Thumbnail Size Calculator
Resize an image proportionally so its longest side fits the maximum thumbnail size.
How to use Thumbnail Size Calculator
1. Enter Original Width, Original Height, Maximum Side. 2. The thumbnail never enlarges its source. Rounding can slightly alter extreme aspect ratios. This is a numeric estimate, not an image transformation. 3. Select Calculate or press Enter. 4. Select Copy result to copy the answer, or Reset to clear the form and result.
Why use this tool?
The thumbnail never enlarges its source. Rounding can slightly alter extreme aspect ratios. This is a numeric estimate, not an image transformation.
How the calculation works
Scale by min(1, maximum side ÷ longest original side), round each dimension and keep at least one pixel. The thumbnail never enlarges its source. Rounding can slightly alter extreme aspect ratios. This is a numeric estimate, not an image transformation. Counts and dimensions use the declared rounding convention; other numbers display up to three decimal places where applicable. Nonzero subsecond durations are labelled. Durations round to the nearest second; nonzero subsecond durations are labelled. File-size output units are decimal. Changing an input label does not convert filled values.
Example
Worked example: Original Width = 400; Original Height = 200; Maximum Side = 100. Result: 100 × 50 px.
Practical tip
Check the input labels and assumptions against the thumbnail size task before reusing the number.
Frequently asked questions
What exactly does Thumbnail Size Calculator calculate?
Scale by min(1, maximum side ÷ longest original side), round each dimension and keep at least one pixel.
Which units or counting rules apply?
The thumbnail never enlarges its source. Rounding can slightly alter extreme aspect ratios. This is a numeric estimate, not an image transformation.
Does this inspect, change or upload my files?
This page calculates the entered values or text in the browser. Numeric image/media tools do not transform files. Check the worked example and stated counting basis; the result does not certify device, search-engine or accessibility outcomes.