Aspect Ratio Calculator: Resize Images and Screens

An aspect ratio calculator finds the proportional relationship between an image’s, screen’s, or video’s width and height, then helps calculate a matching dimension when you resize it. Enter the width and height you know to get the simplified ratio; enter one dimension and a target ratio to work out the other.

Formula

An aspect ratio is width divided by height, conventionally written as width:height. For width w and height h, the decimal ratio is r = w ÷ h. To express the ratio as the smallest whole-number pair, divide both dimensions by their greatest common divisor (GCD): w′ = w ÷ GCD(w,h) and h′ = h ÷ GCD(w,h). The calculator’s simplified width and height are those two values. Width and height must use the same unit, and each must be greater than zero. The inputs here are in pixels, but the ratio itself has no unit.

For resizing to a chosen ratio, use the same relationship in reverse. If the target ratio is a:b, then a known width gives height = width × b ÷ a; a known height gives width = height × a ÷ b. These equations preserve proportions and can be used with pixels, inches, centimetres, or other consistent units.

Worked Example

Using the calculator’s default width of 1,920 px and height of 1,080 px:

1. Divide width by height: 1,920 ÷ 1,080 = 1.777…; rounded to three decimal places, the ratio is 1.778.
2. Find the greatest common divisor of 1,920 and 1,080: it is 120.
3. Divide both dimensions by 120: 1,920 ÷ 120 = 16 and 1,080 ÷ 120 = 9.

The result is approximately 1.778:1, or the exact simplified ratio 16:9. The decimal is rounded for display; 16:9 is the exact whole-number relationship.

What Aspect Ratio Describes

Aspect ratio describes shape, not physical size, resolution, or image quality. A 16:9 image could be 1,920 × 1,080 pixels or 3,840 × 2,160 pixels: the pixel counts differ, but their proportions are the same. Likewise, a display’s diagonal measurement alone does not tell you its aspect ratio.

Ratios are usually written width first and height second. A 4:3 image is four units wide for every three units tall; a 3:2 image is three units wide for every two units tall. A ratio greater than 1:1 is wider than it is tall, while a ratio below 1:1 is taller than it is wide. A square is 1:1.

Common examples include 16:9 for many widescreen displays and video frames, 4:3 for older television and video formats, and 1:1 for square images. These are common conventions, not rules that every device or platform must follow. Always check the required dimensions for the specific destination.

When to Use the Calculator

Use an aspect ratio calculator when you need to identify an existing image’s proportions, prepare artwork for a particular frame, or resize without stretching. It is also useful for working out a matching height or width before cropping or laying out content.

• Identify a ratio: enter both dimensions, such as 1,200 × 800, to find 1.5:1, or 3:2 in simplified form.
• Resize proportionally: for a 16:9 image that must be 1,280 px wide, calculate 1,280 × 9 ÷ 16 = 720 px tall.
• Fit a frame: compare the content’s ratio with the frame’s ratio to see whether it will fit exactly, need cropping, or leave empty space.
• Convert physical dimensions: apply the ratio to measurements in inches or centimetres when planning a print or physical screen layout.

For example, an image that is 1,200 × 800 px has a 3:2 ratio. To retain that shape at 900 px wide, its height should be 900 × 2 ÷ 3 = 600 px. If the destination requires 16:9 instead, forcing the 3:2 image into those exact dimensions would distort it unless you crop or add space.

Resizing, Cropping, and Fitting

Keeping the aspect ratio locked during resizing changes width and height together. This prevents circles from becoming ovals and people from appearing unnaturally wide or narrow. If you change only one dimension while leaving the other fixed, the result is stretched or compressed unless the software automatically crops or pads the image.

When source and destination ratios differ, there are three basic choices:

• Crop: fill the destination frame by removing parts of the image. Check that important subjects or text are not cut off.
• Letterbox or pillarbox: show the entire image and leave unused space at the top and bottom, or at the sides.
• Stretch: fill the frame by changing proportions. This may visibly distort the content and is usually unsuitable for photographs or video.

For a concrete comparison, fitting 4:3 content into a 16:9 frame requires either cropping some of the image’s top and bottom, leaving empty side areas, or distorting its shape. The ratio alone does not decide which option is best; the intended composition and display matter.

Common Mistakes and Limits

The most frequent error is reversing width and height. A 1920 × 1080 image is 16:9, while 1080 × 1920 is 9:16: the same numbers describe opposite orientations when their order changes. Enter dimensions in the stated width-then-height order.

Another common error is mixing units. A width of 10 inches and height of 20 centimetres cannot be divided directly to find a meaningful ratio. Convert both to the same unit first. Pixel dimensions can be compared directly when they come from the same image or frame.

Rounding also matters. The decimal 1.778 is a convenient approximation of 16 ÷ 9, not an exact substitute for every calculation. Keep the original dimensions or use the whole-number ratio for accurate resizing; round final pixel dimensions to whole pixels only when necessary. A calculator can establish proportions, but it cannot determine which content should be cropped, how a particular application handles scaling, or whether a screen’s advertised dimensions include borders.

Frequently Asked Questions

How do I calculate an aspect ratio?
Write the width and height in that order, then simplify the pair by dividing both by their greatest common divisor. For a decimal ratio, divide width by height. For example, 1,200 × 800 simplifies to 3:2 and has a decimal ratio of 1.5:1.

What is the aspect ratio of 1920 × 1080?
It is 16:9. Dividing both values by 120 gives 16 and 9; dividing 1,920 by 1,080 gives approximately 1.778.

How do I resize an image without changing its aspect ratio?
Keep the ratio locked, or calculate the unknown dimension from the known one. For a 4:3 image that is 800 px wide, the matching height is 800 × 3 ÷ 4 = 600 px.

Is aspect ratio the same as resolution?
No. Aspect ratio is the relationship between width and height. Resolution is the number of pixels in the image or display. Different resolutions can share the same ratio.

What happens when an image’s ratio does not match the screen?
It must be cropped, shown with empty space around it, or stretched to fill the screen. Cropping removes content, empty space preserves the full image, and stretching changes its proportions.

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in w | Width | px | 1920 | 0
in h | Height | px | 1080 | 0
out ratio | Aspect ratio | - | w / h | 3
out simplified_width | Ratio width, simplified | - | w / gcd(w,h) | 0
out simplified_height | Ratio height, simplified | - | h / gcd(w,h) | 0