Resize an Image and Keep Its Aspect Ratio

This page starts from an image you already have. Give it the current pixel width and height and it works the ratio out for you, then resizes to whatever target you name. If you are starting from a ratio instead, 16:9 or 4:3 with no existing file, the aspect ratio dimensions page takes the ratio and one dimension and returns the other.

An aspect ratio is the proportional relationship between an image’s width and height, expressed as width:height. It defines the shape of the frame and determines how images print, display on screens, and crop to fit different formats.

Formulas: Aspect Ratio = Width : Height (simplified by GCD) GCD (Greatest Common Divisor) reduces the ratio to its simplest form. Print dimension: Width (inches) = Pixel Width ÷ DPI

Cropping needs a conditional, not a single formula, because which edge you trim depends on which way the mismatch runs. Compare the original ratio with the target:

• Target is wider than the original (target ratio > original ratio): keep the full width, trim top and bottom. Height = Width ÷ Target Ratio
• Target is narrower (target ratio < original ratio): keep the full height, trim the sides. Width = Height × Target Ratio

Get that backwards and you compute a crop larger than the image you started with, which is the single most common mistake in hand-rolled crop math.

Common aspect ratios and their uses:

Worked example, finding crop dimensions: Your camera captures 6000×4000 pixels (3:2 ratio). You want to share on Instagram (1:1 square).

Cropped size = 4000×4000 (centered, losing 1000 pixels on each side of width). Lost area = (6000×4000) − (4000×4000) = 24,000,000 − 16,000,000 = 8,000,000 pixels removed (33% of image)

To print a 4000×4000 crop at 300 DPI: Print size = 4000 ÷ 300 = 13.3 × 13.3 inches

Standard print sizes and their ratios. All stated landscape (long edge first), so they can be compared with each other and with the 3:2 a camera gives you. Turn the print portrait and the ratio simply inverts: 5:4 becomes 4:5.

Note that 10×8 and 20×16 are the same shape at different sizes, which is why an 8×10 crop you already like scales straight up to 16×20 with no further decisions.

How many pixels a print actually needs. At 300 dots per inch, which is the standard for photographic printing:

A 24-megapixel camera gives 6000×4000, so it prints 20×13 inches at full 300 DPI with nothing to spare. Anything larger is fine anyway: nobody views a 30-inch print from ten inches away, and 150 to 200 DPI holds up at normal viewing distance. For web display 72 to 150 DPI is plenty.

Always crop from the original file, never from an exported JPEG. Every save of a lossy format throws away a little more, and the damage compounds. Crop the raw or the master, then export once.

Composition tip: When shooting with a 3:2 camera for eventual 4:5 (Instagram) or 16:9 (YouTube thumbnail) delivery, leave extra space on the sides or top/bottom to give yourself room when cropping. This technique is called shooting loose.