Security

API Key Entropy Calculator

Estimate the entropy of an API key format from its length and character set size.

Last reviewed: April 30, 2026Free toolMethodology

API Key Entropy Calculator

These fields start with sample inputs. Keep them or replace them, then run the tool to show a fresh result.

Number fields accept plain values and common formatted input such as 250000, 250,000, or 1,234.56.

Result

Calculating the sample result.

Why it matters

Many teams define custom API key formats, and entropy math helps check whether the design provides enough random search space.

When to use

  • Reviewing custom key designs
  • Comparing hex, Base64URL, and alphanumeric key formats
  • Checking whether a proposed format is too short

Inputs & Outputs

Inputs

  • Key length is the number of characters in the key.
  • Character set size is the number of possible symbols used in each position.

Outputs

  • Entropy bits estimate the search space complexity of the format.
  • Combination count shows the total number of possible keys under the assumptions entered.

Entropy estimation method

The tool multiplies key length by the base-2 logarithm of the character set size to estimate total entropy bits.

Entropy bits = key length x log2(character set size)

Worked example

1

Key format review

A platform team wants to evaluate a 32-character Base64URL key format using a 64-character alphabet.

Inputs

  • Length: 32
  • Character set size: 64

Steps

  • Entropy = 32 x log2(64) = 32 x 6

Result

  • The estimated entropy is 192 bits.

Edge cases & caveats

  • The estimate assumes the key is generated randomly rather than chosen or derived predictably.
  • Prefixing keys with static identifiers reduces the random portion and should be accounted for separately.

Frequently Asked Questions

Is this different from the password entropy calculator?

The math is the same, but API keys are more often generated randomly, so the assumptions are usually stronger.

Do visible prefixes change the entropy?

Yes. Any static prefix reduces the random part of the key and should be excluded from the random length calculation.

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