Quick answer
An aspect ratio is a frame's width-to-height proportion reduced to its smallest whole numbers, so 1920×1080 and 3840×2160 are both 16:9. Converting a 1920×1080 source to 9:16 with a centred crop gives 608 × 1080, keeping only 31.7% of the width, while a 4:5 crop gives 864 × 1080 and keeps 45%.
What "aspect ratio" actually means
An aspect ratio is just the proportion between a frame's width and its height, written as W:H — 16:9, 4:5, 1:1. It says nothing about the actual pixel count: a 1920×1080 video and a 3840×2160 (4K) video are both 16:9, because 1920/1080 and 3840/2160 both simplify to the same 16:9 fraction. That is exactly what the solver above does — it reduces your width and height to the smallest whole-number ratio that has the same proportion (via the greatest common divisor), and separately checks whether the decimal ratio (width ÷ height) lands close enough to a named industry standard to call it out by name.
Two numbers describe every rectangle, so once you fix the ratio and one dimension, the other is determined: new height = new width × (original height ÷ original width). That is the entire math behind the top calculator — it just solves that equation live in whichever direction you are typing, so resizing an asset never distorts it by accident.
Vertical video: the sizes each publisher gives
Every size below is attributed to the page that prints it. Where a platform's own page was read (YouTube Help, TikTok Ads Manager, X's developer documentation and ad specifications), the row says so; the others are third-party guides, which can lag a platform's own rules. Meta's own Instagram and Facebook pages could not be opened for this page, so every Instagram and Facebook figure is a third party's account of Meta's rules.
| Platform | Ratio | Size | Published by |
|---|---|---|---|
| YouTube Shorts | square or vertical, up to 3 minutes | 1080p at most | YouTube Help (three-minute Shorts, creating Shorts) |
| YouTube Shorts, TikTok, Instagram Reels | 9:16 | 1080 × 1920 | EzClip and anfx, third parties |
| Facebook and Instagram Reels and Stories | 9:16 | 1080 × 1920 | House of Marketers, a third party that attributes it to Meta |
| TikTok in-feed ads (not Spark Ads) | 9:16 recommended | 540 × 960 or larger | TikTok Ads Manager |
| Instagram feed post (a photo specification) | 1.91:1 to 3:4; 4:5 recommended | 1080 × 1350 (4:5), up to 1080 × 1440 (3:4) | PostFast, a third party |
| X video | 16:9 (landscape or portrait) or 1:1 | 1280 × 720 landscape, 720 × 1280 portrait, 720 × 720 square | X developer documentation |
| X video ads (standalone) | 16:9 or 1:1 for the recommended sizes; 9:16, 4:5, 2:3 and 1.91:1 are also listed | 1920 × 1080 (16:9), 1200 × 1200 (1:1) | X creative ad specifications |
| YouTube long-form | 16:9 | 1920 × 1080 is its 1080p size; it lists 426 × 240 up to 7680 × 4320 | YouTube Help |
For the Instagram feed, PostFast says 3:4 (1080 × 1440) is the tallest option when you post from the app, while feed ads and posts scheduled through Instagram's API top out at 4:5, so it calls 4:5 the safe default, which it describes as the tallest ratio that works in the app, in feed ads and in scheduling tools alike. PostFast's page is a specification for feed photos and gives no separate figure for feed video; anfx says the home feed shows Reels cropped to roughly 4:5 (1080 × 1350). The crop helper above offers 4:5 for the feed and has no 3:4 target. EzClip notes that TikTok's own documents publish only minimums, and calls 1080 × 1920 the working standard.
Crop vs. letterbox vs. blurred-pad
Turning a 16:9 recording into a 9:16 upload means picking one of three moves:
- Crop — cut the sides off and fill the full vertical frame. Sharpest-looking result, zero bars, but you permanently lose whatever was near the left or right edge. Works well when a subject stays centered.
