Carousel Splitter

What Instagram actually does to a carousel you upload

A carousel is not ten independent photos. It is one post with one aspect ratio, and every slide is forced into that ratio. In the app, the crop you accept on the first slide is applied to the whole set — that is behaviour you can reproduce in the app rather than something Meta publishes as a spec, but it is consistent enough that you should design around it. If slide one is 4:5 and slide two is square, slide two gets pillarboxed or cropped to match.

Three things happen to each file after you hit share:

• Aspect-ratio enforcement. The feed accepts a window running from 1.91:1 landscape down to 4:5 portrait. Anything outside that window is trimmed to fit before it is ever displayed. For a split-image carousel this matters twice over, because a crop applied after the split is a crop applied unevenly across your panels.
• Resizing. Uploads wider than 1080 px are scaled down. Exporting a 4000 px wide panel does not buy you a sharper post; it buys you one extra resampling pass on Instagram’s servers, on top of the one your export already applied.
• Re-compression. Instagram re-encodes what you send it. The exact encoder settings are not published, and they have visibly changed over the years, so treat any specific quality number you read online as a guess. What is reliably observable is that flat gradients, fine text and high-frequency detail take the biggest hit.

Why 4:5 takes the most vertical space

The feed is a fixed-width column. Whatever the ratio, your post is drawn at the full column width, and the height is simply width divided by ratio. At 4:5 the ratio is 0.8, so the height is 1.25 times the width — the tallest the feed permits. A square post is 1.0 times the width. A 1.91:1 landscape post is about 0.52 times the width, roughly two-fifths the height of a 4:5. That is the whole argument for portrait: a taller post occupies more of the screen as someone scrolls past. It is a geometry claim, not a ranking claim.

Panel specifications

Note the 3:4 row honestly: 0.75 is outside the 1.91:1 to 4:5 feed window, so a 3:4 panel will be trimmed top and bottom when posted as a normal feed carousel. It is in the tool because plenty of design workflows use 3:4 as a working canvas, not because Instagram shows it untouched.

How the splitter cuts the panels

The naive way to split a panorama is to crop the source into n rectangles and resize each one separately. That is where visible seams come from. Each independent resize lands on its own sub-pixel phase, so the last column of pixels in panel 2 and the first column in panel 3 are resampled from slightly different source positions, and you get a faint one-pixel step at every join.

This tool does it the other way round. It scales your source once so its width matches the total carousel width — panel width multiplied by the number of panels — draws that single render onto one canvas, and then copies out exact integer slices at multiples of 1080 px. Every seam is a clean cut through one continuous resample, so the pixel at x=1079 and the pixel at x=1080 are neighbours in the same interpolation, not strangers from two different ones.

Vertical fit is handled after the horizontal scale is locked in:

• If the scaled image is taller than the panel height, it is centre-cropped top and bottom, and the tool tells you what percentage of the height was lost whenever that exceeds 1%.
• If the scaled image is shorter than the panel height, it is centred on a dark #0b0e12 background rather than stretched, and you get a warning naming the percentage of each panel that is bar.
• If the source is narrower than the total width it is upscaled, and any upscale beyond 1.15x raises its own warning with the exact factor.

Getting it right before you export

The fix for almost every carousel problem is choosing the canvas size in your design tool first, not repairing the result afterwards.

• Set the canvas to the full render size. Panels multiplied by 1080 wide, by the panel height. Three 4:5 panels means a 3240 x 1350 artboard. Five square panels means 5400 x 1080. Design the whole panorama on that single canvas.
• Put guides on every multiple of 1080. Those lines are where the swipe breaks fall. A face, a logo or a word sitting across a guide will be sliced in half and read as two half-objects on two separate slides.
• Give each panel a reason to exist. A panorama that is only meaningful when reassembled asks the viewer to do work. Composition that has a subject roughly centred in each 1080 column survives being seen one slide at a time.
• Keep important content away from the vertical edges of the frame. If the post ends up cropped by the feed window, the trim comes off the top and bottom for anything taller than 4:5.
• Export at 1:1 pixel scale. Do not export at 2x and let the tool downscale, and do not export at 540 px wide and let it upscale. Match the render size exactly and there is a single resample in the entire chain.

The honest limits

A wrong-proportion source cannot be fixed by splitting. If your image is proportionally too short for the panel count you chose, the tool letterboxes it on a dark background and tells you what fraction of each panel is bar. It will not stretch your image to fill the frame, because a non-uniform stretch is worse than a bar. If it is proportionally too tall, it centre-crops and reports the percentage lost. Either way the warning is the useful output: it is telling you to change the panel count or re-export the source, not to accept the result.

Seams stay invisible only if you leave the panels alone. The alignment guarantee comes from the fact that every panel is an exact integer slice of one render. The moment you re-crop a panel, nudge it in another editor, or pinch-zoom it inside Instagram’s own crop screen before posting, the panels no longer share a common pixel grid and the seam comes back. Upload them at their exported size and decline every crop the app offers.

Order is manual. Panels are named panel-01.png through panel-10.png. Instagram keeps carousel order as you add slides, so add them in filename order. There is no way to reorder a carousel after publishing.

Practical ceilings. The splitter accepts 2 to 10 panels and files up to 80 MB. Sources above roughly 60 megapixels are refused, because decoding them and holding a full-width canvas at the same time is what makes a browser tab die. The combined render is also capped at 16,000 px on a side, which is where most browsers stop honouring canvas allocations. Panels come out as PNG, individually or as a ZIP, so they are lossless leaving the tool — the only compression is the one Instagram applies later.

Frequently asked questions