Aspect Ratio Calculator
Instantly calculate the proportional relationship between width and height for any display, video frame, or image. Enter width, height, or a target ratio to find the missing dimension — with support for 16:9, 4:3, 21:9, 1:1, and any custom aspect ratio.
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What is an Aspect Ratio?
An aspect ratio describes the proportional relationship between the width and height of a rectangular shape. Written as W:H, it defines the shape of a screen, image, or video frame without specifying the actual dimensions. For example, the 16:9 aspect ratio used by modern HD televisions means that for every 16 horizontal units, there are exactly 9 vertical units — whether the display is 32 inches or 65 inches diagonally.
Common aspect ratios you will encounter include 16:9 for HDTV and YouTube, 4:3 for classic television and presentations, 21:9 for ultrawide cinema displays, 1:1 for Instagram square posts, and 9:16 for TikTok and Stories content. Maintaining the correct aspect ratio when resizing prevents stretching, squashing, or letterboxing that degrades visual quality.
使用される計算公式・方程式
この計算ツールは5つの主要な公式を使用しています：
1 Aspect Ratio from Dimensions ▼
Divide both width and height by their GCD. 1920 × 1080: GCD(1920, 1080) = 120. Ratio = 16 : 9.
2 Height from Width and Ratio ▼
Given a width of 1280 and a 16:9 ratio: Height = 1280 × (9/16) = 720 pixels.
3 Width from Height and Ratio ▼
Given a height of 1080 and a 16:9 ratio: Width = 1080 × (16/9) = 1920 pixels.
Explore all calculation options on the 比率計算ツール home page.
比率計算ツールの使い方
この比率計算ツールは、以下の3ステップで簡単にご利用いただけます：
数値を入力
入力欄に既知の比率の値を入力します。求めたい未知数の入力欄は空欄のままにしておきます。
モードを選択
比率モード（解く、簡素化、スケーリング）を選択します。各モードで異なる計算式が適用されます。
結果を確認
計算するボタンを押します。結果画面に答えと、視覚的な比率バー、円グラフ、詳細なステップバイステップの解決プロセスが表示されます。
実例問題と段階的な解説
本比率計算ツールを使って、以下の3つの例題をステップバイステップで解決するプロセスです：
入力 1 Find aspect ratio of 2560 × 1440
入力 2 Find height for 1920px wide at 4:3
入力 3 Find width for 1080px tall at 21:9
よくある質問 (FAQ)
What is the most common aspect ratio for monitors? ▼
16:9 is the most common aspect ratio for monitors, TVs, and laptops. Standard resolutions include 1920×1080 (Full HD), 2560×1440 (QHD), and 3840×2160 (4K UHD). Ultrawide monitors use 21:9 at resolutions like 3440×1440.
How do I calculate the aspect ratio of my screen? ▼
Divide your screen's pixel width by its height, then simplify by dividing both numbers by their greatest common divisor (GCD). For a 2560×1080 monitor: GCD is 40, giving 64:27 — an ultrawide ratio. Use this calculator to automate the process.
What aspect ratio is best for YouTube videos? ▼
16:9 is the standard YouTube aspect ratio. Upload at 1920×1080 (1080p) or 3840×2160 (4K) for best quality. YouTube will add letterbox bars to videos with different ratios. YouTube Shorts use 9:16 (vertical).
What is the difference between 16:9 and 16:10? ▼
16:9 is wider and flatter, standard for entertainment. 16:10 is slightly taller (about 5.5% more vertical space), common on productivity laptops like MacBook Air (2560×1600). The extra height helps with documents, code, and web browsing.
How do I resize an image without changing its aspect ratio? ▼
Lock the aspect ratio in your editing software (usually a chain-link icon), then change only one dimension. The other adjusts automatically. For example, scaling a 1600×900 image (16:9) to 800px wide automatically sets height to 450px.
What aspect ratio does Instagram use? ▼
Instagram supports multiple ratios: 1:1 (square, 1080×1080), 4:5 (portrait, 1080×1350), and 1.91:1 (landscape, 1080×566). For Reels and Stories, use 9:16 (1080×1920). Portrait 4:5 typically gets the most feed visibility.
How do I convert aspect ratio to pixel dimensions? ▼
Multiply each ratio number by the same scale factor. For 16:9 at 720p: 720 ÷ 9 = 80 (scale factor), then 16 × 80 = 1280. Result: 1280×720. For 4K: 2160 ÷ 9 = 240, then 16 × 240 = 3840. Result: 3840×2160.
What is the aspect ratio of a 4K display? ▼
Standard 4K (UHD) is 3840×2160, which is 16:9. DCI 4K (cinema) is 4096×2160, which is approximately 1.9:1 (256:135). Both are often called '4K' but have different aspect ratios.
Can aspect ratio affect page load speed? ▼
Indirectly, yes. Using the wrong aspect ratio forces browsers to reflow content when images load. Setting explicit width and height attributes (or CSS aspect-ratio) prevents cumulative layout shift (CLS), improving Core Web Vitals scores.
What is anamorphic aspect ratio? ▼
Anamorphic is a cinematography technique that uses special lenses to capture a wider field of view on standard film/sensor. The recorded image is horizontally squeezed and must be stretched during playback. Common anamorphic ratios include 2.39:1 and 2.76:1.
How do I find the aspect ratio of a video file? ▼
Use MediaInfo (free software) or right-click the file and check Properties > Details. Look for 'Frame width' and 'Frame height'. Divide both by their GCD to get the ratio. Most video players also display this in stream information.
What aspect ratio should I use for printing photos? ▼
Standard print sizes use different ratios: 4×6 is 3:2, 5×7 is 5:7, 8×10 is 4:5, and 11×14 is 11:14. Most digital cameras shoot in 3:2, so 4×6 prints require no cropping. Other sizes will need slight cropping.