Aspect Ratio Calculator
Keep an image or video in proportion: enter the original size, pick a preset or your own values, then type a new width or height — the missing side fills in instantly.
Formulas used
| Task | Formula |
|---|---|
| Missing height | H₂ = W₂ × H₁ ÷ W₁ |
| Missing width | W₂ = H₂ × W₁ ÷ H₁ |
| Simplified ratio | W : H, both divided by GCD(W, H) |
| Megapixels | MP = W₂ × H₂ ÷ 1,000,000 |
Frequently asked questions
What is an aspect ratio?
An aspect ratio is the proportional relationship between an image's width and height, written as W:H. A 1920 × 1080 video has an aspect ratio of 16:9 — sixteen units wide for every nine tall.
How do I calculate a missing dimension?
Cross-multiply the equal ratios W₁ ÷ H₁ = W₂ ÷ H₂. To scale 1920 × 1080 down to a width of 1280: H₂ = 1280 × 1080 ÷ 1920 = 720, giving 1280 × 720.
What are the most common aspect ratios?
16:9 is standard for TVs, monitors, and YouTube; 9:16 for vertical video (Shorts, Reels, TikTok); 4:3 for tablets and classic TVs; 1:1 for square social posts; 3:2 for most DSLR photos; and 21:9 for ultrawide monitors.
How do I simplify an aspect ratio?
Divide both numbers by their greatest common divisor. For 1920 × 1080 the GCD is 120, and 1920 ÷ 120 : 1080 ÷ 120 simplifies to 16:9.
Why does my image get letterboxed bars?
Bars appear when the content's aspect ratio differs from the display's — showing a 16:9 video on a 4:3 screen (or vice versa) leaves empty space. Resizing to the display's exact ratio removes them.
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