- Letterbox — keep the entire original frame at its native ratio and fill the rest of the canvas with plain bars (black, white, or a brand color). Nothing is lost, but a lot of the vertical screen goes unused, which reads as low-effort on platforms built around full-bleed video.
- Blurred pad — the same idea as letterboxing, except the bars are a scaled-up, blurred copy of the source frame itself. It fills the canvas edge-to-edge without cutting anything — the tradeoff is that the blurred edges carry no information and can look formulaic if every single clip uses the identical treatment.
The vertical-crop helper above computes the first option — a centered crop — for three common vertical targets, so you know exactly how much width survives before you commit to it in your editor. YouTube Help's advice on resolution and aspect ratios is to avoid adding padding or black bars directly to your video, because padding interferes with the player adjusting to your video's size and the viewer's device; a crop adds none.
Safe zones: what the platform draws on top of your video
Cropping correctly only gets you a clean rectangle — it doesn't guarantee your caption or logo stays visible once TikTok or Instagram lays its own interface over the video. Each app reserves regions of the 1080×1920 canvas for its profile bar, captions, sound credit, and the like/comment/share/bookmark column. No platform's own safe-zone figures were read for this page, so the table gives what three third-party guides publish, each under its own name:
| Published by | Platform | Top | Bottom | Sides |
|---|---|---|---|---|
| Ignite Social Media | TikTok | 108 px | 320 px | 60 px left, 120 px right |
| Ignite Social Media | Instagram and Facebook Reels, boosted content | 220 px | 420 px | a 1010 × 1280 px zone |
| Kreatli | TikTok | about 130 px | about 250 px | about 60 px |
| Kreatli | Instagram Reels | about 108 px | about 320 px | about 60 px |
| House of Marketers | TikTok | no single pixel figure | avoid the lower 20% | avoid the right edge |
| House of Marketers | Facebook and Instagram Reels | about 14% | about 35% | about 6% |
The guides disagree on TikTok's top and bottom margins, and none of these is a platform's own figure. House of Marketers attributes its percentages for Reels to Meta's Business Help Center, which could not be opened for this page. TikTok's in-feed ad page says its safe zone depends on the video's dimensions, the caption length and any additional formats, and offers downloadable template files, which were not downloaded. House of Marketers advises previewing with captions turned on before you export.
Worked example 1 — repurposing a 16:9 talking-head clip
You filmed at 1920×1080 and want a 9:16 cutdown for Shorts. Enter those as the source above and select 9:16: the crop comes out to 608 × 1080, centered, taking a 656px slice off each side. That's only 31.7% of the original width — if your subject wandered more than about 300px from center at any point, part of them will be outside this crop. This is why creators who plan to repurpose horizontal footage frame interviews dead-center rather than using the rule-of-thirds composition typical of landscape video.
Worked example 2 — the same clip for the Instagram feed
Same 1920×1080 source, but this time selecting 4:5 for a main-feed post: the crop is 864 × 1080, keeping 45% of the width — noticeably more forgiving than the 9:16 cut, because 4:5 is a much smaller proportional change from 16:9. This is a good reason to default to 4:5 rather than 9:16 when a full-screen vertical export isn't required: less of the shot is sacrificed for the same "taller" feel.
Worked example 3 — which crops can be Shorts
Take the same 1920×1080 clip, three minutes or shorter, and ask which of its crops YouTube would treat as a Short. YouTube Help says that for standard channels a video with a square or vertical aspect ratio, up to three minutes long and uploaded on or after 15 October 2024, is categorized as a Short (for Official Artist Channels the date is 8 December 2025), and that a wider aspect ratio such as 16:9 keeps a video out of Shorts. So the 16:9 original stays a regular video, while the 9:16 crop (608 × 1080), the 1:1 crop (1080 × 1080) and the 4:5 crop (864 × 1080) are all candidates, each being square or vertical. Select 1:1 in the helper above and it keeps 56.3% of the width, against 45% for 4:5 and 31.7% for 9:16, so a square cut costs the least of the frame. None of them adds padding, which YouTube Help advises against.
