Break down the extra bytes an AEAD or CBC mode adds — nonce, padding and auth tag — and the cost across your message volume.
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.
Encryption Overhead
overhead = nonce or IV + authentication tag + padding, where CBC padding is 16 − (plaintext mod 16) bytes and AEAD modes add none.
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.
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.
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.
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.