Compress Image
Resize an oversized image when needed and recompress it locally as JPEG with a chosen quality level.
Use the Compress Image
Resize an oversized image when needed and recompress it locally as JPEG with a chosen quality level.
Calculations and file processing happen locally in your browser; this batch does not require an external processing service.
Educational calculations show the stated arithmetic only; follow your course or institution rules where they differ. Image files are processed locally in the browser and are not uploaded by this tool.
How to use it
Start with the worked example or settings, replace them with your real data, inspect the result and assumptions, and download or copy only after checking the source information.
How the Compress Image works
Compress Image resizes an uploaded image when necessary and exports it as JPEG using the quality setting selected by the user.
Calculation breakdown
ToolLott will explain the current inputs and displayed result here.
How ToolLott compresses an image
The browser decodes the selected image, limits the width while preserving aspect ratio, redraws it to a canvas, and encodes the canvas as JPEG with the selected quality value.
new height = original height x (new width / original width)Applied only when the image is wider than the selected maximum width.
| Symbol / input | Meaning | Unit |
|---|---|---|
original width/height | Decoded source-image dimensions | pixels |
max width | Largest allowed output width | pixels |
quality | JPEG encoder quality input selected by the user | 10-100% in the ToolLott UI |
Step-by-step method
- Decode the selected image in the browser.
- If its width exceeds the maximum, calculate a proportional output height.
- Draw the image to a canvas at the target dimensions.
- Encode the canvas as JPEG using the selected quality and expose the generated file for download.
Worked example
The QA workflow loads a deterministic 1000 x 700 PNG, sets maximum width to 800 px and uses the JPEG compression path.
Example inputs
- Source fixture = 1000 x 700 PNG
- Maximum width = 800 px
- JPEG quality target = 75%
Calculation / processing
Scale factor = 800 / 1000 = 0.8Output height = 700 x 0.8 = 560 pxCanvas output = 800 x 560 pxEncode as image/jpeg at the selected quality setting
JPEG quality is an encoder setting, not a promise that the output file will be exactly 75% of the original file size.
Assumptions
- The browser can decode the selected input format.
- Aspect ratio is preserved when resizing to the selected maximum width.
- JPEG is an appropriate lossy output format for the user's image.
Limitations
- Exact file-size reduction depends on image content, source format, dimensions and browser encoder behaviour.
- JPEG does not preserve alpha transparency.
- Repeated lossy recompression can reduce visual quality; retain the original when quality matters.
Common questions
Does 75% quality mean the file will be 25% smaller?
No. The quality value controls the JPEG encoder. Output file size depends heavily on image content and dimensions.
Will a smaller maximum width reduce file size?
Usually, because fewer pixels need to be encoded, but the exact reduction is not fixed.
Is the image uploaded?
No. This compression tool processes the image locally in the browser.
Methodology sources
Related ToolLott tools
A realistic way Noah could use this tool
Noah is a freelance designer.
A site editor receives a 2400-pixel-wide site photo that is larger than needed for a content card. They want a local JPEG export capped at 1600 px wide and 75% quality before comparing the actual file sizes.
Noah fills the tool with the case values—quality = 75; max width = 1600. Running those values produces Compression target: JPEG 75% - max width 1600px, giving the scenario a concrete output that can be recalculated or regenerated when the inputs change.
The resulting figure/text/output is Compression target: JPEG 75% - max width 1600px. Noah now has a reproducible checkpoint for the task: resize an oversized image when needed and recompress it locally as JPEG with a chosen quality level. The page also keeps this limitation explicit: Educational outputs use the stated mathematical or formatting model and may differ from institution-specific conventions.
Fictional scenario using realistic example data. For Ready tools, the worked result is tied to the tested example shown in the tool. Replace the figures with your own inputs and independently verify important professional, financial, legal, health or safety decisions.
What this tool is for
Use it to resize an oversized image when needed and recompress it locally as JPEG with a chosen quality level.
It sits within ToolLott’s Image Tools collection, where you can also pick colours, compress, crop, resize, rotate, watermark, convert and prepare images for common web and document uses.