Free Online Aspect Ratio Calculator
Find the ratio for any dimensions or resize with locked proportions
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Why use Aspect Ratio Calculator
- Reduces any dimensions to lowest terms -- no manual GCD math.
- Resize mode computes the missing dimension so you only enter one number.
- Swap button flips orientation in one click for cross-platform repurposing.
- Presets cover the five formats people actually use: 16:9, 4:3, 1:1, 21:9, 9:16.
- Results are pixel-precise -- no rounding surprises when you export.
How it works
The calculator computes the greatest common divisor (GCD) using the Euclidean algorithm -- repeatedly dividing the larger number by the smaller until the remainder is zero. Dividing both dimensions by the GCD produces the simplified ratio. In Resize mode, cross-multiplication solves for the missing dimension: newWidth * H / W, rounded to the nearest pixel.
About this tool
Enter 1920 and 1080 and the tool tells you 16:9. Enter a target width of 1280 in Resize mode and it returns 720 for the height -- proportions locked, no math. Quick-select presets for 16:9, 4:3, 1:1, 21:9, and 9:16 cover the formats people actually use: YouTube, TikTok, Instagram, ultrawide monitors, and presentation slides. A swap button flips landscape to portrait in one click, saving time when repurposing a YouTube thumbnail for Instagram Reels. You need this when setting the CSS aspect-ratio property on responsive containers, exporting video to platforms with different frame requirements, cropping photos to standard print sizes (4x6 is 3:2), or configuring width and height attributes on img tags to prevent layout shift.
How to use Aspect Ratio Calculator
- Enter width and height. Type your dimensions. The simplified ratio appears instantly -- 1920 x 1080 resolves to 16:9.
- Or pick a preset. Click 16:9, 4:3, 1:1, 21:9, or 9:16 to load a ratio directly.
- Use Resize mode. Toggle Resize, enter one dimension, and the other is calculated with proportions locked.
- Swap orientation. Click Swap to flip landscape to portrait (or vice versa) in one click.
Use cases
- Resizing a 4K frame to 1280px wide for a YouTube thumbnail while keeping the height at exactly 720.
- Checking whether a 5184 x 3456 RAW file matches 3:2 for 4x6 inch prints without cropping.
- Finding the correct height for a 21:9 widescreen embed container to set CSS padding-top.
- Flipping a landscape blog banner to 9:16 for Instagram Stories.
- Determining the exact pixel crop for a podcast cover that needs 1:1 at 3000x3000 from a rectangular source.
- Calculating the aspect-ratio CSS value for a responsive video wrapper.
Frequently Asked Questions
The proportional relationship between width and height, written as two numbers separated by a colon (e.g., 16:9). A 1280x720 image and a 1920x1080 image share the same 16:9 ratio -- same shape, different resolutions.
Divide both width and height by their greatest common divisor (GCD). For 1920x1080: GCD = 120, so 1920/120 = 16, 1080/120 = 9. Result: 16:9. This tool does it automatically.
To keep the aspect ratio when scaling, multiply both dimensions by the same factor. For example, to scale 1920×1080 down to 1280 width: 1280 ÷ 1920 = 0.667. Multiply the height: 1080 × 0.667 ≈ 720. Result: 1280×720 (still 16:9). This tool's Resize mode does this automatically -enter the known dimension and get the other.
16:9 is standard for HD video, YouTube, most monitors, and presentations. 4:3 is used by older monitors, standard-definition video, and many tablet screens. 1:1 is used by Instagram posts, profile photos, and album covers. 21:9 (ultrawide) is used by cinema films and ultrawide monitors. 9:16 is the portrait format for TikTok, Instagram Reels, and YouTube Shorts.
16:9 (1.78:1) is the standard widescreen format used by televisions, most monitors, and streaming platforms. 16:10 (1.6:1) is slightly taller and was popular on laptops and professional monitors in the late 2000s for extra vertical space when reading and coding. Many modern laptops are returning to 16:10 or even 3:2 for improved usability.
It depends on the platform and placement. Instagram feed posts work best at 1:1 (square) or 4:5 (portrait, takes more screen space). Instagram Reels, TikTok, and YouTube Shorts all use 9:16 (1080 × 1920). YouTube videos are 16:9 (1920 × 1080 or 1280 × 720). Facebook and LinkedIn link previews prefer 1.91:1 (approximately 1200 × 628). Twitter/X card images use 2:1. Check each platform's current upload guidelines, as they update periodically.
The modern approach uses the aspect-ratio CSS property: set width: 100% and aspect-ratio: 16 / 9 on the container. For broader browser support, the older padding-top trick works: set position: relative, padding-top: 56.25% (which is 9/16 × 100%), and position the child element absolutely inside it. The CSS aspect-ratio property is supported in all browsers since 2021 and is now the recommended method.
Aspect ratio describes the proportional shape (16:9, 4:3) without specifying pixel count. Resolution specifies the actual pixel dimensions (1920 × 1080). Two images can share the same aspect ratio but have different resolutions -1280 × 720 and 3840 × 2160 are both 16:9 but vary significantly in file size and sharpness. Aspect ratio determines how an image fits a frame; resolution determines how sharp it looks.
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