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How many bits of precision does a standard 64-bit double (IEEE 754 binary64) number have?

53 bits, about 15 to 17 significant decimal digits.

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  1. 64 bits
  2. 53 bits ✔
  3. 32 bits

Why: A 64-bit double splits its bits between a sign, an exponent and the fraction. The fraction plus a hidden leading 1 gives 53 bits of precision. The other bits set the size, not the precision.

Huh, didn't know that: Integers above 2 to the 53rd (about 9 quadrillion) can no longer all be stored exactly in a double.

The dad joke
Why did the double get kicked out of the precision club? It only had 53 of 64 members show up.
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References

Answer: 53 bits. A 64-bit double splits its bits between a sign, an exponent and the fraction. The fraction plus a hidden leading 1 gives 53 bits of precision. The other bits set the size, not the precision.

  1. Python docs: Floating-Point Arithmetic, Issues and Limitations docs.python.org/3/tutorial/floatingpoint.html
    IEEE 754 binary64 values contain 53 bits of precision
    Checked 2026-10-10.

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