RSA Key Size Calculator

Map an RSA modulus size to its NIST SP 800-57 equivalent security strength, ECC equivalent and signature size.

Inputs

RSA modulus size

Protect data until year

Equivalent Security Strength

112bits

ECC Equivalent

ECC 224-bit curve (~112-bit strength)

Signature Size

256bytes

DER Public Key Size

294bytes

Assessment

Upgrade to RSA-3072 for 128-bit strength

Step by step

Values used
RSA modulus size = RSA-2048; Protect data until year = 2,035
RSA Key Size
NIST SP 800-57 equivalences: RSA-2048 ≈ 112-bit, RSA-3072 ≈ 128-bit, RSA-7680 ≈ 192-bit, RSA-15360 ≈ 256-bit; a raw signature is modulus ÷ 8 bytes.
Equivalent Security Strength
= 112 bits
ECC Equivalent
= ECC 224-bit curve (~112-bit strength)
Signature Size
= 256 bytes
DER Public Key Size
= 294 bytes
Assessment
= Upgrade to RSA-3072 for 128-bit strength

How it works

RSA strength grows far slower than its key length because the general number field sieve is sub-exponential: quadrupling the work factor from 112 to 128 bits needs the modulus to jump from 2048 to 3072 bits. Signature and ciphertext blocks are always exactly the modulus length in bytes. NIST disallowed 112-bit strength after 2030, so any RSA-2048 certificate or signing key with a lifetime past that date is already on a migration clock.

Formula

RSA Key Size

NIST SP 800-57 equivalences: RSA-2048 ≈ 112-bit, RSA-3072 ≈ 128-bit, RSA-7680 ≈ 192-bit, RSA-15360 ≈ 256-bit; a raw signature is modulus ÷ 8 bytes.

modulus: RSA modulus length in bits
equivalent bits: Symmetric-key strength that offers the same work factor

Frequently Asked Questions

How is RSA Key Size calculated?

NIST SP 800-57 equivalences: RSA-2048 ≈ 112-bit, RSA-3072 ≈ 128-bit, RSA-7680 ≈ 192-bit, RSA-15360 ≈ 256-bit; a raw signature is modulus ÷ 8 bytes. RSA strength grows far slower than its key length because the general number field sieve is sub-exponential: quadrupling the work factor from 112 to 128 bits needs the modulus to jump from 2048 to 3072 bits. Signature and ciphertext blocks are always exactly the modulus length in bytes.

Why does RSA Key Size matter?

NIST disallowed 112-bit strength after 2030, so any RSA-2048 certificate or signing key with a lifetime past that date is already on a migration clock.

What values do I need to enter?

This calculator takes 2 inputs: RSA modulus size, Protect data until year. 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 is RSA-4096 not worth twice RSA-2048?

RSA-4096 only lifts the strength from 112 to roughly 140 bits while making private-key operations about 6–7× slower. If you need more than 128-bit strength, ECC P-384 or a post-quantum algorithm is a far better trade than a larger RSA modulus.

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ECC Security Level Calculator AES Key Strength Calculator Digital Signature Size Calculator