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Estimate how small a JPG, PNG, or WebP will get at any quality setting. Plan your batch compression before you run it. No upload required - just enter image specs.
Before you run a batch compression job across 500 product images, wouldn't you want to know the result? FastCompressor's File Size Calculator estimates the output file size of any image at a given quality setting - before you commit to compression.
Enter your image dimensions, format, and target quality, and get an estimated output size in seconds.
No file upload required.
Moving 10,000 product images to a CDN? The calculator estimates your total compressed storage footprint before you run the job - useful for CDN cost planning.
Need to email 12 photos but your client's email cap is 25MB? Estimate compressed sizes across formats and quality levels to find the right settings before sending.
Platform upload limits (Mailchimp max image size, WordPress media upload limit) are fixed.
The calculator helps you find the right quality setting to stay under the limit.
Which is smaller for your specific image - JPG at 80%, WebP at 75%, or AVIF at 70%? The calculator gives you format comparison data instantly, without running multiple compressions.
Built for professionals who need speed, privacy, and quality.
Learn how it works
Why guessing image sizes is costing you time and money
Every web developer has done this: compress a batch of images, upload them, run Lighthouse, discover the images are still too large, go back, re-compress at a lower quality, re-upload. Repeat twice more.
The Image File Size Calculator eliminates this loop. Before you process a single file, you can estimate what compression settings will achieve.
The calculator uses the established relationship between format, dimensions, colour depth, and quality level to produce accurate estimates. These are not exact byte counts - actual file sizes vary based on image content (a photograph compresses differently from a flat graphic). But the estimates are accurate within 10-20% for planning purposes.
Use cases:
See why teams choose FastCompressor
| Feature | FastCompressor | Typical Online Tool |
|---|---|---|
| No upload needed | Yes - estimate only | Requires upload |
| Multi-format compare | Yes | One format at a time |
| Batch estimate | Yes - folder total | Single file |
| Format options | JPG / PNG / WebP / AVIF | Limited |
| Free to use | Yes | Sometimes paywalled |
| Desktop app version | Yes | No |
Then compress unlimited files with Pro - $19 once
Or enter dimensions below for a quick estimate