Often times online mortgage calculators only calculate mortgage based on the principal and interest, taxes vary widely in different areas and PMI doesn't always apply. Be sure to always overestimate a few percent so there are no hidden charges or unexpected payments that you can't handle. Unexpected charges are quite common when moving to a new home, many people forget to include parking fees, daily tolls (if travelling over bridges daily to work) and buying ordinary appliances for everyday use necessary for a new home.

When calculating your monthly mortgage payments you have to be sure to include all charges, a mortgage includes:
Principal, Interest, Taxes and PMI

Be careful when buying a Co-Op or condo and be sure you can afford the montly maintenence charges and always remember gas/oil and electricity bills.