Why Format Conversion Beats Compression for Email
When you need to compress files for email, your first instinct might be to use a compression tool. But converting to a different format often achieves dramatically better results. The format you choose determines the fundamental efficiency of how image data is stored.
A 5 MB PNG photograph converted to JPG is often under 500 KB—a 90% reduction—without any additional compression settings applied.
PNG uses lossless compression, preserving every pixel perfectly. This is ideal for graphics with sharp edges or transparency, but wasteful for photographs where subtle variations don't matter. JPG uses lossy compression designed specifically for photos, discarding data your eyes won't miss.
| Conversion | Typical Reduction | Best For |
|---|---|---|
| PNG → JPG | 70-90% | Photographs, complex images |
| HEIC → JPG | 10-30% increase | iPhone photos needing universal compatibility (not for size reduction) |
| RAW → JPG | 80-95% | Camera files for sharing, not editing |
| BMP → JPG | 85-95% | Legacy uncompressed images |
The key insight: format conversion changes what data is stored, while compression changes how efficiently that data is stored. For email attachments, changing the format is usually the higher-impact move.
Email Attachment Size Limits
Email attachment limits haven't changed much in a decade, but file sizes have. Gmail and Outlook cap attachments at 25 MB, and many corporate servers set lower limits at 10-15 MB. A single modern smartphone photo can easily exceed 5 MB.
This creates a practical constraint: if you're sending multiple images, they need to be small. A folder of 20 vacation photos at 5 MB each totals 100 MB—four times the Gmail limit. Converting these PNG screenshots or HEIC iPhone photos to compressed JPGs can bring the entire batch under 10 MB.
Provider limits at a glance
- Gmail — 25 MB total attachment size
- Outlook — 25 MB for Outlook.com, varies for corporate
- Yahoo Mail — 25 MB per message
- Proton Mail — 25 MB limit
For reliable delivery, aim for individual attachments under 5 MB and total message size under 15 MB. This ensures your email won't bounce, even if the recipient's server has stricter limits than yours.
Best Format Conversions for Email
Different source formats require different conversion strategies. Here are the most common scenarios when preparing images for email.
PNG to JPG
PNG files are often screenshots, exported graphics, or photos saved with transparency. If the image doesn't need transparency and is photographic in nature, converting PNG to JPG is almost always worth it.
Quick workflow
Use PNG to JPG conversion with the 1080p resize preset and lossy compression. This handles screenshots and exported graphics in one step.
HEIC to JPG
Modern iPhones save photos as HEIC by default. While efficient, HEIC isn't universally supported—many Windows PCs and older email clients can't open these files. Converting HEIC to JPG ensures everyone can view your photos.
The conversion typically reduces file size by 40-60% while gaining universal compatibility. Use HEIC to JPG conversion with the 1080p preset for email-friendly results.
RAW to JPG
Camera RAW files (CR2, NEF, ARW, etc.) are 25-50 MB each—far too large for email. These files contain unprocessed sensor data meant for editing. For sharing finished photos, convert RAW to JPG.
RAW conversion workflow
- For email sharing — Convert to JPG at 1080p or 720p preset with lossy compression
- For print-quality sharing — Convert to JPG at 4K preset with quality 85+
- File size impact — A 30 MB RAW becomes a 200-400 KB JPG at 1080p
WebP to JPG
WebP is excellent for the web but has inconsistent support in email clients. Some versions of Outlook and older email software won't display WebP images. For maximum compatibility, convert WebP to JPG before attaching to emails.
This conversion also increases file size slightly—WebP is more efficient than JPG—so pair it with the 720p resize preset to keep attachments small.
Using Resize and Compression Options
After choosing your conversion format, resize and compression settings fine-tune the result. Convert.FAST applies these automatically during conversion. For more details on file size reduction techniques, see our guide on how to reduce image file size.
Resize Presets for Email
Modern phone cameras produce images at 3000x4000 pixels or larger—far larger than needed for email viewing. Resizing to display dimensions before converting eliminates wasted data.
| Preset | Longest Edge | Best For |
|---|---|---|
| 720p | 1280 pixels | Email thumbnails, quick sharing |
| 1080p | 1920 pixels | Standard email viewing, most photos |
| 4K | 3840 pixels | High-resolution needs, minimal reduction |
For most email attachments, the 1080p preset strikes the right balance. Images remain sharp on high-DPI displays but are small enough to send multiple per email.
