Width & Height Ratio Calculator

Get the aspect ratio from any width and height, or lock a ratio like 16:9 and enter one dimension to find the other. A live preview shows the shape, and the nearest standard ratio is identified when your numbers are close but not exact.

Formula W : H

Common resolutions
Method

Reduce to find it, multiply to apply it

Take both dimensions in the same unit

Usually pixels, but centimetres or inches work identically — an aspect ratio is dimensionless, so the unit cancels. What you cannot do is mix them: 1920 pixels against 10 centimetres is meaningless.

Divide both by their greatest common divisor

gcd(1920, 1080) = 120, so the ratio is 1920/120 : 1080/120 = 16 : 9. If the GCD turns out to be small and the reduced numbers are large and unfamiliar, your dimensions are probably an approximation of a standard ratio rather than an exact match.

Compute the decimal value as a cross-check

1920 ÷ 1080 = 1.7778. This single number is easier to compare than a pair, and it is what tells you that 683 : 384 and 16 : 9 are nearly the same shape.

To resize, scale both dimensions by one factor

From 1920 wide to 1280 wide the factor is 1280 ÷ 1920 = 2/3, so the height goes from 1080 to 1080 × 2/3 = 720. Changing only the width is what distorts an image.

Or divide the new width by the ratio value

1280 ÷ 1.7778 = 720. Same answer, one step, and it works from a ratio alone without needing the original dimensions — which is what the resize mode here does.

ratio = W/g : H/g   (g = gcd)

Finding the ratio. The same GCD reduction as any other ratio — see the simplify ratio calculator for the general case.

H = W ÷ (rW/rH)

Resizing. Divide the width by the ratio's decimal value to get the matching height. Multiply to go the other way.

Fit versus fill

Putting a 16:9 image into a 4:3 frame means choosing. Fit scales until the whole image is inside and leaves bars top and bottom. Fill scales until the frame is covered and crops the sides. You cannot have the whole image and the exact frame shape at once.

Worked examples

Screens, photos and approximations

Exact standard ratio

What aspect ratio is 1920 × 1080?

Euclidean GCD
gcd(1920, 1080) = 120
Divide width
1920 ÷ 120 = 16
Divide height
1080 ÷ 120 = 9
Decimal
1920 ÷ 1080 = 1.7778
Shape
landscape, since > 1

16 : 9 exactly

Resizing

A 16:9 video needs to be 1280 pixels wide. What height?

Ratio value
16 ÷ 9 = 1.7778
Divide the width
1280 ÷ 1.7778
Height
720
Check
1280 ÷ 720 = 1.7778 ✓
Known as
720p

1280 × 720

Not quite standard

What ratio is 1366 × 768?

GCD
gcd(1366, 768) = 2
Reduced
683 : 384
Decimal
1.7786
16 : 9 is
1.7778
Difference
0.05% — near but not exact

683 : 384, approximately 16 : 9

Photo to portrait post

A 6000 × 4000 photo has to become a 4:5 portrait post 1080 wide.

Photo ratio
6000 : 4000 = 3 : 2 (1.5)
Target ratio
4 : 5 = 0.8
Target height
1080 ÷ 0.8 = 1350
Shapes differ
1.5 vs 0.8, so cropping is needed
Fill approach
crop the sides heavily

1080 × 1350, with a crop

Reference

Standard aspect ratios

Ratio Decimal Orientation Example sizes Used for
16 : 91.7778Landscape1280×720, 1920×1080, 3840×2160HD video, most screens, YouTube
4 : 31.3333Landscape640×480, 1024×768, 1600×1200Older monitors, iPad, some cameras
3 : 21.5Landscape1080×720, 3000×2000, 6000×400035 mm film, most DSLRs
21 : 92.3333Landscape2560×1080, 3440×1440Ultrawide monitors
2.39 : 12.39Landscape2048×858, 4096×1716Anamorphic cinema
1 : 11Square1080×1080, 2048×2048Square social posts, album art
4 : 50.8Portrait1080×1350Tall feed posts
2 : 30.6667Portrait1000×1500, 2000×3000Portrait photos, posters
9 : 160.5625Portrait1080×1920Vertical video, stories, reels
5 : 41.25Landscape1280×1024Older square-ish monitors
√2 : 11.4142LandscapeA-series paperA4, A3 — halves to the same ratio
Background

Why aspect ratios are so rarely round numbers

Screen and image dimensions have to be whole numbers of pixels, but the shapes people want are not always expressible as a small whole-number ratio at a convenient pixel count. The result is a lot of near-misses. 1366 × 768 was a standard laptop resolution for years and is not 16:9 — it is 1.7786 against 1.7778. The manufacturers chose 1366 because 768 × 16/9 is 1365.33, and you cannot have a third of a pixel.

