Numbers, units, and bits
Resize an image while controlling proportions and rounding
Tools to use
You want to specify only an image's width, but an editor also asks for height. Preserving the ratio produces a fraction. The mathematical answer and the dimensions accepted by an image file are different questions. Use Ratio Calculator first, then decide integer pixels and fitting behaviour so stretching or cropping is deliberate.
Reduce 1920×1080 to 16:9
1920 : 1080 = 16 : 9Select ratio simplification and enter 1920 and 1080 separately. The result 16:9 describes shape, not display size. Both 320×180 and 1920×1080 have that ratio but different pixel counts. Equal proportions do not imply equal detail or image quality.
For portrait images keep the order straight: 1080×1920 is 9:16. Supplying a height while retaining 16:9 can produce a valid calculation for the wrong shape. Use width:height consistently and check the dimensions after any rotation.
Fix the width and calculate the other side
ratio = 16:9
width = 320
height = 320 × 9 ÷ 16 = 180A ratio determines the missing side, but the current ratio screen cannot accept ratio notation in its missing-dimension input. Use Decimal Calculator with 320*9/16 instead: height is 180, and 180*16/9 returns width 320. No unavailable known-side selector is needed. Reverse calculation helps catch swapped dimensions.
The calculator does not inspect an uploaded image. You supply the ratio. Do not assume every video is 16:9; check the actual asset. The tool cannot know that a square icon or portrait clip was given the wrong ratio. Correct arithmetic cannot validate an incorrect premise.
At width 321, the height is fractional
321 × 9 ÷ 16 = 180.5625
floor → 180
nearest → 181
ceil → 181Enter 321*9/16 in Decimal Calculator to obtain 180.5625. If integer image pixels are required, explicitly choose floor, nearest or ceiling at the destination. Calculator significant-digit precision is not the same as rounding to whole pixels. Height 181 slightly changes the ratio; retain original dimensions rather than expecting rounded values to reverse exactly.
For exact integer 16:9 dimensions, use matching multiples of 16 and 9: after 320×180 comes 336×189. But 336 will not fit a maximum width of 321. Decide which constraints are required; padding or cropping may be needed when exact proportions and bounds conflict.
Fitting inside a box differs from filling it
width 300 → height 168.75
height 200 → width 355.555…Fitting width 300 gives height 168.75, leaving vertical space in a 300×200 box. Matching height 200 gives width about 355.56, requiring horizontal cropping to fill it. Forcing 300×200 changes 16:9 to 3:2. A surprising layout may be an unresolved choice between padding, cropping and distortion rather than wrong arithmetic.
The calculator does not resize or crop files. Use it to estimate dimensions, then inspect the actual image or CSS layout. CSS aspect-ratio expresses a ratio alongside other sizing constraints. Verify important faces or text in the rendered result instead of assuming the arithmetic guarantees composition.
Hand over the rounding rule and purpose too
A handoff saying only 'height 181' omits the intent. Specify 'original 16:9, width 321, calculated height 180.5625 rounded to 181, no crop'. Distinguish CSS display pixels from exported image pixels when they differ.
Keep the source ratio, resulting width and height, and rounding rule together. Small sample numbers suffice to check the relationship. Because the tool calculates from dimensions without uploading the image, you can discuss sizing without sending an unpublished or internal asset.