How to Compress an Image for Email Attachment

ハウツー

このガイドの使い方

1

Upload your image to ImgKit

Open ImgKit's image compressor at gift.harpd.com/tools/image-compressor. Click 'Choose File' or drag your image onto the upload area. JPEG, PNG, and WebP files are all supported.

2

Adjust the quality setting

Use the quality slider to find the right balance. For photos, start at 80 % — the preview will show you the result before you commit. For logos and graphics, use lossless mode to preserve sharp edges.

3

Download the compressed file

Click 'Download' to save the compressed image. ImgKit shows the original and new file sizes so you can confirm the reduction. Attach the downloaded file to your email.

Email attachment limits catch people off guard. A single uncompressed photo from a modern smartphone can be 8–15 MB — over the limit for many mail servers. Compressing with ImgKit takes under a minute and keeps images looking sharp.

Email Attachment Size Limits

Different email providers enforce different limits:

Provider	Attachment limit
Gmail	25 MB total
Outlook.com / Microsoft 365	20 MB total
Yahoo Mail	25 MB total
iCloud Mail	20 MB total
Corporate mail servers	Often 10 MB or less

These limits apply to the entire message, not just one file. A message with five photos needs to fit within the total limit. Email also encodes attachments in Base64, which adds approximately 33 % overhead — a 15 MB set of images becomes ~20 MB on the wire.

Practical targets: Keep each image under 1–2 MB for safe delivery. If you’re sending multiple photos, aim for 500 KB per image.

Recommended File Sizes by Use Case

• Email thumbnail / reference image: 100–300 KB
• Full-size photo for review: 500 KB – 1 MB
• High-quality photo for print or production use: 1–3 MB (only when the recipient specifically needs it)

Best Format Choices for Email

Photos → JPEG. JPEG’s lossy compression is highly efficient for photographs. At quality 80–85 %, file sizes are typically 200–800 KB for a full-resolution smartphone photo — well within any provider’s limits.

Screenshots and graphics → PNG. PNG’s lossless compression keeps text sharp and prevents color banding in flat-color graphics. A screenshot compressed with an optimizer like ImgKit is usually 50–200 KB.

Avoid HEIC for email. iPhone photos are saved as HEIC by default, but most email clients and Windows systems cannot open them. Convert to JPG first using ImgKit’s HEIC to JPG converter.

How to Compress with ImgKit

Open ImgKit’s compressor and follow the three steps above. The entire process runs in your browser — nothing is uploaded to a server, so your images stay private.

After downloading, attach the compressed file directly to your email. If you still need to reduce the dimensions (for example, a 4000 × 3000 photo that only needs to be 1200 × 900 for review purposes), use the resize tool before or after compression. Resizing down also reduces file size significantly on its own.

For more detail on targeting specific file sizes, see the related guides on compressing images to 500 KB and below.

よくある質問

What is the file size limit for email attachments?

Gmail allows up to 25 MB per message. Outlook.com and Microsoft 365 allow up to 20 MB. Apple Mail follows your email provider's limits. Corporate mail servers often impose stricter limits (10 MB or less). As a practical rule, keep individual image attachments under 5 MB and total message size under 10 MB to ensure reliable delivery.

What is the best image format for email attachments?

JPEG is the best format for photo attachments — it achieves small file sizes with acceptable quality. PNG is better for screenshots, diagrams, and graphics with text. Avoid sending HEIC, TIFF, or RAW files by email; most recipients cannot open them without additional software.

How do I reduce image file size without losing visible quality?

For JPEG photos, a quality setting of 80–85 % is visually indistinguishable from the original at normal viewing sizes but typically cuts file size by 60–75 % compared to a quality-100 export. For PNG graphics, use ImgKit's lossless PNG compressor to remove metadata and optimize encoding without touching a pixel.

Can I compress images on a mobile device?

Yes. ImgKit works in any modern mobile browser on iOS and Android. Open gift.harpd.com in Safari or Chrome, upload your image, compress it, and download the result — no app installation required.

Does compression change the image dimensions?

Compression alone does not change pixel dimensions — it only reduces file size by encoding the existing pixels more efficiently. If you also want to reduce the physical dimensions (width and height in pixels), use a resize tool in addition to or instead of compression.