THE NUMBERS, EXPLAINED
How to use this calculator.
For each slot, choose the smallest supplied source width at least as large as slot × pixel ratio. Report a missing source when none fits. This deterministic planning rule does not reproduce every browser image-selection decision; it ignores art direction and network preferences.
Start with your inputs
Enter rendered slot widths (css px), device pixel ratios for those slots, available source widths (image px). Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Rendered slot widths (CSS px) | 320,600 |
| Device pixel ratios for those slots | 2,1 |
| Available source widths (image px) | 400,640,800 |
A worked example
With the example inputs above, required source widths (px) is 640, 600. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| Required source widths (px) | 640, 600 |
| Selected source widths or missing targets | 640, 640 |
How this is calculated
Required source widths (px)
Round up each CSS slot width × its device pixel ratio.
Selected source widths or missing targets
Pick the smallest available source at least that wide; otherwise show the missing minimum.
Symbols and units
- a
- Rendered slot widths (CSS px)
- b
- Device pixel ratios for those slots
- c
- Available source widths (image px)
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions and limits
For each slot, choose the smallest supplied source width at least as large as slot × pixel ratio. Report a missing source when none fits. This deterministic planning rule does not reproduce every browser image-selection decision; it ignores art direction and network preferences.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
Required source widths (px); Selected source widths or missing targets. The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: For each slot, choose the smallest supplied source width at least as large as slot × pixel ratio. Report a missing source when none fits. This deterministic planning rule does not reproduce every browser image-selection decision; it ignores art direction and network preferences.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Further reading
MDN: responsive images and pixel density ↗
Background reading; this does not imply endorsement or independent review of SumFizz.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
Read the quality report →