Proportional resizing

Aspect Ratio Calculator

Enter an original width and height, then type either a new width or a new height — the missing dimension is filled in instantly so your image, video or layout keeps exactly the same proportions.

Original size

New size — fill in one side

—New dimensions
—Simplified ratio
—Decimal ratio
—Common name
—Scale factor
—Pixel count change

Nothing leaves your device — the calculation runs in the page.

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Solve either way

Fill in a new width or a new height and the other side is calculated for you.

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Ratio in lowest terms

See 1920 x 1080 reduced to 16:9, with the decimal value alongside.

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Named presets

Jump straight to widescreen, cinema, square or vertical video proportions.

How to use the aspect ratio calculator

Aspect ratio maths is a single proportion, but doing it by hand for every asset gets tedious. This tool keeps both sides in sync as you type.

  1. Enter the original sizeType the width and height you are starting from, or tap a ratio preset.
  2. Type one new dimensionEnter either the new width or the new height — leave the other blank.
  3. Copy the resultThe missing dimension appears immediately, along with the simplified ratio and pixel counts.

About aspect ratios

An aspect ratio describes the relationship between width and height, independent of size. Preserving it while resizing is what stops photographs from looking stretched and video from being letterboxed badly. Because the relationship is a simple proportion, one known dimension is always enough to derive the other.

Common video ratios

16:9 is standard HD, 21:9 (really 64:27) is ultrawide, 2.39:1 is anamorphic cinema, and 9:16 is vertical short-form video.

Common photo ratios

3:2 comes from 35 mm film and most DSLRs, 4:3 from Micro Four Thirds and older sensors, and 1:1 from medium format and square social posts.

Letterboxing and pillarboxing

Fitting content of one ratio into a frame of another leaves bars: horizontal bars are letterboxing, vertical bars are pillarboxing.

Rounding to whole pixels

Video encoders often prefer even numbers or multiples of eight, so a calculated 719 px height is usually rounded to 720.

Aspect ratio calculator FAQ

Quick answers about resizing proportionally and reading ratio notation.

How do I calculate a new height for a given width?

Multiply the new width by the original height and divide by the original width. For a 1920 x 1080 source scaled to 1280 wide, the height is 1280 x 1080 / 1920 = 720.

What does 16:9 mean?

It means the image is 16 units wide for every 9 units tall, a ratio of about 1.778:1. It is the standard for HD and 4K video, YouTube and almost all modern televisions.

Why is my calculated dimension not a whole number?

Not every target size divides cleanly. The calculator rounds to the nearest pixel and shows the exact decimal, so you can decide whether to nudge the other dimension by one pixel instead.

What is the difference between aspect ratio and resolution?

Resolution is the actual pixel count, while aspect ratio is the shape those pixels form. 1920 x 1080 and 1280 x 720 are different resolutions but share the same 16:9 aspect ratio.

Which ratio should I use for social media?

1:1 suits square feed posts, 4:5 is the tallest allowed in most feeds, 9:16 fits full-screen stories and short video, and 16:9 remains standard for landscape video and thumbnails.

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