Image Capacity Calculation for Steganography
Understand how to calculate the maximum hidden data capacity of any image based on its dimensions, color channels, and the LSB embedding method.
Detailed Explanation
How Much Data Can an Image Hide?
Before embedding a message, you need to know whether the cover image is large enough. Capacity calculation is straightforward but involves several factors.
Basic Formula
capacity_bits = width × height × channels_used × bits_per_channel
capacity_bytes = capacity_bits / 8
For standard LSB embedding using all three RGB channels at 1 bit per channel:
capacity_bytes = (W × H × 3) / 8
Practical Examples
| Image Size | Pixels | Raw Capacity | Usable (minus header) |
|---|---|---|---|
| 100 × 100 | 10,000 | 3,750 bytes | 3,746 bytes |
| 640 × 480 | 307,200 | 115,200 bytes (112 KB) | 115,196 bytes |
| 1920 × 1080 | 2,073,600 | 777,600 bytes (759 KB) | 777,596 bytes |
| 4000 × 3000 | 12,000,000 | 4,500,000 bytes (4.3 MB) | 4,499,996 bytes |
Overhead: The Length Header
A 32-bit (4-byte) length header is embedded before the message to indicate payload size. This is subtracted from the total capacity:
usable_capacity = capacity_bytes - 4
For all but the smallest images, this overhead is negligible.
UTF-8 Considerations
Not all characters are 1 byte. When estimating whether your text fits:
- ASCII (English letters, digits): 1 byte each
- Latin extended (accents, umlauts): 2 bytes each
- CJK characters (Chinese, Japanese, Korean): 3 bytes each
- Emoji: 4 bytes each
A 500-character Japanese message requires roughly 1,500 bytes — three times what the same character count in English would need.
Capacity Display
The Invisible Watermark tool automatically calculates and displays the available capacity when you load an image. It shows both the maximum byte count and an estimated character count based on your input encoding. If your message exceeds capacity, the tool warns you before attempting to embed.
Use Case
A digital artist wants to embed a 2,000-character artist statement into their work and needs to verify that the image resolution provides enough capacity before embedding.