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RSA Key Size Calculator

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

About RSA Key Size Calculator

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

Worked example

With the default values below, RSA Key Size Calculator returns Equivalent Security Strength of 112. Change any input above and every figure here is recalculated in your browser.

Inputs used

RSA modulus sizeone of RSA-1024 (legacy), RSA-2048, RSA-3072, RSA-4096, RSA-7680, RSA-15360
RSA-2048
Protect data until yearaccepts 2,024–2,075
2,035

Results

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

Step by step

  1. 1Values used
  2. 2RSA Key Size
  3. 3Equivalent Security Strength= 112 bits
  4. 4ECC Equivalent= ECC 224-bit curve (~112-bit strength)
  5. 5Signature Size= 256 bytes
  6. 6DER Public Key Size= 294 bytes
  7. 7Assessment= Upgrade to RSA-3072 for 128-bit strength

Assumptions and limits

  • The result is derived only from the 2 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 whole numbers, 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 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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