I have 2 incredible sites that get it almost perfect!!
First, if you are putting down less than 20%, you're going to need to figure out your PMI. For that go here:
Then go to:
You plug in the total loan amount, the amount down, the interest rate, taxes (you can find out what they are in your area online, and you insurance.
When it comes up with the total, subtract the $80 PMI and add the one you go from the first website. VIOLA! You'll be darn close to your payment!! (especially if you've called your insurance company and have a figure for that!)
Taxes: We add our county and city taxes together and know it 1.08 per $100 (we also confirmed with our Ryan Rep who can look it up).
Take the value of the loan/total amount before down payment divide by 100 then multiply by the taxes.
example: 205,000 / 100 * 1.08 = 2214 in taxes a year. (just picked a number)