Aspect ratio calculator with crop and letterbox preview

Calculate aspect ratios and preview how your image fits Instagram, Open Graph or A4 - before you export, not after.

Runs in your browser Your inputs are processed in your browser and not sent to a server.

An aspect ratio calculator answers two questions at once: what ratio your image has now, and what happens when you fit it to a different format - Instagram, Open Graph, A4. The preview below shows you live whether the fit costs pixels or adds padding, before you export rather than after. Your image stays in the browser; only the pixel dimensions are read.

What's the image for? Instagram, TikTok, Reels
Source
px
· 1.50
Target
px
Source · 1,920 × 1,276Target frame · 1,021 × 1,276Crop: 899 px width
Sample by Sean Oulashin on Unsplash
Compare all formats Here's how your image lands in every ratio for this mode.
Target size 1,021 1,276 px 4:5 1.3 MP −47% 1,276 px × 4:5 = 1,021 px width
Code snippet 
/* Crop 1920×1276 → 1021×1276 */
.target {
  aspect-ratio: 1021 / 1276;
  width: 100%;
  max-width: 1021px;
}
.target img {
  width: 100%;
  height: 100%;
  object-fit: cover;
  object-position: 50% 50%;
}

Overview

Which format goes where? Click any row to load its pixel dimensions as your source.

FormatPixelsRatioGrid behaviour
Instagram portrait Tall feed format. Profile grid switched to 4:5 in January 2025 (was 1:1).
Verified May 2026 Source

Runs in your browser. Images are never uploaded - only their dimensions are read.

Crop or letterbox? The only two honest answers

When your source is 16:9 and the target is 1:1, there are exactly two mathematically clean answers - and one wrong one.

Crop cuts off the overhang. 1920×1080 becomes a 1080×1080 frame in the middle of the source; 420 pixels are lost on each side. That's the social-media standard: visual impact beats completeness. Exactly the area shaded red in the visualizer - what's gone.

Letterbox does the opposite: the entire source content stays visible; the missing area is padded with bars. 1920×1080 in 1:1 becomes 1920×1920 with 420 pixels of bar top and bottom. That's a film in a 16:9 TV with black bars, or a 4:3 video in a 16:9 player as pillarbox (vertical bars instead of horizontal). Clean choice for archives, slideshows, or whenever the composition is sacred.

Stretching is the third option you almost never want. The image is squashed into the target ratio - faces become egg-shaped, logos lopsided, lines wobbly. The calculator shows stretch as a visible option but flags it amber as an anti-pattern. We don't hide it (it's useful as a comparison) but we don't let anyone sit in it without warning.

The visualizer is the answer. Type 1920 and 1080 into the source and pick Instagram square, and you'll see immediately: red hatch on left and right (crop) plus the exact pixel count −840 px width. Switch to Letterbox and the same source content gets padded with 840 px of height. The decision is visual, not arithmetic. → Try it: 1920×1080 → 1:1

Why your Instagram post gets cropped - and what changed in 2025

Instagram runs several formats in parallel. The Toolflux presets curate three:

  • Feed square (1:1, 1080×1080) - the classic shape. Safe in the feed; fitted into the new profile grid.
  • Feed portrait (4:5, 1080×1350) - the tallest feed crop. Since January 2025 this is also the profile-grid format.
  • Story / Reels (9:16, 1080×1920) - vertical, full-screen, its own world.

Per help.instagram.com, feed photos go as tall as 3:4. The historical cut lived at the profile grid: until early 2025 every post got centre-cropped to 1:1 there - a 4:5 post lost 135 pixels off the top and 135 off the bottom on the profile page. Adam Mosseri (Head of Instagram) switched the grid from squares to 4:5 rectangles in January 2025: 4:5 posts now sit fully in the grid, while older 1:1 posts get fitted into 4:5.

Pick the Instagram portrait preset (4:5) to see how your image sits in the new grid - or type 1080×1080 as source to check how an old square gets fitted into 4:5. Sources: help.instagram.com, Mosseri announcement January 2025, verified May 2026 (date stamped on every preset chip). → Try it: 1080×1080 in 4:5 grid

Pixels ≠ aspect ratio - same shape, different size

Common confusion: 4K must be a different aspect ratio from 1080p. It isn't. Both are 16:9.

ResolutionPixelsMPRatio
720p (HD)1280×7200.916:9
1080p (FHD)1920×10802.116:9
1440p (QHD)2560×14403.716:9
4K UHD3840×21608.316:9
8K UHD7680×432033.216:9
DCI 4K Cinema4096×21608.8≈ 1.9:1

The ratio stays 16:9; only the pixel count grows. From 1080p to 4K the pixels quadruple (2.1 MP to 8.3 MP) - same image, higher resolution. Cinema 4K (DCI) is the odd one out: 4096×2160 is marginally wider (≈1.9:1) because cinema distribution uses a wider standard than home TVs.

