SWI-Prolog uses rational number arithmetic if the Prolog flag
prefer_rationals
is `true`

and if this is defined for a function on the given
operants. This results in perfectly precise answers. Unfortunately
rational numbers can get really large and, if a precise answer is not
needed, a big waste of memory and CPU time. In such cases one should use
floating point arithmetic. The Prolog flag
max_rational_size
provides a *tripwire* to detect cases where rational numbers get
big and react on these events.

Floating point arithmetic can be forced by forcing a float into an argument at any point, i.e., the result of a function with at least one float is always float except for the float-to-integer rounding and truncating functions such as round/1, rational/1 or float_integer_part/1.

Float arithmetic is typically forced by using a floating point constant as initial value or operant. Alternatively, the float/1 fun