How can I figure the percentage of increased value of a home 10 years old?

I'm not sure if you mean you want to estimate the current value based
on this percentage increase... or if you want to calculate the
percentage increase based on the current value.

If you have the origional purchase price and the current market value
(an appraised value or a reasonable estimated value) then:
% Increase = (Current Price - Origional Price) / Origional Price

If you do not have the current market value and want to estimate it
based on the % increase over 10 years then you will need to do market
analysis.  See how much other homes around your home increased in
value over the past 10 years to find a rough estimate.

Check out zillow.com

Enter the address of the house.  You will get an photo map of the
area.  Click on the property.   After a while you will get more
information, including a chart showing 12 month price moves.  You can
click on 10 years, enter the code number shown on the new screen, and
then you will get a ten year review of price changes.

There are some questions about the sources and validity of this price
information, but perhaps it will help you.

Go to www.mmamortgage.com/HomePriceIndex.aspx. You will need to know
the month and year the property was built or last purchased and the
purchase price at that time. After you enter the required information
the site will calculate the current value based on information from
data on property values nationwide in a database maintained by the
Federal Home Loan Mortgage Corporation (Freddie Mac). It will also
provide a chart showing the quarterly increase from the last purchase
date until now.

You determine the percentage of increased value by subtracting the old
purchase price from the current value. You then divide that result by
the old purchase price. The result is your percent of increased value.

Example:

Bought in May, 1996, for $100,000. Calculated current value is
$150,000. $150,000-100,000=$50,000. $50,000 divided by $100,000= 0.50
or 50%, which is your percent of increased value.

Your porperty must be in one of the 160 largest metropolitan areas in
the nation for this to work because Freddie Mac does not maintain data
on smaller metro areas.