SWI-Prolog -- Manual

Documentation
- Reference manual
  - Built-in Predicates
    - Arithmetic
      - General purpose arithmetic
        
        >/2
        
        </2
        
        =</2
        
        >=/2
        
        =\=/2
        
        =:=/2
        
        is/2
        
        Arithmetic types
        
        Rational number examples
        
        Rational numbers or floats
        
        IEEE 754 floating point arithmetic
        
        Floating point arithmetic precision
        
        Arithmetic Functions
- Packages

4.27.2.5 Floating point arithmetic precision

SWI-Prolog represents floats using the C double type. On virtually all modern hardware this implies it uses 64-bit IEEE 754 floating point numbers. See also section 4.27.2.4. All floating point arithmetic is performed using C. Different C compilers, different C math libraries and different hardware floating point support may yield different results for the same expression on different instances of SWI-Prolog.