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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.

About Encryption Overhead Calculator

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

Worked example

With the default values below, Encryption Overhead Calculator returns Overhead per Message of 28. Change any input above and every figure here is recalculated in your browser.

Inputs used

Cipher modeone of AES-GCM (12 B nonce, 16 B tag), ChaCha20-Poly1305 (12 B nonce, 16 B tag), AES-CBC (16 B IV, PKCS#7), AES-CBC + HMAC-SHA-256 (16 B IV, 32 B MAC), AES-CCM (13 B nonce, 16 B tag)
AES-GCM (12 B nonce, 16 B tag)
Plaintext sizeminimum 0 bytes
1,024 bytes
Messages encryptedminimum 1 messages
1,000,000 messages

Results

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

Step by step

  1. 1Values used
  2. 2Encryption Overhead
  3. 3Overhead per Message= 28 bytes
  4. 4Total Ciphertext Size= 1,052 bytes
  5. 5Overhead vs Plaintext= 2.73
  6. 6Padding Added= 0 bytes
  7. 7Overhead Across All Messages= 26.70 MiB
  8. 8Byte Breakdown= 12 B IV/nonce + 0 B padding + 16 B tag/MAC

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

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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