OptiDrop

Aspect Ratio Calculator — Resize Dimensions

Calculate missing width or height while keeping the aspect ratio. Choose from common ratios or enter a custom one. 100% free, no sign-up.

What is an Aspect Ratio?

An aspect ratio describes the proportional relationship between the width and height of an image, screen, or video. It is written as two numbers separated by a colon, such as 16:9 or 4:3. The aspect ratio does not tell you the actual pixel dimensions — it tells you how the width relates to the height.

Common Aspect Ratios

  • 16:9 — Standard widescreen for YouTube, monitors, and TVs
  • 4:3 — Traditional monitor and presentation ratio
  • 1:1 — Square format for Instagram posts and profile pictures
  • 4:5 — Instagram portrait feed posts
  • 9:16 — Vertical video for TikTok, Instagram Reels, YouTube Shorts
  • 3:2 — Standard DSLR photo ratio
  • 21:9 — Ultrawide cinema monitors

How to Use This Calculator

Enter your desired width in the width field, and the calculator will automatically compute the matching height to maintain the selected aspect ratio. You can also enter a height first and get the width. Use the preset buttons for common ratios, or enter a custom ratio.

Frequently Asked Questions

An aspect ratio describes the proportional relationship between width and height. For example, 16:9 means the width is 16 units for every 9 units of height. Common ratios include 16:9 (widescreen), 4:3 (standard), and 1:1 (square).
Instagram uses several ratios: Square posts are 1:1 (1080x1080), Portrait posts are 4:5 (1080x1350), Stories and Reels are 9:16 (1080x1920). This calculator helps you resize images to fit these dimensions.
Divide both width and height by their greatest common divisor. For 1920x1080: GCD is 120, so 1920/120 = 16 and 1080/120 = 9, giving 16:9. To resize while keeping the ratio: new height = original height × (new width / original width).
16:9 (widescreen) is wider and used for modern displays and YouTube. 4:3 (standard) is more square and used for older monitors and presentations. At 1920px wide, 16:9 is 1080px tall while 4:3 is 1440px tall.

Related Tools

Photo Resizer Resize photos to passport dimensions. Format Converter Convert between JPG, PNG, WebP, BMP. Unit Converter Convert length, weight, temperature, speed.

Last updated: June 2026