Cropping to an exact aspect ratio
A 4:5 crop that is "nearly" 4:5 gets letterboxed or trimmed by the platform. Here is the arithmetic, the tools on your Mac that lock a ratio, and the one step people skip.
You need a photograph at exactly 4:5 for a feed, or 16:9 for a slide, or 1:1 for a profile picture. You drag a crop box until it looks right, upload it, and the platform adds a thin bar down one side or trims a sliver off the top — because "looks right" was 4:5.03.
An aspect ratio is a precise thing, and getting it exactly right takes a little arithmetic and one extra step.
Ratio and size are different questions
An aspect ratio is the shape: width divided by height. 4:5 is 0.8. A 1080 × 1350 image is 4:5; so is 2160 × 2700; so is 800 × 1000.
Pixel dimensions are the size. Most platforms specify both — a shape, and a size they will scale to — and they will fix a wrong shape for you, badly.
So there are always two steps:
- Crop to the right shape.
- Resize to the right size.
Doing them in that order matters. Resizing first and then cropping gives you an image of the right shape at an arbitrary size.
The arithmetic, once
Your camera shoots 6000 × 4000, which is 3:2. You want 4:5.
4:5 is taller than 3:2, so height is the limiting side. Keep the full height and work out the width:
width = 4000 × 4 ÷ 5 = 3200
So the crop is 3200 × 4000, positioned wherever you like horizontally. Then resize to 1080 × 1350.
Going the other way — 3:2 source to 16:9 — width is the limiting side:
height = 6000 × 9 ÷ 16 = 3375
A crop of 6000 × 3375, then resize to 1920 × 1080.
When the result is not a whole number, round it and accept a sub-pixel error. It will not be visible, and it is far smaller than the error in a hand-dragged box.
The Photos app does locked ratios well
For photographs already in your library, Photos is the easiest route:
- Open the photo and press Edit.
- Click Crop in the toolbar.
- Pick an aspect ratio in the sidebar — presets include Square, 4:5 and 16:9 — or choose Custom to type any ratio.
- Drag the image inside the frame to position it.
Photos keeps the original, so you can change the crop later. The limitation is that it crops but does not let you set exact output pixels in the same step; you choose a size when you export, and the options are coarse.
Preview, with a little care
Preview has no ratio lock beyond a square (hold Shift while dragging a selection). It does show the selection's size in pixels as you drag, so you can aim for the numbers you calculated above, then Tools → Crop.
Then Tools → Adjust Size, leave Scale proportionally ticked, and type the exact width. If the crop was right, the height will come out exactly right too. If it comes out 1351 instead of 1350, your crop was a few pixels off — go back rather than unlinking width and height, which would stretch the image.
That last point is the step people skip. Never unlock the proportions to force a size. A stretched face is far more noticeable than a thin border.
Where it gets fiddly
The manual route works for one image. It gets tedious when:
- You need the same image in three shapes — 1:1, 4:5 and 16:9 — each framed differently.
- You want to keep the uncropped original alongside the crop without managing copies.
- The image is a graphic with text near the edges, and a different ratio means moving the text, not just trimming.
For the last case, cropping is the wrong tool. Build the graphic on a canvas of the target size instead, and lay the photograph and text out inside it.
What Miyu Paint does here
Miyu Paint is a native image editor for Mac, and crop and resize are among its free tools, along with drawing and selections. Free export is PNG or JPEG up to 2048 pixels on the longest edge, with no watermark — comfortably above 1080 × 1350 and most other feed sizes.
For the multi-shape and text cases, full access adds layers: the photograph on one layer, text and shapes on others, all still editable, so changing the frame means moving things rather than starting over. Editable .mpaint projects keep that work for next time. Full access comes from a fourteen-day trial you start yourself, which does not renew into a charge, or a one-time Lifetime Unlock.
Everything happens on your Mac, with no account, advertising or analytics.