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.
- The GCD is the core operation - both dimensions divide by it to reach the simplest whole-number form. 1920 and 1080 share a GCD of 120, giving 16:9.
- Same ratio means same shape. 3840×2160 and 640×360 look identical on the same screen - both are 16:9, just different sizes.
- Resizing is strictly linear: doubling the width means doubling the height. Any scale factor works the same way.
- Platforms have fixed requirements: 16:9 for YouTube, 9:16 for TikTok/Reels, 1:1 for Instagram grid, 4:5 for Instagram portrait feed.
- The decimal ratio (W÷H) is a fast identity check: 1.778 = 16:9, 1.333 = 4:3, 1.000 = square.
How to use our width to height ratio 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.
Aspect ratio explained to a beginner
Think of aspect ratio like the shape of a sandwich box - not its size, just its proportions. A box that is 16 cm wide and 9 cm tall has the same shape as a 1920-pixel-wide screen that is 1080 pixels tall. Scale it up or down and the shape stays identical. The ratio captures the shape, not the size.
To find the ratio, you need the GCD - the largest number that divides evenly into both dimensions. For 1920 and 1080, that number is 120. Divide: 1920÷120 = 16, 1080÷120 = 9. Result: 16:9. A quick shortcut: compute W÷H as a decimal. If it equals 1.778, the rectangle is 16:9 regardless of the exact pixel count. That single number tells you the shape instantly.
Dimension ratio 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
A handful of ratios dominate modern screens and cameras. Knowing the decimal equivalent of each one lets you identify a ratio instantly - without calculating the GCD - just by dividing width by height.
The table below covers the ratios you'll encounter most often, along with their primary use cases and real-world resolutions.
| 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 |
When I'm checking whether an image will fit a specific screen, I skip the GCD step entirely and just calculate the decimal ratio (W÷H). If it matches the target's decimal - 1.778 for 16:9, 1.333 for 4:3 - the image fits without cropping.
The simplified W:H notation is more readable, but the decimal is faster for quick comparisons when you're working through a batch of assets.
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.
Aspect ratio 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 |
In my experience, the place where aspect ratio errors cause the most pain is social media uploads. TikTok and Instagram Reels demand 9:16, but most cameras default to 16:9.
Uploading 16:9 footage to a 9:16 slot doesn't just letterbox it - the platform auto-crops to center, often cutting off the subject's head or feet. Checking the target ratio before you shoot saves far more time than re-editing after the fact.
Frequently asked questions about aspect ratio
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.
Quiz: how well do you know aspect ratios?
1. What is the decimal ratio (W÷H) for a standard 16:9 aspect ratio?
2. The GCD of 1920 and 1080 is used to simplify the ratio to 16:9. What is that GCD?
3. Using the 16:9 resize table on this page, what height corresponds to a new width of 1280 pixels?
4. According to the applications table, which domain typically uses the 3:2 aspect ratio?
5. The page warns that uploading 16:9 footage to a 9:16 slot (TikTok, Reels) most commonly results in: