Mortgage calculators found on the Internet are generally reliable and accurate for determining the principal and interest portion on a monthly mortgage payment. However, because P&I is just one of several parts of the mortgage equation, it can be a bit more challenging to determine the total monthly payment.
There are several parts to the typical mortgage payment: principal and interest, private mortgage insurance and property taxes. Mortgage bankers typically refer to these components as PITI (principal, interest, taxes, insurance). With most online mortgage calculators, it's fairly easy to determine the P&I payment using current interest rates, but that only gives part of the total monthly payment. Private mortgage insurance (PMI) usually accounts for about 0.5 percent of the loan total, and it is another expense one must enter into the calculator.
Most online calculators figure PMI based on the down payment. The last part of the PITI equation is the property taxes. Depending on the home's location, property taxes can be a significant part of the monthly payment, so it's important to obtain an accurate estimate. Once property taxes are entered into the mortgage calculator, one has a reasonably accurate idea of one's total monthly mortgage payment.
One last thing to investigate when buying a home is whether it's located in an area governed by a home owner's association, or HOA. Most HOAs require home owners to pay an annual amount for upkeep of common areas in the neighborhood, and this expense is often not calculated by online mortgage calculators. A mortgage banker or realtor can help determine if the property is subject to an HOA.