How is an aspect ratio simplified?

The calculator scales both numbers to integers and divides them by their greatest common divisor, so 1920 × 1080 reduces to 16:9 and 1440 × 960 reduces to 3:2. If the sides share no useful common factor, the ratio stays as entered, which is why the decimal value is shown alongside — it is always comparable regardless of how the pair simplifies.

How do I resize an image without stretching it?

Enter the original width and height, then type your target width in the resize section. The matching height is computed as new width × original height ÷ original width, which preserves the exact proportions. Use that pair in any image editor or CSS and nothing will look squashed or stretched.

What does the fit-inside-box section do?

It scales your source dimensions by the largest factor that keeps both sides within the box you specify — the same math video players use for letterboxing. For example, a 1920 × 1080 video inside a 1280 × 1280 box becomes 1280 × 720. The scale factor is shown so you know whether the result was reduced or enlarged.

Which aspect ratios should I use for video and social media?

16:9 is the standard for YouTube, TVs and most monitors; 9:16 is used for vertical formats like Reels, Shorts and TikTok. 1:1 suits square feed posts, 4:3 matches older footage and many tablets, 3:2 is common for photography, and 21:9 is the typical ultrawide cinema format. The preset chips fill these in with one tap.

Is anything I enter uploaded to a server?

No. Every calculation runs in JavaScript inside your browser and no network request is made. The page also works offline once loaded, and dimensions you type are never stored or transmitted anywhere.

Aspect Ratio Calculator

Enter a width and height to see the simplified ratio, resize while keeping proportions, or fit dimensions inside a maximum box.

WidthHeight

16:9

1.78 : 1 · one unit of height per 1.78 units of width

shape at 16:9

Resize while keeping the ratio

Enter a new width or height — the other side is computed from the ratio above.

New widthNew height

1280 × 720

same proportions as 1920 × 1080

Fit inside a box

Largest size of your 1920 × 1080 source that fits inside a maximum frame without cropping.

Max widthMax height

Computed dimensions may be rounded to whole pixels; video encoders often require even numbers.

How it works

Type the original width and height in pixels, or tap a preset chip such as 16:9 or 4:3 to fill them in.
Read the simplified ratio and decimal value, then enter a new width or height in the resize section — the other dimension is computed automatically.
To letterbox media, open the fit-inside-box section and enter the maximum width and height; the tool returns the largest dimensions that fit while keeping the ratio.

Frequently asked questions

How is an aspect ratio simplified?: The calculator scales both numbers to integers and divides them by their greatest common divisor, so 1920 × 1080 reduces to 16:9 and 1440 × 960 reduces to 3:2. If the sides share no useful common factor, the ratio stays as entered, which is why the decimal value is shown alongside — it is always comparable regardless of how the pair simplifies.
How do I resize an image without stretching it?: Enter the original width and height, then type your target width in the resize section. The matching height is computed as new width × original height ÷ original width, which preserves the exact proportions. Use that pair in any image editor or CSS and nothing will look squashed or stretched.
What does the fit-inside-box section do?: It scales your source dimensions by the largest factor that keeps both sides within the box you specify — the same math video players use for letterboxing. For example, a 1920 × 1080 video inside a 1280 × 1280 box becomes 1280 × 720. The scale factor is shown so you know whether the result was reduced or enlarged.
Which aspect ratios should I use for video and social media?: 16:9 is the standard for YouTube, TVs and most monitors; 9:16 is used for vertical formats like Reels, Shorts and TikTok. 1:1 suits square feed posts, 4:3 matches older footage and many tablets, 3:2 is common for photography, and 21:9 is the typical ultrawide cinema format. The preset chips fill these in with one tap.
Is anything I enter uploaded to a server?: No. Every calculation runs in JavaScript inside your browser and no network request is made. The page also works offline once loaded, and dimensions you type are never stored or transmitted anywhere.

About this tool

This aspect ratio calculator turns any width and height into a simplified ratio and its decimal equivalent, updating live as you type. Enter 1920 × 1080 and you immediately see 16:9 and 1.78; enter 1080 × 1350 and you get 4:5 — the ratio Instagram uses for portrait posts. A small rectangle preview drawn to scale shows the actual shape, which is often quicker to judge than the numbers themselves, and preset chips for 16:9, 4:3, 21:9, 1:1, 9:16 and 3:2 fill in the most common formats with one tap.

The math runs entirely in your browser. Ratios are simplified by scaling both sides to integers and dividing by their greatest common divisor, and the resize section applies straightforward cross-multiplication: type a new width and the height that preserves the ratio appears instantly, or type a height to solve for width. Nothing is uploaded, so the tool responds on every keystroke with no waiting.

The fit-inside-box section answers a slightly different question: what is the largest version of my source that fits inside a given frame? It multiplies your source dimensions by the smaller of the two possible scale factors — box width over source width, or box height over source height — which is exactly how players letterbox video and how CSS object-fit: contain positions images. Use it to size a thumbnail into a fixed card, place a video inside a page section, or prepare artwork for a print area without cropping.

Practical tips: work from the original pixel dimensions rather than an already-simplified ratio, since rounding compounds when you chain calculations. When a computed dimension comes out fractional, round to the nearest even number — many video encoders require both sides to be divisible by two. And if two sources show the same simplified ratio but different decimals, trust the decimal; it reflects the exact proportions before any rounding.