The answer of enthusiastic netizens is not very accurate. The formulation of "calculation rules" is not common in the patent field, and it is generally called "rules and methods of intellectual activities". The rules and methods of intellectual activities can't be patented, but if it involves algorithm inventions related to computer software or programs, it is more likely to be authorized, which needs specific case analysis and proper writing. Regarding the openness of algorithm patents, a simple standard is that as an ordinary programmer, if the description you provide can be converted into line by line instructions, ok, otherwise, you need to continue to enrich, and usually describe the flow chart related to the algorithm clearly.
Off-topic, "can't apply for a patent" is also inaccurate. It is your own business to apply for a patent, and whether it is approved or not is up to the State Patent Office.