Practical guidance
- Shoot for the crop you'll need. If a horizontal recording is going to be cut down to 9:16 later, keep the subject in the center third of the frame from the start.
- 4:5 is the forgiving middle ground. When a platform accepts it, it survives a 16:9-to-vertical crop far better than 9:16 does.
- Design captions after the crop, not before. A caption placed for the full 16:9 frame will very likely fall inside the cropped-out region on a 9:16 export.
- Leave the safe zone empty. Keep logos, subtitles and CTAs out of the regions the platform's own interface covers (see the safe zones above); that space belongs to the platform.
- Ultrawide and cinema ratios (21:9, 2.39:1) are creative choices, not platform requirements — use them for look and feel, not because a platform asks for them.
This tool does the geometry only. It cannot see your footage, so it can't tell you whether a centered crop will actually keep your subject in frame — check the preview in your editor before exporting.
FAQ
What aspect ratio should I use for YouTube Shorts, TikTok and Instagram Reels?
Third-party guides (EzClip and anfx) give 9:16 at 1080×1920 as the canvas for YouTube Shorts, TikTok and Instagram Reels, so one vertical export at that size is the simplest default if you only want to render one file. The platforms' own pages say less: YouTube Help says a square or vertical video of up to three minutes is categorized as a Short and that Shorts can be uploaded at up to 1080p, TikTok's in-feed ad specification, which covers ads and not ordinary posts, recommends vertical video at 9:16, 540×960 or larger, and Meta's pages could not be opened for this page.
Should I crop, letterbox, or blur-pad when converting horizontal footage to vertical?
Crop when your subject stays centered and you can afford to lose the sides — it fills the whole vertical frame with no bars. Letterbox (plain color bars) when nothing in the shot can be cropped, such as a screen recording or a two-person side-by-side. Blurred pad — a scaled, blurred copy of the same frame filling the bars — fills the canvas edge to edge without cutting anything, at the cost of decorative-only edges that can look templated if every clip uses it. For YouTube uploads, YouTube Help advises avoiding padding or black bars added directly to your video.
What is a safe zone and why does it matter for vertical video?
The safe zone is the part of a 1080×1920 vertical frame not covered by app UI — profile bars, captions, like/comment/share icons, and (on ads) a CTA button. No platform's own pixel figures were read for this page, and the third-party guides disagree. For TikTok, Ignite Social Media says to leave 108 px at the top and 320 px at the bottom, Kreatli about 130 px and about 250 px. For Instagram and Facebook Reels, Ignite gives 220 px and 420 px for boosted content, Kreatli about 108 px and about 320 px for Instagram Reels, and House of Marketers, which attributes its figures to Meta, about 14% at the top, 35% at the bottom and 6% at the sides. Keep captions, logos and faces away from those regions or the platform's own UI will sit on top of them.
What is the difference between 9:16 and 4:5?
9:16 (1080×1920) is the tall, full-screen ratio that third-party guides give for Shorts, Reels and TikTok. 4:5 (1080×1350) is shorter and wider; PostFast recommends it for the Instagram feed, which it says accepts photos at ratios from 1.91:1 up to 3:4 (1080×1440) when you post from the app. A 4:5 crop keeps noticeably more of a 16:9 source than a 9:16 crop does, because it is a less extreme ratio change.
What does 2.39:1 mean and where is it used?
2.39:1 (sometimes shown as 2.40:1) is an anamorphic widescreen ratio used in theatrical cinema, roughly 34% wider relative to its height than standard 16:9. It shows up as black bars above and below the picture on a 16:9 TV or monitor, and editors sometimes crop regular 16:9 footage to 2.39:1 deliberately to give it a cinematic look.
Will cropping to vertical cut off important parts of my shot?
A 9:16 crop of a 16:9 frame keeps only about 32% of the original width, centered — anything near the left or right edge of the source is gone. If you plan to repurpose landscape footage as vertical, frame your subject in the center third of the shot while filming so the crop has room to work with, or shoot a separate vertical take of anything that must not be cut.