A splitter divides the width of the photo by the number of slides. Most of the time that division is not exact. A 1170-pixel-wide photo across four slides is 292.5 pixels each, and half a pixel is not a thing a canvas can cut.
What happens then is worse than a crooked seam. Asked for a 292.5-pixel slice, the browser does not round — it resamples, rebuilding every pixel in the tile from a weighted average of its neighbours. We measured it on a source of alternating one-pixel black and white stripes:
| Slice | Pixels that came back | Edge contrast |
|---|---|---|
| Whole pixel | 0, 255, 0, 255 | 255 — exact |
| 292.5 | 111, 143, 111, 143 | 32 |
Eighty-seven per cent of the edge contrast, gone — and not only at the seam. The whole tile is rebuilt, so fine detail everywhere in it goes soft. On a photograph you will not see stripes; you will see a carousel that looks slightly blurrier than the picture you started with, and no obvious reason why.
Trim the crop to a width that divides exactly, before cutting anything. This tool takes the largest whole-pixel width that is divisible by the slide count, centres it, and cuts there. Every slice lands on an integer boundary, so nothing is resampled, and every slide comes out the same size — which a carousel needs anyway, because all slides share one aspect ratio.
The cost is a few pixels of width. Measured on five common phone widths:
| Photo width | Slides | Naive slice | Whole-pixel slice | Pixels dropped |
|---|---|---|---|---|
| 1170 | 4 | 292.5 | 292 | 2 |
| 1080 | 3 | 360 | 360 | 0 |
| 4032 | 3 | 1344 | 1344 | 0 |
| 3024 | 5 | 604.8 | 604 | 4 |
| 1290 | 7 | 184.29 | 184 | 2 |
At most four pixels, and on the two widths that already divide exactly, none at all. That is the whole trade: four pixels of width against every pixel in every tile being rebuilt.
Every slide in a carousel shares one aspect ratio, so the shape you pick decides how much of the photo survives. For a given number of slides, the widest strip you can use is the slide ratio multiplied by the slide count:
A 16:9 photo is 1.78:1. Against a 2.4:1 target it is too short rather than too narrow, so the tool crops the sides. Against 3:1 it crops harder still. The preview shows exactly what is kept before you download anything, and Full height of the photo skips the ratio entirely and uses the whole height, which is useful when you are posting somewhere that does not force one.
The measurement behind all of this — what a browser actually returns when you ask it for half a pixel, and how much of a photograph survives each slide shape — is written up in Half a Pixel Is Why Your Carousel Looks Soft.
No. The file is read by your browser, cut on a canvas in the page, and offered back to you as downloads. It never reaches a server, which is also why it works with the tab offline once the page has loaded.
Because that division usually is not a whole number, and a fractional slice makes the browser resample the tile. The tool trims the crop to a width that divides exactly — at most a few pixels — so every slice is pixel-exact instead.
This tool goes up to ten, which is the limit Instagram has published for a carousel post. Other feeds differ, and the arithmetic does not care: any count from two upward works the same way.
The pixels will line up exactly, which is the part a splitter controls. What you cannot control is the app: some feeds add a small gap or a rounded corner between slides, so a hard edge running across the seam can still read as a join. Photos with detail across the seam hide it better than flat colour or a straight line does.
Portrait 4:5 keeps the most of a normal photo, because taller slides need a narrower strip to fill the same number of slides. Square 1:1 is the safest for a mixed feed. Landscape 1.91:1 needs an extremely wide source and is mostly useful for panoramas.
Yes. The cutting is done by the same canvas the desktop uses. Very large photos are limited by the memory of the browser rather than by the tool; if a download comes back blank, the photo is larger than the browser could hold and resizing it first will fix it.
PNG, which is lossless, so nothing is lost a second time on the way out. If you would rather have JPEG, run the slides through the image converter afterwards.
Not here — this tool cuts a single row of slides, which is what a carousel swipes through. A grid is a different layout with different arithmetic.