Measure the result of a real WebP recompression rather than typing hypothetical sizes into a percentage formula. The calculator exports both the derivative and an audit-friendly report showing original bytes, output bytes, signed savings, and percentage change.
How to use Compression Savings
- Add a static WebP whose current size is the baseline.
- Select an output quality and calculate the savings with a genuine local encode.
- Preview the derivative, confirm its visual quality, and keep the JSON measurement with your optimization record.
What this tool is good at
When to use Compression Savings
Prepare a deliberate compression-savings asset
Create a reviewable derivative and measure signed byte savings for a selected quality before claiming an optimization improvement. Work from the selected WebP's decoded pixels and measured output instead of assuming a filename, generic preset, or browser preview answers the production question.
Compare real local results
Use the paired WebP and JSON outputs to document original bytes, output bytes, percentage change, and cases where the result is honestly larger. Keep the derivative or JSON report with a content ticket, optimization review, design handoff, or regression fixture so another person can reproduce the decision.
Protect private production imagery
Process unreleased campaigns, product photos, client media, interface assets, and internal fixtures in the current browser tab without submitting selected image bytes to a remote conversion or analysis endpoint.
How to choose the right settings
Match the destination requirement
Create a reviewable derivative and measure signed byte savings for a selected quality before claiming an optimization improvement. Confirm dimensions, background, alpha behavior, quality budget, analytical resolution, and consuming application before applying the same setting to a larger batch.
Inspect a representative result
Use the paired WebP and JSON outputs to document original bytes, output bytes, percentage change, and cases where the result is honestly larger. Check detailed and flat-color samples, transparent edges, gradients, text, and varied aspect ratios because one convenient default cannot represent every source.
Retain the authoritative original
The percentage includes every source-versus-output difference, including metadata removal, and it cannot prove acceptable sharpness, texture, or text rendering. Keep source WebP files until generated assets and reports have been reviewed in the real destination and accepted by the responsible editor or developer.
Practical workflow and output details
A genuine browser WebP encode is completed first, then its Blob size is compared with the source using a transparent signed calculation. Processing uses bounded browser Canvas operations, typed-array pixel analysis, local WebP or PNG encoding, and Blob downloads; selected file bytes are never posted for image processing.
The percentage includes every source-versus-output difference, including metadata removal, and it cannot prove acceptable sharpness, texture, or text rendering. The tool states this boundary so a focused local operation is not mistaken for a color-managed editing suite, universal performance benchmark, perceptual-quality guarantee, animation editor, or reversible source-of-truth workflow.
Format behavior and limitations
Negative savings means the derivative is larger than the source; the calculator never hides or clamps that result.
Byte reduction alone cannot establish acceptable quality, and metadata stripping can contribute to size change alongside pixel re-encoding.
Frequently asked questions
What does a negative percentage mean?
It means the selected encode produced more bytes than the original, which is common for sources already optimized efficiently.
Is this only a mathematical calculator?
No. It first creates a real WebP output and then compares its Blob size with the selected source.
Does the report prove quality is preserved?
No. Review the image visually and use a quality metric when the workflow requires quantitative distortion analysis.
Related WebP tools
Quality vs Size Tester
Encode a static WebP at five real quality levels and compare measured bytes, size ratios, and browser encoding time.
Bulk WebP Compressor
Compress a queue of static WebP files with one consistent quality policy and local per-file downloads.
Target Size Compressor
Search multiple WebP quality levels locally and choose the highest tested result at or below a target size when possible.