Most classical liberals use a combination of utilitarianism and natural rights in their philosophical framework. Some, like Jeremy Bentham, reject rights altogether in favor of a purely “greatest good for the greatest number” approach. This is all well and good, except for the fact that it is impossible to calculate a person’s utility function.
Even if you could, you can’t meaningfully compare different people’s utility functions to each other. You can make ordinal comparisons (order of preference), but not cardinal (strength of preference) comparisons. This throws a rather large wrench into the pure utilitarian enterprise.
Bishop Butler expressed a similar concern when he said that although God is probably a utilitarian, it is better that men not be, for they are likely to commit serious errors in calculating what will promote the greatest happiness for the greatest number.