Encryption Overhead Calculator
Break down the extra bytes an AEAD or CBC mode adds — nonce, padding and auth tag — and the cost across your message volume.
Inputs
Overhead per Message
28bytes
Total Ciphertext Size
1,052bytes
Overhead vs Plaintext
2.73%
Padding Added
0bytes
Overhead Across All Messages
26.70MiB
Byte Breakdown
12 B IV/nonce + 0 B padding + 16 B tag/MAC
Step by step
Values used
Cipher mode = AES-GCM (12 B nonce, 16 B tag); Plaintext size = 1,024 bytes; Messages encrypted = 1,000,000 messages
Encryption Overhead
overhead = nonce or IV + authentication tag + padding, where CBC padding is 16 − (plaintext mod 16) bytes and AEAD modes add none.
Overhead per Message
= 28 bytes
Total Ciphertext Size
= 1,052 bytes
Overhead vs Plaintext
= 2.73
Padding Added
= 0 bytes
Overhead Across All Messages
= 26.70 MiB
Byte Breakdown
= 12 B IV/nonce + 0 B padding + 16 B tag/MAC
How it works
AEAD modes are stream-based, so the ciphertext matches the plaintext length and the only additions are the nonce you must transmit and the fixed 16-byte tag. CBC has to pad to a whole 16-byte block, and PKCS#7 adds a full block when the plaintext is already aligned so the receiver can always strip padding unambiguously. On small records the overhead dominates: encrypting 64-byte telemetry with AES-GCM inflates it by 44%, which decides whether a payload still fits inside an MTU or a queue message limit.
Formula
Encryption Overhead
overhead = nonce or IV + authentication tag + padding, where CBC padding is 16 − (plaintext mod 16) bytes and AEAD modes add none.
- nonce
- 12 bytes for GCM and ChaCha20-Poly1305, 16 for CBC
- tag
- 16 bytes for a GCM/Poly1305 tag, 32 for HMAC-SHA-256
- padding
- PKCS#7 always adds 1–16 bytes so the length is unambiguous
Frequently Asked Questions
How is Encryption Overhead calculated?
overhead = nonce or IV + authentication tag + padding, where CBC padding is 16 − (plaintext mod 16) bytes and AEAD modes add none. AEAD modes are stream-based, so the ciphertext matches the plaintext length and the only additions are the nonce you must transmit and the fixed 16-byte tag. CBC has to pad to a whole 16-byte block, and PKCS#7 adds a full block when the plaintext is already aligned so the receiver can always strip padding unambiguously.
Why does Encryption Overhead matter?
On small records the overhead dominates: encrypting 64-byte telemetry with AES-GCM inflates it by 44%, which decides whether a payload still fits inside an MTU or a queue message limit.
What values do I need to enter?
This calculator takes 3 inputs: Cipher mode, Plaintext size, Messages encrypted. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Why does AES-GCM add nothing for padding?
GCM runs AES in counter mode, turning the block cipher into a keystream generator. The keystream is truncated to the plaintext length, so no padding is needed and no padding-oracle attack surface exists.
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