Aspect Ratio Calculator – Image Ratio and Resize
Enter the width and height of an image, screen or video to find its aspect ratio, its common name (like 16:9), and the matching dimension for any proportional resize.
Find the Aspect Ratio of Width and Height
Aspect Ratio
Where:
- \(W\) — original width of the image, screen or video
- \(H\) — original height, in the same unit as the width
- \(g\) — greatest common divisor of \(W\) and \(H\), used to reduce the ratio to smallest whole numbers
- \(W'\), \(H'\) — the new width or height you want to scale to
The first part expresses the aspect ratio as the quotient of width by height, reduced to the smallest whole-number pair (for example 1920:1080 reduces to 16:9). The second part gives the missing dimension that keeps the same proportions when one dimension is changed.
Aspect Ratio – Calculation Example
Example: a Full HD video measures 1920 pixels wide and 1080 pixels high. You want to resize it to a width of 1280 pixels without distorting it.
\[ g = \gcd(1920, 1080) = 120 \quad \Rightarrow \quad \frac{1920/120}{1080/120} = \frac{16}{9} \]
\[ \frac{W}{H} = \frac{1920}{1080} = 1.7778 \]
\[ H' = 1280 \cdot \frac{1080}{1920} = 720 \]
Result: the aspect ratio is 16:9 (1.7778:1), the common widescreen HD format. Scaling the width to 1280 px gives a matching height of 720 px, i.e. a 1280 × 720 frame.
How to Use the Calculator
- Enter the Original Width of your image, screen or video. Any unit works — pixels, centimetres, inches — as long as both dimensions use the same unit.
- Enter the Original Height in the same unit.
- Optionally, enter a New Width to find the height that keeps the ratio, or a New Height to find the matching width. Fill in only one of the two, never both.
- Read the results: the Simplified Aspect Ratio in smallest whole numbers, the decimal Width ÷ Height value, the Common Name if the ratio matches a standard format, and the Proportionally Resized Dimension.
What Is an Aspect Ratio?
The aspect ratio describes the shape of a rectangle — an image, a screen, a video frame, a printed photo — as the relationship between its width and its height. It is written as two numbers separated by a colon, such as 16:9, and read as "16 units wide for every 9 units tall." The actual size does not matter: a 1920 × 1080 monitor, a 3840 × 2160 (4K) TV and a 1280 × 720 video all share the same 16:9 shape.
Aspect ratios matter because content created for one shape looks wrong on another. Displaying a 4:3 classic-TV picture on a 16:9 widescreen leaves black bars on the sides; the reverse stretches or crops the image. Knowing the ratio lets you resize, crop, print or embed media so that proportions stay correct and nothing appears squashed or stretched.
The decimal form (width ÷ height) is useful for comparison: 16:9 ≈ 1.7778, 4:3 ≈ 1.3333 and a square is 1.0. The larger the value, the wider the rectangle. Portrait or vertical formats, such as 9:16 phone video, have values below 1.
Common Aspect Ratios Reference
| Ratio | Decimal | Typical Use |
|---|---|---|
| 1:1 | 1.0000 | Square images, profile pictures |
| 5:4 | 1.2500 | Older LCD monitors |
| 4:3 | 1.3333 | Classic TV, iPads, point-and-shoot photos |
| 3:2 | 1.5000 | 35 mm photography, DSLR sensors |
| 16:10 | 1.6000 | Laptop and tablet displays |
| 16:9 | 1.7778 | HD, Full HD, 4K UHD video and TV |
| 1.85:1 | 1.8500 | Cinema film |
| 21:9 | 2.3333 | Ultrawide monitors |
| 2.39:1 | 2.3900 | Anamorphic cinema scope |
| 9:16 | 0.5625 | Vertical video, phone portrait |
Assumptions and Tips
- Same units required. The width and height must be expressed in the same unit; the ratio itself is unitless. Pixels are the most common choice for screens and images.
- Whole-number reduction works only for whole-number inputs. For decimal dimensions the calculator finds the nearest exact simple fraction; if none exists, it shows the decimal ratio with four decimals instead.
- Recognition tolerance is 1%. A ratio is labelled with a common name when it matches a standard format within 1% — this accounts for slight manufacturing variations in screens and sensors.
- Fill in only one new dimension. Entering both a new width and a new height is ambiguous (the pair may not preserve the ratio), so the calculator asks for exactly one and computes the other.
- Scaling does not sharpen. Proportional resizing preserves shape, but enlarging a small image also enlarges its blur. For video work, also check what bitrate your connection can carry with a data transfer rate converter, or how long a render will take with a time unit converter.
Frequently Asked Questions
How do I calculate an aspect ratio by hand?
Divide the width by the height using the same unit for both, then reduce the two numbers by their greatest common divisor. For 1920 × 1080: \(\gcd(1920, 1080) = 120\), so \(1920/120 = 16\) and \(1080/120 = 9\), giving 16:9. The decimal form is simply \(1920 \div 1080 \approx 1.7778\). You can check the result by multiplying back: 16 × 120 = 1920.
How do I find the missing width or height when resizing?
Use cross-multiplication. If the original is \(W \times H\) and you want a new width \(W'\), then \(H' = W' \cdot H / W\). For example, scaling 1920 × 1080 to a width of 800 gives \(800 \cdot 1080 / 1920 = 450\). Never adjust both dimensions independently — that changes the shape and makes the image look stretched or squashed.
What is the difference between aspect ratio and resolution?
Aspect ratio describes the shape of the frame (e.g. 16:9), while resolution describes its size in pixels (e.g. 1920 × 1080). Many different resolutions share one aspect ratio: 1280 × 720, 1920 × 1080 and 3840 × 2160 are all 16:9. A resolution only tells you the ratio after you divide width by height and simplify.
Does it matter whether I use pixels, inches or centimetres?
No, as long as both dimensions use the same unit. The aspect ratio is a unitless relationship, so 16 inches by 9 inches and 1600 by 900 pixels both give 16:9. Mixing units (width in inches, height in millimetres) will produce a wrong result — convert one of them first.
What is a good or normal aspect ratio?
There is no single "best" value — it depends on the use. 16:9 is standard for TV, YouTube and monitors; 9:16 for vertical phone video like TikTok and Reels; 1:1 for profile pictures; 3:2 or 4:3 for photography; 21:9 for ultrawide displays. Choose the ratio of the platform you are creating for, not the one of your source screen.
Why does my screen not show an exact common ratio like 16:9?
Manufacturing tolerances, analogue standards and cropping often make real dimensions slightly off. Old CRT-derived formats, HD broadcast maths and camera sensors frequently produce values like 1.7777 instead of 1.7778. Differences under about 1% are usually the same format in practice, which is why the calculator recognises standard ratios within a 1% tolerance.
How do I convert between landscape and portrait orientations?
Simply swap the two numbers: 16:9 landscape becomes 9:16 portrait. The decimal form inverts too — 16:9 ≈ 1.7778, while 9:16 ≈ 0.5625. When you rotate content, remember that a 1920 × 1080 horizontal video becomes 1080 × 1920 when held vertically; the shape is the same, only the orientation changes.