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Aspect ratio calculator (16:9, 4:3, 21:9)

Find the aspect ratio of any resolution, such as 1920 × 1080 = 16:9, resize it to a new width or height, or fit it in a screen with letterbox bars.

Drafted with AI writing toolsUpdated Checked examples: 9

px
px
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Simplified ratio
16:9
Simplified ratio: 16:9
Width ÷ height
1.7778
Closest common ratio
16:9

1,920 × 1,080 simplifies to 16:9: the width is 1.7778 times the height.

Shape

16:91,920 px1,080 px
How it's calculated S

  1. Divide both sides by their greatest common divisor

    gcd⁡(1920,1080)=120,1920120:1080120=16:9\gcd(1920, 1080) = 120,\quad \frac{1920}{120} : \frac{1080}{120} = 16 : 9

  2. As a single number

    1,9201,080=1.7778\frac{1{,}920}{1{,}080} = 1.7778

About aspect ratio calculator (16

The aspect ratio is the width divided by the height, written in lowest terms by dividing both sides by their greatest common divisor: 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, so 1920 × 1080 is 16:9. Resizing keeps that quotient fixed, so the new height is the new width × H ÷ W. Fitting a picture inside another frame uses the largest scale that fits both ways, s = min(frame width ÷ W, frame height ÷ H), and the leftover space becomes bars.

Scaling 1920 × 1080 to 1280 wide gives 1280 × 720. Showing that 16:9 picture on a 1024 × 768 screen gives a 1024 × 576 picture with 96 px letterbox bars above and below, so the picture uses 75% of the screen.

Ratio reduction uses integer arithmetic; scaling uses 50-digit decimal arithmetic. Scaled sizes are then rounded to whole pixels, which can shift the ratio slightly.

How to calculate an aspect ratio

Divide the width and the height by their greatest common divisor. For 1920 × 1080 the GCD is 120, so the ratio is (1920 ÷ 120):(1080 ÷ 120) = 16:9, a width 1.7778 times the height. Every screen or image with that shape, from 1280 × 720 to 3840 × 2160, reduces to the same 16:9.

Finding the GCD by hand

Euclid's algorithm divides and keeps the remainder until the remainder is zero:

  1. 1920 ÷ 1080 leaves 840.
  2. 1080 ÷ 840 leaves 240.
  3. 840 ÷ 240 leaves 120.
  4. 240 ÷ 120 leaves 0, so the GCD is 120.

Decimal sides work the same way once written as fractions, so 2.39 × 1 reduces to 239:100.

Common resolutions and their exact ratios

  • 1280 × 720, 1920 × 1080, 2560 × 1440 and 3840 × 2160: 16:9
  • 1366 × 768: 683:384, which is 0.05% wider than 16:9
  • 1920 × 1200 and 2560 × 1600: 8:5, usually written 16:10
  • 1024 × 768: 4:3
  • 2560 × 1080: 64:27; 3440 × 1440: 43:18; 3840 × 1600: 12:5, all sold as "21:9"
  • 1080 × 1920: 9:16, vertical phone video
  • 1080 × 1350: 4:5, a portrait social post
  • 1998 × 1080 and 2048 × 858: the cinema flat (1.85:1, exactly 37:20) and scope (2.39:1, exactly 1024:429) containers

The scope label 2.39:1 is rounded: 2048 ÷ 858 = 2.387. The DCI compliance test plan lists both container sizes.

Resizing without distortion

Keep the ratio fixed and solve for the missing side: new height = new width × H ÷ W. Scaling 1920 × 1080 to 1280 wide gives 1280 × 1080 ÷ 1920 = 720.

Round only at the end. Resizing to 1000 wide gives a height of 562.5, so you choose 563 or 562. Video encoded with 4:2:0 chroma, the usual H.264 setting, needs even dimensions, which makes 1000 × 562 the practical video size.

Fitting a picture inside a screen

Scale by the smaller of frame width ÷ picture width and frame height ÷ picture height. The whole picture then fits, and the leftover space becomes bars:

  • 16:9 video on a 1024 × 768 screen: s = min(1024 ÷ 1920, 768 ÷ 1080) = 0.5333, giving 1024 × 576 with 96 px letterbox bars above and below. The picture fills 75% of the screen.
  • 4:3 content on a 1920 × 1080 screen: 1440 × 1080, with 240 px pillarbox bars on each side.
  • A 2.39:1 film on a 1920 × 1080 TV: 1920 × 803, with letterbox bars of about 138 px above and below.
  • Vertical 9:16 video on a 1920 × 1080 TV: 607.5 × 1080, filling only 31.6% of the screen.

Scaling by the larger factor fills the frame instead and crops the overflow. For 4:3 content on a 16:9 screen, that crops 25% of the picture.

Reduce, resize and fit examples

ScenarioSource ratioResult
Default16:9—
Resize default16:91280 × 720 px
Letterbox default16:91,024 × 576 px with 96 px bars
Pillarbox 4:34:31,440 × 1,080 px with 240 px bars
True 21:9 on ultrawide7:33,360 × 1,440 px with 40 px bars

Default only reduces 1920 × 1080. Resize default scales it to 1280 wide, and Letterbox default fits it in 1024 × 768. Pillarbox 4:3 fits 4:3 content in 1920 × 1080, and the last row fits true 21:9 video, which reduces to 7:3, in a 3440 × 1440 ultrawide.

