Portrait Aspect Ratio Calculator

Enter a width and height to get the simplified portrait aspect ratio (reduced with the GCD), its decimal value, and a proportionally scaled dimension for resizing.

Quick Facts

Method
Width and height divided by their greatest common divisor (GCD)
1080 x 1920 shares a GCD of 120, so it reduces to 9:16.

Your Results

Calculated
Simplified ratio
-
Width : Height, lowest terms
Decimal ratio
-
Width ÷ Height (under 1 = portrait)
Scaled height
-
Height at the new width, same ratio
Closest standard ratio
-
Nearest common portrait ratio

Ready

Enter a width and height, then press Calculate.

About the Portrait Aspect Ratio Calculator

An aspect ratio describes the proportional relationship between an image or screen's width and height, written as width:height. A ratio is called portrait when the height is greater than the width — the opposite of landscape. This calculator takes the width and height you enter, reduces them to the simplest whole-number ratio using the greatest common divisor (GCD), reports the decimal equivalent, and — if you supply a new width — scales the height proportionally so a resized image or crop is not stretched or squashed.

The formula

Given a width W and height H:

  • Simplified ratio: divide both W and H by their GCD — W ÷ gcd(W,H) : H ÷ gcd(W,H). For 1080 × 1920, gcd(1080,1920) = 120, so the ratio reduces to 9:16.
  • Decimal ratio: W ÷ H. A value under 1 confirms portrait orientation (taller than wide); exactly 1 is square; over 1 is landscape.
  • Proportional scaling: to resize without distortion, a new width W′ needs a new height of H′ = W′ × (H ÷ W). This keeps W′ ÷ H′ equal to the original W ÷ H.

Common portrait aspect ratios

  • 1:1 — square format; Instagram feed posts, profile photos
  • 4:5 — the tallest standard Instagram feed post format
  • 3:4 — classic photo prints and many tablet screens in portrait mode
  • 2:3 — 35mm film and common 4×6 photo prints
  • 9:16 — vertical phone screens; Instagram Reels, TikTok, YouTube Shorts

Practical uses

  • Finding the simplified ratio name for a set of pixel or print dimensions
  • Calculating the height of an image at a new target width (or vice versa) without distortion
  • Checking whether a photo or design matches a platform's required portrait ratio before upload

Frequently Asked Questions

How is an aspect ratio simplified?
An aspect ratio is simplified by dividing both the width and height by their greatest common divisor (GCD). For example, 1080 × 1920 pixels shares a GCD of 120, so 1080/120 : 1920/120 reduces to 9:16. The same method works for print or screen dimensions in any unit.
What counts as a portrait aspect ratio?
A ratio is portrait when the height is greater than the width, so the simplified width:height ratio is less than 1 as a decimal (for example 9:16 = 0.5625). If width and height are equal it is square (1:1), and if width exceeds height it is landscape.
How do I resize an image without distorting it?
Pick one dimension you want to change, then multiply it by the ratio of the original height to the original width (or width to height) to get the matching value for the other dimension. This cross-multiplication keeps width divided by height constant, so the resized image is not stretched or squashed.

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