In the reference tab, click any row to load it as the source - handy when you want to check whether a 4K master is enough for a 1:1 Instagram crop (yes, easily, with 8.3 MP). → Load 4K UHD as source

Letterbox = horizontal bars, pillarbox = vertical bars

Two terms that get mixed up:

  • Letterbox: black bars on top and bottom. Happens when a wider image (e.g. cinema 21:9) is shown in a narrower player (16:9 TV) without cutting the content.
  • Pillarbox: black bars on left and right. Happens when a narrower image (e.g. classic 4:3 TV stock) is shown in a wider player (16:9 screen).

The names come from the early widescreen home-cinema era: letterbox because the image looked like a letter through a mailbox slot; pillarbox because it resembled an old British pillar-style postbox with vertical slots. The visualizer labels all padding as Letterbox (umbrella term) and shows direction via the axis: height padding means bars top and bottom (classic letterbox), width padding means bars left and right (technically pillarbox). → Letterbox cinema 21:9 in 16:9

What the calculator does

  • Six modes for six use-cases: Social (Instagram, TikTok), Link preview (OG, X, LinkedIn), Video & Screens (720p through 8K, cinema), Photo (full-frame, APS-C, MFT), Print (A4, A3, photo prints with DPI), and Custom (free ratio). Each mode shows only its own presets, units, and conventions - no DPI in Social, megapixels emphasised in Photo, paper sizes in Print.
  • Three-way input for the source: edit width, height, or ratio - the others recompute live. Type 16:9 into the ratio field and the height recomputes. Type a new width and the ratio chip updates.
  • Drop or click to upload: drag an image onto the preview, or click the upload icon. The image renders directly inside the source rectangle, with the crop area tinted red so you see what's about to be cut, not just an abstract hatch. Dimensions are read in the browser - the image never leaves the page.
  • Dated preset library per mode: Instagram (1:1 / 4:5 / 9:16), TikTok, Open Graph (1.91:1, 1200×630), X card, LinkedIn, YouTube, Cinema, A4 portrait and landscape, A3, A5, full-frame, APS-C, MFT, Hasselblad square. Every preset carries a Verified date and a source link to its underlying spec - platform docs (IG, TikTok, OG, X, LinkedIn, YouTube) for the social/link/video presets, ISO or Wikipedia references for the photo, print and cinema formats.
  • Crop / Letterbox / Stretch as three answers to "how should it fit?", as pills bolted to the top edge of the preview. Crop is the default, letterbox is its equal, stretch stays visible but flagged anti-pattern in amber.
  • Mode-specific overview table: each mode's table below the preview shows the right view of your use-case. In Print mode it lists papers A6 through A2 with fits cleanly / source too small computed for your source and DPI. In Photo mode it lists sensor formats with megapixels and print advice. Click a row to load it as the source.
  • Orientation toggle in Social, Photo, and Print: Preserve, Landscape or Portrait forces a ratio's orientation without changing the preset.
  • URL round-trip: mode, preset, fit mode, orientation, and DPI live in the URL as query parameters. Source pixel values only get written when you click the Share button explicitly - they're never sent to the address bar automatically.

Local logic, no server round-trip. Crop or letterbox - see what'll happen before it does. Once the format is set, contrast is next - that's what the colour converter is for, with OKLCH lightness and live WCAG.

Frequently asked

What aspect ratio is Instagram?

Instagram supports multiple ratios in parallel. The classic three are feed square (1:1), feed portrait (4:5), and Story / Reels (9:16); feed photos go up to 3:4 per help.instagram.com. The profile grid switched from squares to 4:5 rectangles in January 2025 - 4:5 posts now sit fully in the grid.

What's the aspect ratio of 4K video?

Consumer 4K (UHD) is 16:9, same as 1080p, with four times the pixels. Cinema 4K (DCI) is approximately 1.9:1.

How do I calculate aspect ratio?

Enter source width and height, pick a target preset or type a custom ratio, choose crop or letterbox. The preview shows what gets cut or padded with pixel deltas.

What's the difference between letterbox and pillarbox?

Letterbox = horizontal bars (a wider image inside a narrower frame). Pillarbox = vertical bars (a narrower image inside a wider frame).

How many pixels do I need for A4 at 300 DPI?

2480×3508 pixels. The print tab shows whether your source can print cleanly at A4, 4×6 photo and 13×18 photo at the chosen DPI. → Check A4 at 300 DPI

Are uploaded images stored?

No. Your image stays local in the browser - the calculator only reads its pixel dimensions for the preview and the ratio math. No upload, no server, no cache. The analytics platform receives three anonymous numbers per upload: width, height and file size - no image content.