How to calculate monthly mortgage payment affordability?

I assume you are looking to calculate how much house you can afford
(both in per-month mortgage payments and in the total value of a home)
based on a number of factors.  Luckily in this age of computers, there
are a number of calculators out there that make it easy to calculate a
mortgage payment affordability based on how much you make, your debts,
the length of the loan, and the loan's term-length.

I've listed several below:

Mortgage 101's calculator "How much can I afford?"
  - has detailed options to play with
  [http://nt.mortgage101.com/partner-scripts/1070.asp?p=mtg101&pw=760]

Lycos "Affordability Calculator"
  - simpler than Mortgage 101's
  [http://mortgages.lycos.com/Mortgage_and_Finance/Calculators/AffordabilityCalc/]


Basically, what lenders (and these calculators) do to calculate your
affordability (e.g. how much you can spend monthly and how much house
you can afford) is calculate your "debt-service ratio".  To do this,
add up all of your monthly debts (including credit card debts, car
payments, student loans, etc.)  The sum of your monthly debts plus
your monthly mortgage payment plus any mortgage taxes, fees, etc.
should add up to *no more* than 40% of your monthly gross (pre-tax)
income.  Additionally, the mortgage and its associated taxes and fees
by itself should add up to no more than 30% of your monthly gross
income.
[more info: http://www.fyilondon.com/londonroyallepage/mortgageinfo.shtml]

Of course, this is just a rule of thumb so that you don't end up with
too little each month to afford food, clothes, random niceities, and
the unfortunate occasional unexpected repair expenses, etc.

I hope this works out for you, and please let me know if I can be of
further assistance.

Take care.

--Joey

The Motley Fool (www.fool.com) also has some very useful calculators
as well as advice on buying a home.  I am about to close on my first
home, and I made extensive use of the tools available on the Motley
Fool.  Hope this helps.