
| 2:3 | 0.667 | Portrait photography prints | 4000×6000 |
Resizing with aspect ratio
When resizing an image or video frame, one dimension determines the other. Changing width while preserving a 16:9 ratio:
| New width | Calculated height (16:9) | Common name |
|---|---|---|
| 3840 | 2160 | 4K UHD |
| 2560 | 1440 | QHD / 2K |
| 1920 | 1080 | Full HD |
| 1280 | 720 | HD |
| 854 | 480 | SD |
| 640 | 360 | 360p |
The relationship is linear: double the width and the height doubles too. This means you can scale by any factor and the ratio remains constant.
Applications by domain
| Domain | Typical ratio | Why it matters |
|---|---|---|
| Web design | 16:9 | Hero images, video embeds, responsive layouts |
| Social media | 1:1, 4:5, 9:16 | Platform-specific crop requirements |
| Photography | 3:2 | Standard DSLR sensor format for prints |
| Film / cinema | 2.39:1 (widescreen) | Anamorphic lens format |
| Print design | 1:√2 (A4 paper) | ISO paper series — scales without distortion |
| UI/UX design | Custom | Modal dialogs, card components, thumbnails |
Frequently asked questions
What is an aspect ratio?
The proportional width-to-height relationship of a rectangle, simplified to the lowest whole-number form using the GCD. Example: 1920:1080 = 16:9.
How do you calculate an aspect ratio?
Find GCD(width, height), then divide both by it. GCD(1920, 1080) = 120; 1920÷120 = 16, 1080÷120 = 9 → 16:9.
How do you resize an image while keeping the aspect ratio?
Multiply the new width by H/W (for height), or the new height by W/H (for width). For 16:9 at width 1280: height = 1280 × 9/16 = 720 px.
What is a 16:9 aspect ratio in pixels?
Any resolution where width ÷ height = 16 ÷ 9 ≈ 1.778. Common examples: 3840×2160, 2560×1440, 1920×1080, 1280×720.
Is 4:3 or 16:9 better?
Neither is inherently better — it depends on the use case. 16:9 matches modern screens and video; 4:3 suits older content, tablets, and certain print formats. The right ratio is the one that fits your output medium.