Geometry Syntax Reference

How dimension strings like 800x600>, 50%, and WxH! control resize, crop, extent, and other spatial operations.

Geometry is a string format that specifies dimensions, offsets, and resize behavior. It appears in -resize, -crop, -extent, -thumbnail, -scale, -sample, and other options that manipulate image dimensions. The authoritative overview is Command-line processing (geometry) on imagemagick.org; this page summarizes the parts doc readers use most often.

Base syntax

Unless noted, examples assume a 1600×1200 source (4∶3 aspect ratio). For “fit inside” sizes, ImageMagick uses scale = min(W/w, H/h) so neither output dimension exceeds the box.

Format	Meaning	Example (1600×1200 source)
WxH	Fit inside W×H, keep aspect ratio (scale = min of width/height ratios)	800x400 → 533×400; height binds first because 400÷1200 is a smaller scale than 800÷1600; not 800×600 (height would exceed 400).
W	Set width W, height follows aspect ratio	800 → 800×600
xH	Set height H, width follows aspect ratio	x600 → 800×600
W%	Scale both width and height by the same percentage	50% → 800×600
W%xH%	Scale width and height independently (aspect ratio usually changes)	50%x25% → 800×300 (half width, quarter height. A common mistake is expecting 50% on both axes; use plain 50% for uniform shrink).
WxH+X+Y	Width×height plus offset (typical with -crop)	400x300+100+50 → 400×300 region with top-left of the crop at pixel (100, 50)
A@	Resize to approximate total pixel area A	640000@ → 924×693 (area ≈ 640,332)

Geometry dimensions are in pixels. ImageMagick does not factor in DPI or print resolution when interpreting geometry strings. An image at 300 DPI and an image at 72 DPI with the same pixel dimensions receive the same resize result.

Resize modifier flags

Modifier flags are appended to the end of a geometry string. They change how ImageMagick handles aspect ratio and sizing constraints.

Independent percentages on both axes (W%xH%)

Strings like 50%x25% scale width and height by different percentages, so the aspect ratio changes (unlike a single 50%, which scales both axes together). See the W%xH% row in the base syntax table. That trips people who expect 50%x25% to mean “50% overall.”

All examples below use a 1600x1200 input image and a target of 800x400.

No flag (default)

magick input.jpg -resize 800x400 output.jpg

Fits the image within 800x400 while preserving aspect ratio. The result never exceeds either dimension. Output: 533x400 (height is the limiting dimension).

! Force exact dimensions

magick input.jpg -resize '800x400!' output.jpg

Ignores aspect ratio and forces the image to exactly 800x400. Output: 800x400.

The ! flag distorts the image if the target aspect ratio differs from the source. A 4:3 image forced into 2:1 dimensions will appear horizontally stretched.

> Only shrink

magick input.jpg -resize '800x600>' output.jpg

Resizes the image only if it is larger than the target. If the image already fits within the dimensions, it stays unchanged.

Input size	Target	Output
1600x1200	800x600>	800x600 (shrunk)
400x300	800x600>	400x300 (unchanged)

< Only enlarge

magick input.jpg -resize '800x600<' output.jpg

Resizes the image only if it is smaller than the target. Larger images stay unchanged.

Input size	Target	Output
400x300	800x600<	800x600 (enlarged)
1600x1200	800x600<	1600x1200 (unchanged)

^ Fill area

magick input.jpg -resize '800x400^' output.jpg

^ means “cover” the box, not “fit inside.” ImageMagick scales until both dimensions are at least the target (max(W/w, H/h)). For 1600×1200 into an 800×400 box, the width hits 800 first, producing 800×600. The image is 150px taller than the 400px target. That overflow is expected; you still need a crop or extent step if the deliverable must be exactly 800×400.

This is typically paired with -gravity center -extent (or -crop) to trim the extra:

magick input.jpg -resize '800x400^' -gravity center -extent 800x400 output.jpg

Final output: 800×400 (scaled to cover, then canvas cropped from the center).

Offset syntax

Offsets specify a position within the image, written as +X+Y or -X-Y after the dimensions. They are primarily used with -crop and -extent.

magick input.jpg -crop 400x300+100+50 +repage output.jpg

This extracts a 400x300 region starting 100 pixels from the left edge and 50 pixels from the top edge. The +repage resets the virtual canvas to match the cropped dimensions.

Negative offsets shift in the opposite direction:

magick input.jpg -crop 400x300-50-25 +repage output.jpg

This shifts the crop region 50 pixels left and 25 pixels up from the default origin. Pixels outside the image boundary are not included; the result may be smaller than 400x300.

Gravity and crop offsets

When -gravity is set, crop offsets are measured from the gravity anchor point instead of the top-left corner.

Gravity	Anchor for +X+Y (on an 800×800 image, 0-based pixels)	-crop 400x400+0+0 result
NorthWest (default)	Top-left corner (0, 0)	Top-left 400×400 block
NorthEast	Top-right corner (799, 0); column width−1, row 0	Top-right 400×400 block
Center	Image center (400, 400); half of 800 in each axis	Centered 400×400
SouthEast	Bottom-right (799, 799); column and row size−1	Bottom-right 400×400 block
South	Bottom edge center (400, 799); middle column, bottom row	Bottom-centered 400×400 strip

Offsets add to the anchor in the direction ImageMagick defines for that gravity (see the official geometry page for edge cases).

-gravity persists for all subsequent spatial operations until explicitly reset. To reset it:

magick input.jpg -gravity Center -crop 400x400+0+0 +repage -gravity None -crop 200x200+0+0 +repage output.jpg

The first crop uses Center gravity. The second crop uses the default top-left origin after -gravity None resets it.

Options that accept geometry

Resize operations (all preserve aspect ratio by default, all accept modifier flags):

Option	Behavior	Use when
-resize	High-quality resize with filtering	Default: best visual quality for photos and general output.
-thumbnail	Same geometry as -resize, plus strips metadata and profiles	Web assets and thumbnails where smaller files and privacy matter.
-scale	Fast resize without resize filters (pixel averaging)	Speed over quality; large downscales where a softer -resize is unnecessary.
-sample	Nearest-neighbor only (no interpolation)	Pixel art, crisp UI captures, masks; avoid on photos (stair‑edges / aliasing).

Crop and extent operations (use WxH+X+Y offset syntax):

Option	Behavior
-crop	Extract a region; add +repage to reset virtual canvas
-extent	Set canvas size; pads with -background color if larger than image

Edge cases

x600 vs 0x600

These are different. x600 omits the width, so ImageMagick calculates it proportionally from the height. 0x600 specifies a width of zero, which produces a 1x1 pixel output. Always omit the dimension rather than setting it to zero.

Two-axis percentages (50%x25%)

Covered in the base table above: width and height scale independently, so the picture stretches unless both percentages match.

Decimal percentages

Values like 33.3% are valid. ImageMagick rounds the result to the nearest whole pixel. A 1600x1200 image at 33.3% becomes 533x400.

Area resize (@)

The @ flag targets an approximate pixel count rather than specific dimensions. 640000@ on a 1600x1200 image produces 924x693 (area = 640,332). The result preserves aspect ratio and gets as close to the target area as possible.

Shell escaping

The >, <, ^, and ! characters have special meaning in most shells. Wrap geometry strings in single quotes on Linux/macOS ('800x600>') or double quotes on Windows ("800x600>"). The backslash escape (800x600\>) also works on Linux/macOS.

See also

• Common options reference for -strip, -quality, -gravity, -background, -composite, and related flags used together with geometry in real commands.