Aspect Ratio Calculator

Aspect Ratio Calculator

Type a width and height to get the simplified ratio and its decimal form, lock any ratio to resize without distortion, and preview exactly how much letterboxing or cropping a mismatched frame costs you.

1 · Find a ratio from pixels

16:9= 1.7778:1 · landscape

Exact match for the standard 16:9 ratio.

2 · Resize with a locked ratio

Ratio (custom allowed)
:

= 16:9 · 1.7778:1

New size (edit either side)
×

width × height, px

Exact fit — both sides are whole pixels at this ratio.

3 · Preview: your pixels inside the locked ratio

Content 1920×1080 (16:9) placed in a 16:9 frame:

Exact match — no bars.

What an aspect ratio actually is

An aspect ratio is the relationship between an image's width and height, written as two numbers separated by a colon. It says nothing about size: 1920×1080, 1280×720 and 640×360 are three very different resolutions with the same 16:9 ratio, which is why a 720p video fills a 4K screen without bars or distortion. The math behind the tool is deliberately simple, and it is worth knowing so you can sanity-check any result:

  • Simplifying: divide width and height by their greatest common divisor (GCD). This is exact, not an approximation.
  • Decimal form: width ÷ height. 16:9 = 1.7778, 4:3 = 1.3333, 9:16 = 0.5625. Anything above 1 is landscape, below 1 is portrait.
  • Resizing on a locked ratio: new height = new width × (ratio height ÷ ratio width), then rounded to a whole pixel — with a warning when rounding actually changed the value.

Three worked examples

1. Simplify 1920×1080

The GCD of 1920 and 1080 is 120. Dividing both sides gives 16:9, decimal 1.7778 — an exact match for the most common video standard. Every mainstream 16:9 resolution is just a multiple of this pair: 1280×720 is 80×(16:9), 3840×2160 is 240×(16:9). The full ladder from 640×360 to 8K is on the 16:9 resolution chart.

2. Simplify 1366×768 — the classic trap

The GCD of 1366 and 768 is just 2, so the exact answer is the unhelpful-looking 683:384. Its decimal, 1.7786, sits about 0.05% away from true 16:9. That tiny gap is why this laptop resolution confuses people: it is sold as 16:9, behaves like 16:9, but a locked-16:9 resize starting from 1366 will always land on a fractional height (768.375). The calculator flags both facts instead of picking one. More reverse-lookup cases like this are on the pixels-to-ratio page.

3. Resize 16:9 to 1000 px wide

Lock 16:9 and type 1000 as the width: the exact height is 1000 × 9 ÷ 16 = 562.5 px. Half a pixel does not exist, so the tool rounds to 562 using round-half-to-even and says so. It also tells you the fix — 16:9 widths divisible by 16 (such as 992 or 1008) always produce whole-pixel heights. Silent rounding is how images end up one pixel off-ratio and video encoders end up complaining about odd dimensions.

Common aspect ratios at a glance

Reference ratios with exact decimal values and example pixel sizes. Decimals rounded to 4 places.
Ratio Decimal Orientation Example resolutions Typical use
9:160.5625Portrait1080×1920, 720×1280TikTok, Reels, Shorts, Stories
4:50.8Portrait1080×1350Instagram portrait posts
1:11Square1080×1080Profile pictures, square posts
5:41.25Landscape1280×1024Older SXGA monitors
4:31.3333Landscape1024×768, 4032×3024Classic TV, phone/compact camera sensors
3:21.5Landscape6000×4000DSLR/mirrorless photos, 35mm film
16:101.6Landscape1920×1200, 2560×1600Productivity laptops
16:91.7778Landscape1920×1080, 3840×2160HD/4K video, YouTube, TV
1.85:11.85Landscape1998×1080 (DCI flat)Cinema "flat" projection
21:9-class2.3333*Landscape3440×1440, 2560×1080Ultrawide monitors
2.39:12.39Landscape2048×858 (DCI scope)Widescreen cinema "scope"

*Marketing value — actual ultrawide panels are 2.37 to 2.40, none exactly 21:9. The ultrawide page lists the exact ratio of each panel resolution.