Common aspect ratio mistakes

  • Taking "21:9" literally. True 21:9 is 2.333, while ultrawide panels run from 2.370 to 2.4. That is why 21:9 video on a 3440 × 1440 panel still gets 40 px side bars.
  • Swapping width and height. 1080 × 1920 is 9:16, a different shape from 16:9.
  • Assuming square pixels. NTSC DVD frames are 720 × 480 pixels, a 3:2 grid, yet play at 4:3 or 16:9 because their pixels are not square. HD video under ITU-R BT.709 uses square pixels, so its pixel ratio is its picture ratio.
  • Rounding in the middle. Rounding an intermediate size and scaling again compounds the error. Keep exact fractions until the last step.

Sizes, presets and rounding

  • Width comes first, and decimal sides are accepted.
  • Preset source ratios, such as 16:9, 4:3, 21:9, 9:16, 1.85:1 and 2.39:1, cover the case where you know the shape but not the pixel size.
  • Whole-pixel results round halves up, and the exact fractional sizes are kept as well.

For physical screen size and pixel density, use the screen size and PPI calculator.

Worked examples

Full HD

I want to
Find the ratio
Width
1920 px
Height
1080 px
Simplified ratio
16:9
Width ÷ height
1.7778
Closest common ratio
16:9

Checked against: Independent calculation

Ultrawide 2560 × 1080

I want to
Find the ratio
Width
2560 px
Height
1080 px
Simplified ratio
64:27
Width ÷ height
2.3704

Checked against: Independent calculation

1366 × 768 laptop is not exactly 16:9 (edge)

I want to
Find the ratio
Width
1366 px
Height
768 px
Simplified ratio
683:384
Width ÷ height
1.7786
Closest common ratio
16:9 (0.05% wider)

Checked against: (683/384) / (16/9) − 1 = 0.0488%

Decimal cinema ratio

I want to
Find the ratio
Width
2.39 px
Height
1 px
Simplified ratio
239:100

Checked against: Independent calculation

Questions

What is the aspect ratio of 1920 × 1080?

1920 × 1080 is 16:9: both sides divide by their greatest common divisor, 120, to give 16 and 9, and the width is 1.7778 times the height. ITU-R BT.709 defines 1920 × 1080 as the HDTV picture format, and the same 16:9 shape covers 1280 × 720, 2560 × 1440 and 3840 × 2160 (4K UHD).

Is a 21:9 ultrawide monitor really 21:9?

No. Ultrawide monitors sold as 21:9 are 2560 × 1080 (64:27, 2.370), 3440 × 1440 (43:18, 2.389) or 3840 × 1600 (12:5, 2.4), while 21:9 itself is 2.333. The label is rounded marketing, and the gap shows up in practice: true 21:9 video on a 3440 × 1440 panel fills 3360 × 1440 and leaves a 40 px pillarbox bar on each side.

How do I calculate the new height when resizing an image?

Multiply the new width by the original height over the original width: new height = new width × H ÷ W. Scaling 1920 × 1080 to 1280 wide gives 1280 × 1080 ÷ 1920 = 720. The other way round, new width = new height × W ÷ H, so a 4:3 picture 1080 tall is 1440 wide. Round to whole pixels at the end, and to even numbers for most video encoders.

What is the difference between letterbox and pillarbox?

Letterboxing adds bars above and below when the picture is wider than the screen, such as 16:9 video on a 1024 × 768 display, which gets 96 px bars. Pillarboxing adds bars on the left and right when the picture is narrower, such as 4:3 content on a 1920 × 1080 screen, which leaves 240 px on each side. Zooming to fill removes the bars but crops the picture, by 25% in that 4:3 case.

What aspect ratios do cinema films use?

Digital cinema uses two DCI container shapes: flat at 1.85:1 (1998 × 1080 in the 2K container) and scope at 2.39:1 (2048 × 858). On a 1920 × 1080 TV, a 2.39:1 film shows letterbox bars of about 138 px above and below, and a 1.85:1 film bars of about 21 px, because both are wider than the TV's 16:9 (1.78:1).

About this calculator

W:H=Wgcd⁡(W,H):Hgcd⁡(W,H)s=min⁡ ⁣(WfW,HfH)\begin{aligned} W{:}H &= \frac{W}{\gcd(W,H)} : \frac{H}{\gcd(W,H)} \\[4pt] s &= \min\!\left(\frac{W_f}{W}, \frac{H_f}{H}\right)\end{aligned}

Sources

  1. ITU-R BT.709-6 — HDTV picture format 1920 × 1080 at 16:9
  2. DCI Digital Cinema System Specification v1.2 (2008), Table 3 — flat 1998 × 1080 (1.85:1) and scope 2048 × 858 (2.39:1) containers

Checked against references

This calculator includes 9 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.

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