Responsive Image Dimensions Calculator
This tool preserves a source image’s aspect ratio while estimating the pixel dimensions needed for a target display width and device pixel ratio. Use it to plan responsive image variants.
How to use this calculator
- Enter positive integer source dimensions and the intended rendered CSS width and DPR.
- Check that the input values use the same units and reporting period before comparing scenarios.
- Change one assumption at a time to see which input drives the result.
Formula and method
Target pixel width = CSS width × device pixel ratio; target height = target width × source height / source width.
- Target bitmap width = 1,200 × 2 = 2,400 px; height = 2,400 × 3,000 ÷ 4,000 = 1,800 px.
- Browser selection also depends on `srcset`, `sizes`, layout changes and actual device conditions. Upscaling a source does not create real detail.
Worked examples
Worked example
A 4,000 × 3,000 image is intended for a 1,200 CSS-pixel slot at DPR 2.
Target bitmap width = 1,200 × 2 = 2,400 px; height = 2,400 × 3,000 ÷ 4,000 = 1,800 px.
A proportional 2,400 × 1,800 bitmap meets the entered target.
Understanding the result
A proportional 2,400 × 1,800 bitmap meets the entered target.
Browser selection also depends on `srcset`, `sizes`, layout changes and actual device conditions. Upscaling a source does not create real detail.
Important notes
Enter positive integer source dimensions and the intended rendered CSS width and DPR.
The displayed result uses the entered assumptions; validate important estimates against source measurements, specifications or qualified guidance.
Frequently asked questions
Should every image be served at DPR 3?
Not automatically. File size, likely devices and browser source selection all matter.
Does this crop the image?
No. It preserves the original aspect ratio; cropping requires a separate design decision.
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Related explanations
Learn the method behind the number or compare the terminology before using the calculator.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.