年齢計算
画像と動画のアスペクト比を計算し、任意の比率の寸法を求め、画像を比例的に拡大縮小します。
このツールについて
特定のアスペクト比を維持する寸法を計算します。幅と高さを入力して比率（16:9、4:3など）を取得するか、比率と一方の寸法を入力してもう一方を計算します。一般的な画面サイズ、ソーシャルメディア、動画フォーマットのプリセット比率が含まれています。
使い方
- 1 幅と高さを入力してアスペクト比を計算してください。
- 2 またはプリセット比率（16:9、4:3など）を選択して一方の寸法を入力してください。
- 3 もう一方の寸法が自動的に計算されます。
- 4 「比率」表示を使用して簡略化された比率を確認してください。
What an aspect ratio is, and why it matters
An aspect ratio is the proportional relationship between width and height, written as width:height. A 16:9 image is 16 units wide for every 9 tall — about 1.78 times wider than it is high. The ratio is independent of actual size: 1280×720, 1920×1080, and 3840×2160 are all 16:9. Getting it right is what stops images from looking stretched or squashed when they move between a phone, a slide deck, a YouTube player, and a print. This calculator does two distinct jobs: it derives the simplified ratio from any pair of dimensions, and it scales one dimension to match a target while preserving the proportion.
How the ratio is simplified
To turn 1920×1080 into 16:9, the tool divides both numbers by their greatest common divisor (GCD) using the Euclidean algorithm. The GCD of 1920 and 1080 is 120, and 1920 ÷ 120 = 16 while 1080 ÷ 120 = 9. It also shows the decimal form (here 1.7778:1), which is useful when a ratio doesn't reduce to small whole numbers. A cinema frame of 2.39:1, for instance, has an ugly integer ratio but a clean decimal, so the decimal is the value you compare against.
The scaling formula, worked out
The "Scale to New Size" panel keeps your original proportion locked and fills in the missing dimension. The rule is simply new_height = new_width ÷ ratio (or new_width = new_height × ratio), where the ratio comes from your original width divided by your original height. Suppose your source is 1920×1080 (ratio 1.7778) and you need it 1280 wide. The tool computes 1280 ÷ 1.7778 ≈ 720, giving 1280×720 — the same shape, smaller. Ask instead for a height of 600 and it returns 600 × 1.7778 ≈ 1067 wide. Results are rounded to whole pixels, so on rare ratios the reduced figure can be off by a pixel; that's expected, since you can't have a fractional pixel.
Preset ratios and where they belong
| Ratio | Decimal | Typical use |
|---|---|---|
| 16:9 | 1.78 | HD/4K video, YouTube, most monitors |
| 4:3 | 1.33 | Classic TV, older photos, some slides |
| 1:1 | 1.00 | Square social posts, avatars |
| 21:9 | 2.33 | Ultrawide monitors, cinematic crops |
| 3:2 | 1.50 | DSLR/mirrorless stills, prints |
| 4:5 | 0.80 | Instagram portrait (1080×1350) |
| 9:16 | 0.56 | Phone stories, Reels, TikTok |
| 2.39:1 | 2.39 | Anamorphic widescreen film |
Selecting a preset fills the dimension fields with that ratio scaled by 100 (so 16:9 becomes 1600×900) and updates the live preview rectangle, giving you a quick visual sense of how tall or wide the frame is.
Practical use cases
- Resizing without distortion. Lock the original ratio, type the width your layout allows, and read off the exact height — no guessing, no manual division.
- Designing responsive embeds. A 16:9 video in a fluid container needs a height equal to width × 9 ÷ 16; this is the number CSS
aspect-ratioor the old padding-top trick (56.25%) encodes. - Choosing export sizes per platform. Confirm that 1080×1350 really is 4:5 before exporting a batch for Instagram, or that a thumbnail hits 16:9 for YouTube.
- Cropping decisions. Compare your source ratio to the target: if your photo is 3:2 and the slot is 16:9, you'll lose height, so frame the crop accordingly.
Common mistakes
- Changing only one dimension. Setting a new width without recomputing height is the classic cause of squashed images. Always scale both together.
- Confusing ratio with resolution. 720p and 1080p are different resolutions but the same 16:9 ratio; matching the ratio doesn't guarantee matching sharpness.
- Forgetting orientation. 9:16 and 16:9 are the same numbers swapped but opposite shapes — a portrait story is not a landscape video.
- Rounding too early. If you chain several resizes, small rounding errors accumulate; scale once from the original whenever possible.
- Letterboxing without a plan. Forcing a 4:3 photo into a 16:9 frame leaves black bars on the sides; decide upfront whether to crop, pad, or pick a different target ratio.
A quick note on letterboxing versus cropping
When a source ratio doesn't match its destination, you have two honest choices and one bad one. You can letterbox (add bars to keep the whole image) or crop (fill the frame and lose some edges) — both preserve the proportions of what remains. The bad choice is stretching, which changes the ratio and visibly distorts faces and circles. Use this calculator to see the gap before you decide: compare your source decimal ratio to the target's. A 1.50 (3:2) photo headed into a 1.78 (16:9) slot is wider on paper but proportionally taller, so filling the slot means trimming the top and bottom.
Everything runs in your browser
The GCD reduction, the decimal ratio, and the scaling math are plain JavaScript that execute on your device — no dimensions are uploaded and no image is processed here; this tool works purely with the numbers you enter. It functions offline once the page has loaded.