This is why the decimal value matters more than the reduced pair in practice. Two resolutions that reduce to wildly different-looking ratios can be visually identical in shape, and the only way to see that is to compare the decimals. A difference under about 0.5% is invisible; anything over a couple of percent shows as noticeable letterboxing or cropping.

The A-series paper trick

A4 paper has an aspect ratio of √2 : 1, approximately 1.4142, and it is the only ratio with a special property: cut it in half across the long side and each half has the same aspect ratio as the original. A3 halved is A4, A4 halved is A5, and so on indefinitely. That is only true for √2, because halving swaps the ratio to 1 : r/2, and setting r = 2/r gives r² = 2. It is the reason photocopier enlargement is a clean 141% and reduction is 71%.

Cropping decisions

When the source and target ratios differ you lose something. The arithmetic of how much is straightforward: to fill a target of ratio t from a source of ratio s, if s > t the source is too wide and you crop 1 − t/s of the width; if s < t you crop 1 − s/t of the height. Going from a 3:2 photo to a 4:5 post means cropping about 47% of the width, which is why photos framed for print rarely survive that conversion intact.

Related tools

For the general ratio reduction behind this, the simplify ratio calculator handles two to six terms. To scale physical dimensions with unit conversion, and to get the area and volume factors, use the scale factor calculator. And to generate a series of matching sizes at one ratio, the equivalent ratio calculator produces the table.

Questions

Aspect ratios — questions

Resizing without distortion, and why so many real resolutions are only approximately standard.

How do you calculate an aspect ratio from width and height?
Divide both numbers by their greatest common divisor. For 1920 × 1080 the GCD is 120, giving 16 : 9. The decimal form is simply width divided by height — 1920 ÷ 1080 = 1.7778 — which is what you compare when two ratios reduce to unfamiliar numbers.
How do I resize an image without distorting it?
Keep the ratio fixed and scale both dimensions by the same factor. If a 1920 × 1080 image needs to be 1280 wide, the factor is 1280 ÷ 1920 = 0.6667, so the height becomes 1080 × 0.6667 = 720. Equivalently, divide the new width by the ratio value: 1280 ÷ 1.7778 = 720. Changing only one dimension is what stretches an image.
What is 16:9 as a decimal?
16 ÷ 9 = 1.7778 (recurring). Other common values: 4:3 is 1.3333, 3:2 is 1.5, 21:9 is 2.3333, 1:1 is 1, and 9:16 — portrait video — is 0.5625. A value above 1 is landscape, below 1 is portrait, and exactly 1 is square.
Why does 1366 × 768 not reduce to 16 : 9?
Because it is not exactly 16:9. 1366 ÷ 768 = 1.7786 against 16:9's 1.7778 — very close but not equal, and the GCD of 1366 and 768 is 2, giving the unhelpful 683 : 384. Many real screen resolutions are approximations of a standard ratio, chosen because the exact dimensions had to be whole numbers. This calculator reports the nearest standard preset for exactly this reason.
What is the difference between aspect ratio and resolution?
Resolution is the actual pixel count — 1920 × 1080. Aspect ratio is the shape those pixels make — 16 : 9. Many different resolutions share one aspect ratio: 1280×720, 1920×1080 and 3840×2160 are all 16:9. Ratio tells you the shape; resolution tells you the detail.
How do I fit an image into a frame of a different ratio?
You have to choose what to sacrifice. To fit the whole image inside, scale so the more constrained dimension matches and accept blank space on the other axis (letterboxing). To fill the frame, scale so the less constrained dimension matches and accept that the excess is cropped. There is no third option that keeps both the whole image and the exact frame shape.
What aspect ratio should I use for social media?
It varies by platform and placement, but the common shapes are 1:1 square for feed posts, 4:5 for a taller feed image, 9:16 for full-screen vertical video and stories, and 16:9 for landscape video. Because platforms change their specifications, the safest approach is to design at the ratio and export at whatever pixel dimensions are currently required — which is what the resize mode here is for.
Can an aspect ratio have decimals?
The decimal value always can — 1.7778 for 16:9. The ratio itself is conventionally written as whole numbers because that is easier to read and to recognise. The one common exception is 2.39:1 for anamorphic cinema, where the decimal is retained precisely because no small whole-number pair comes close enough.