Calculate the ratio before selecting dimensions

For an 1800 by 1200 source, the width-to-height ratio is 3:2. An output of 900 by 600 preserves that ratio. A requested 900 by 900 square does not, so you must choose whether to crop part of the scene or place the full scene inside a square background. Recording that decision prevents someone from later stretching the image to fill an incompatible slot.

Treat two numbers in pixels differently from a maximum long edge. If a requirement says no more than 1000 pixels on the long edge, a 1000 by 667 result may be appropriate for a 3:2 source. If it says exactly 1000 by 1000, the canvas must be square. Print dimensions introduce another input, pixels per inch, and should not be inferred from these pixel values alone. After exporting, inspect orientation, edge content and the actual dimensions recorded for the file.

Start with the shape, not the number

A square target, a wide banner, and a tiny icon each need a different decision. The browser can create the requested canvas, but it cannot make a rectangular source fit a different shape without cropping, padding, or distortion.

Crop-to-fill is useful when the destination must be completely covered. Fit-with-background is safer when the whole image must remain visible. Stretching is the fast path to distorted faces, logos, circles, and text.

Use exact-size pages for real constraints

Exact sizes are most useful when a form, profile field, app icon, banner slot, or upload portal asks for a specific dimension. A page should explain that constraint rather than pretending every pixel pair is equally important.

For very small sizes such as icons, legibility matters more than detail. For larger banner sizes, the important question is which area remains visible after cropping.

Verify the generated output

A trustworthy resizer should show the generated dimensions from the actual output, not just repeat the requested settings. That catches orientation problems and accidental changes before the file is used.

Pixel size is separate from file size. If a form also limits KB or MB, resize first and then compress the generated image if needed.