Image File Size Calculator
Know the Output Size Before You Compress.
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.
100% Private
Zero cloud uploads
True Color Fidelity
Original colors stay vibrant
Instant Speed
Fast in-browser processing
Free Online
No file limits
Upload an image (optional)
Or enter dimensions below for a quick estimate
Compression settings
Wondering what size your image will be after compression? Set format and quality, then get a realistic estimate before you export.
Why a File Size Calculator Is Useful
Plan Storage Before Migrating
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.
Check Before Attaching
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.
Hit Exact Upload Limits
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.
Compare Format Efficiency
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.
How to compress File Size Calculator files
Instant, private compression in three quick steps.
Select Files
Drag and drop your File Size Calculator files into the browser or click Browse.
Choose Quality
Pick Balanced (recommended), Lossless, or Maximum compression presets.
Instant Download
Save files individually or download all converted files in a single ZIP archive.
Built for professionals who need speed, privacy, and quality.
E-commerce managers estimating CDN storage costs before a product image migration
Developers setting image quality thresholds in build pipelines
Email marketers checking whether an image batch fits within attachment limits
Photographers quoting file delivery sizes to clients
Web designers presenting image optimisation potential to clients
How We Compare
Why privacy-focused professionals 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 |
Why use FastCompressor Desktop for File Size Calculator?
Online tools are great for quick, single-image edits. But when you need to process hundreds of files or maintain strict privacy, native software is the only way to go.
Image File Size Calculator
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:
- CDN and bandwidth planning: If you're switching 10,000 product images from JPEG to WebP, the calculator estimates your storage savings and bandwidth reduction before migration.
- Core Web Vitals planning: Google recommends images contributing to LCP be under 200 KB. Calculate which quality settings achieve this for your typical image dimensions.
- Email marketing: Most email clients display images but load them on delivery. Email images over 200 KB increase delivery time and spam score. Calculate the right quality target before your next campaign.
- Client delivery estimates: "These 150 retouched photos will be approximately 2.3 GB as TIFF, or 450 MB as compressed JPG at quality 85." Exact answers before the job is done.
Frequently Asked Questions
Common questions about in-browser File Size Calculator optimization.
Try the File Size Calculator Free - No Upload, No Signup
Then compress unlimited files with Pro - $19 once