What people use this for

  • Exporting video for a specific platform — checking that an edit will land on 9:16 for vertical feeds or 16:9 for YouTube before rendering.
  • Cropping photos for Instagram — seeing exactly how much of a shot survives the 4:5 portrait crop before committing.
  • CSS and responsive design — getting the aspect-ratio value for an embed container, or the padding-top percentage for older fallbacks (9÷16 = 56.25%).
  • Resizing images without distortion — locking the original ratio and computing the missing dimension instead of eyeballing it.
  • Identifying odd files — pasting the pixel size of a scan, screenshot or sensor output into the reverse lookup to find what standard it is (or isn't).

Letterbox vs crop, in numbers

When content and frame ratios differ, you pay in one of two currencies: screen area (letterbox) or content (crop). The preview above computes the exact price. Two examples worth memorising: 4:3 content on a 16:9 screen wastes 25% of the screen width as side bars, or loses 25% of its height if you crop to fill. And 16:9 footage placed in a 9:16 vertical frame loses a brutal 68.4% of its width when cropped — which is why vertical-first platforms keep telling creators to shoot vertical rather than crop landscape footage.

Link to this tool

If this calculator saved you some arithmetic, a link helps other people find it (and is the only "payment" this free tool ever asks for): 

<a href="https://aspectcalc.pages.dev/">Aspect Ratio Calculator</a>

Frequently asked questions

How do I calculate an aspect ratio from a width and height?

Divide both numbers by their greatest common divisor (GCD). For 1920×1080 the GCD is 120, so 1920÷120 = 16 and 1080÷120 = 9 — the ratio is 16:9. The calculator does exactly this, and also shows the decimal form (1920÷1080 ≈ 1.7778), which is easier to compare across ratios than fractions are.

Why does 1366×768 give 683:384 instead of 16:9?

Because 1366×768 is genuinely not 16:9. Its GCD is only 2, so the simplest exact form is 683:384, which equals about 1.7786 — roughly 0.05% wider than true 16:9 (1.7778). The panel was marketed as widescreen and everything displays fine on it, but a pixel-exact 16:9 frame at 1366 wide would need a height of 768.375 pixels, which does not exist. The calculator shows both the exact ratio and the nearest common ratio so you can decide which answer you actually need.

What does the "not a whole pixel" warning mean?

When you lock a ratio and type one dimension, the other side is computed by multiplication and division — and the exact result is often fractional. 16:9 at 1000 px wide gives a height of exactly 562.5 px. Pixels are indivisible, so the tool rounds and tells you it did, instead of silently printing a number that is not exactly on-ratio. If you need pixel-perfect output, the warning also tells you which widths (or heights) avoid the problem — for 16:9, any width divisible by 16.

Why round half to even instead of always rounding up?

Round-half-to-even ("banker's rounding", the IEEE 754 default) sends 562.5 to 562 but 563.5 to 564 — ties go to the even neighbour. Always rounding .5 up biases every tie in the same direction, which matters when dimensions get scaled repeatedly in a pipeline. For a single resize the difference is at most one pixel either way; the point is that the rule is stated, not hidden.

Should I letterbox or crop when the ratios don't match?

Letterboxing (fit) keeps every pixel of your content and fills the leftover frame with bars; cropping (fill) uses the whole frame but throws pixels away. The right choice depends on what is near the edges. The preview in section 3 shows both with the exact percentage: 16:9 footage in a 9:16 vertical frame keeps everything but shrinks it drastically when letterboxed, or loses about 68% of its width when cropped. Numbers like that usually make the decision obvious.

Is 21:9 a real aspect ratio?

As marketing, yes; as an exact panel ratio, almost never. True 21:9 is about 2.333, but the common ultrawide resolutions are 2560×1080 and 5120×2160 (64:27 ≈ 2.370), 3440×1440 (43:18 ≈ 2.389) and 3840×1600 (12:5 = 2.400). Details and the math are on the 21:9 ultrawide page.

Do you store or see the numbers I type?

No. The whole tool is JavaScript running in your browser. There is no server-side calculation, no account and no database — the dimensions you enter never leave your device, and we never receive them.