Fixed-rate mortgages include same interest-rate which can be continued at least for first few years. This is how your monthly payments will remain the same without any change in figure. This is a long-term mortgage and thus can be continued even for 30 years.
In this case, ARM can be easily and quickly adjusted and this is one of the leading reasons that this mortgage system has gained the highest popularity in the present age. You can now make proper estimation as well as keep the expenses separately on the basis of this mortgage.