Smart Compression Modes
After resizing and format conversion, compression further reduces file size:
- Lossy — Uses format-specific optimization (MozJPEG for JPG, palette quantization for PNG). Best for photographs where small file size matters. Quality 70-85 is typically "visually lossless"—differences are imperceptible in normal viewing.
- Lossless — Reduces size without discarding image data. Use for screenshots, diagrams, or graphics with text where artifacts would be noticeable.
Recommended settings for email
- Format: Convert PNG/HEIC/RAW to JPG
- Resize: 1080p preset for most photos, 720p for thumbnails
- Compression: Lossy mode for photographs
- Metadata: Strip EXIF data to save a few KB and remove location info
Batch Processing for Multiple Files
Processing images one at time isn't practical for email batch preparation. Convert.FAST handles up to 1,000 files at once with identical settings.
Batch conversion workflow
- Configure resize (1080p) and compression (lossy) settings first
- Drop or select multiple images—different formats are fine
- Each file converts with your preset settings automatically
- Download individual files or the complete batch as a ZIP
Settings persist across file additions within a session. Add more files after the first batch completes without reconfiguring options. This is useful when you've forgotten a few images and need them processed identically.
Real-world batch example
A designer sends 25 product photos to a client. Original files are HEIC from an iPhone, averaging 4 MB each (100 MB total). After batch conversion to JPG at 1080p with lossy compression: 25 files averaging 180 KB each (4.5 MB total)—small enough for a single email.
Security and Privacy
Processing files through a cloud service requires trust in data handling. Files are encrypted in transit (TLS), encrypted at rest (AES), and automatically deleted within one hour. Filename metadata is redacted from the database after deletion.
For sensitive images—client photos, confidential screenshots, personal documents—this automatic deletion is essential. You can read more about encryption practices and data retention policies.
When to Use Compression Instead
Format conversion is the right choice when you're changing from an inefficient format (PNG, HEIC, RAW, BMP) to an email-friendly one (JPG). But sometimes you want to keep the original format and just make it smaller.
Use compression-only when:
- You need to keep PNG transparency — Converting PNG to JPG removes transparency. If you need transparent backgrounds, compress the PNG instead.
- You're working with documents — PDF, Word, and PowerPoint files need document-specific compression, not format conversion.
- The file is already JPG — Re-compressing a JPG introduces generation loss. If quality is already acceptable, sending as-is may be better than re-compressing.
For these scenarios, use Compress.FAST to shrink files without changing formats. It specializes in document compression and image optimization while keeping original file types intact.
Frequently Asked Questions
Will converting PNG to JPG reduce image quality?
Yes, but the impact is usually imperceptible for photographs. JPG uses lossy compression that discards data, while PNG preserves every pixel. For photos with smooth gradients and natural textures, this loss isn't visible at normal quality settings (70-85). For graphics with sharp edges or text, PNG may be preferable.
What's the best format for email attachments?
JPG is the most compatible format for email—every device and email client supports it. For photographs, it's also highly efficient. For graphics needing transparency, PNG is the standard but results in larger files. WebP and AVIF are excellent for the web but have spotty email client support.
How much can I reduce a file by converting formats?
Typical reductions: PNG to JPG (70-90%), HEIC to JPG (40-60%), RAW to JPG (80-95%), BMP to JPG (85-95%). Adding resize and compression on top can achieve 90-95% total reduction. A 10 MB PNG converted to 1080p JPG often becomes 200-400 KB.
Should I resize before or after converting?
Resize first for the smallest output. Resizing removes unnecessary pixel data before compression, giving the algorithm less to process. Convert.FAST applies resize before format conversion automatically when both options are enabled, ensuring optimal results.
Convert.FAST supports batch conversion up to 1,000 files with resize presets and smart compression. Convert HEIC, PNG, RAW, and WebP to email-friendly JPG. Files are encrypted and auto-deleted within one hour.
Stewart Celani
Founder
15+ years in enterprise infrastructure and web development. Stewart built Tools.FAST after repeatedly hitting the same problem at work: bulk file processing felt either slow, unreliable, or unsafe. Convert.FAST is the tool he wished existed—now available for anyone who needs to get through real workloads, quickly and safely.
Read more about Stewart