Aspect ratio calculator

A 3840 x 2160 file and a 1920 x 1080 file look different in a folder and identical on a screen. They share a shape, and that shape is the aspect ratio: the width of the frame divided by its height. It decides where the black bars fall when a scope feature plays on a television, why archive footage arrives with bars at the sides, and how much of a wide interview survives when someone asks for a vertical cut.

How the aspect ratio calculator works

You give it a width and a height in pixels. It gives back four answers.

• Frame ratio. Both sides are divided by their greatest common divisor, so 3840 x 2160 becomes 16:9. Width divided by height gives the same ratio as a decimal against 1 (1.778:1).
• Closest named ratio. That decimal is compared with the fifteen named ratios in the cheat sheet. If the nearest one is more than half a per cent away, the tool says by how much.
• Same ratio at a new width. New height = new width ÷ (width ÷ height). Video frames are almost always an even number of pixels, because 4:2:0 colour sampling works on pairs of pixels, so the answer is rounded to the nearest even number.
• A picture inside the frame. If the picture is wider than the frame, it takes the full width and its height = frame width ÷ picture ratio. That is a letterbox, and each bar = (frame height minus picture height) ÷ 2, top and bottom. If the picture is narrower, it takes the full height, its width = frame height × picture ratio, and the bars sit left and right: a pillarbox.

Worked example: a scope picture in a UHD frame

The tool opens with a frame of 3840 x 2160 pixels, a 2.39:1 picture to fit inside it, and a resize width of 1920. The results read:

• Frame ratio: 16:9, or 1.778:1.
• Closest named ratio: 16:9 (HD and UHD television, most online video). It is an exact match, so no difference is shown.
• Same ratio at the new width: 1920 x 1080 px.
• 2.39:1 picture inside the frame: 3840 x 1606 px.
• Each bar: 277 px top and bottom.

The last two lines come from 3840 ÷ 2.39 = 1606.7, rounded to the even 1606. The frame is 2160 lines tall, which leaves 554 spare lines: 277 above the picture and 277 below. The preview draws it as a letterbox.

Change the inputs and the other cases appear. A DCI 4K frame of 4096 x 2160 reduces to 256:135, or 1.896:1. No standard name fits it exactly, and the tool reports 1.85:1 as the closest, 2.5 per cent off.

Set the frame to 1920 x 1080 and the picture to 4:3, the shape of most standard-definition material, and you get 1440 x 1080 with pillarbox bars of 240 px left and right. Documentary editors meet this one constantly in archive footage. A 9:16 vertical clip in the same frame is 608 x 1080, with bars of 656 px on each side. More than two thirds of the screen is black.

Aspect ratio cheat sheet

These are the fifteen named ratios the tool knows, widest first, with the size each one takes inside a 1920 x 1080 frame.

Scope gets written as 2.35:1, 2.39:1 or 2.40:1, depending on the era and on who is talking. The tool uses 2.39:1. In a UHD frame the gap between the three is a few dozen lines.

For phones, the shapes to know are 9:16 and 4:5. A vertical frame 1080 pixels wide is 1080 x 1920 at 9:16 and 1080 x 1350 at 4:5. Enter those as the frame size and the tool confirms them.

Cutting a vertical version from a 16:9 film

Crop, never squeeze. The 9:16 slice of a 1920 x 1080 frame is only 608 pixels wide, so filling a phone screen means enlarging it about 1.78 times, and it looks soft. From UHD the slice is 1216 x 2160, which scales down cleanly. If an interview will be cut both ways, shoot UHD and leave room beside the speaker; the interview guide covers the rest of the setup.

What the calculator cannot tell you

It assumes square pixels. Anamorphic lenses squeeze the picture sideways, and some standard-definition formats used non-square pixels: a 720 x 576 PAL frame was shown as 4:3, though its pixel counts reduce to 5:4. For those, work from the display shape rather than the pixel count.

It knows nothing about title-safe margins or the parts of the screen a platform covers with buttons and captions. With an odd frame height a bar can come out as half a pixel; the encoder puts the extra line on one side. And it cannot choose a ratio for your film. That is a creative decision. For lens framing, try the crop factor calculator.

Questions

What is aspect ratio?

The proportion of a picture's width to its height, written as two whole numbers (16:9) or as a decimal against 1 (1.78:1). It describes shape rather than size, so 1280 x 720, 1920 x 1080 and 3840 x 2160 all share the same ratio.

What aspect ratio is 1920 x 1080?

16:9. Divide both numbers by 120 and you get 16 and 9. As a decimal it is 1.778:1, the standard shape for HD and UHD television and most online video.

What is the difference between letterbox and pillarbox?

A letterbox puts bars above and below the picture, because the picture is wider than the screen, as when a 2.39:1 film plays on a 16:9 set. A pillarbox puts bars left and right, because the picture is narrower, as with 4:3 archive or a 9:16 phone clip in a 16:9 edit.

Is 2.39:1 the same as 2.35:1?

For practical purposes, yes. Both names describe anamorphic scope, and 2.40:1 is used for it too. In a 3840 x 2160 frame, 2.39:1 gives a picture 1606 lines tall with bars of 277; follow whatever your delivery specification asks for.