growth rate but not average

[kotlon]

DATE VALUE
1959-01-01 286.6
1959-02-01 287.7
1959-03-01 289.2
1959-04-01 290.1
1959-05-01 292.2
1959-06-01 294.1
1959-07-01 295.2
1959-08-01 296.4
1959-09-01 296.7
1959-10-01 296.5
1959-11-01 297.1
1959-12-01 297.8
1960-01-01 298.2
1960-02-01 298.5
1960-03-01 299.4
1960-04-01 300.1
1960-05-01 300.9
1960-06-01 302.3
1960-07-01 304.1
1960-08-01 306.9
1960-09-01 308.4
1960-10-01 309.5
1960-11-01 310.9
1960-12-01 312.4
1961-01-01 314.1
1961-02-01 316.5

This is my monthy data and it continues up to 2006. I wanna calculate
the growth rate (% annual rate), that is the growth rate from one year
to the next (for example 1959 to 1960, then from 1960 to
1961...etc...up until 2006). NOT the average growth rate for all these
years!
So the growth rate for 1959 to 1960 would be (312.4-297.8)/297.8. My
question is: Is there a faster way to make this calculation or should I
type this formula for every year? (45 times).
Excuse my bad english and thanks in advance

 

[Toppers]

Assuming data is in colums A & B, then in C2:


=(INDEX($B$2:$B$800,ROW()*12)-INDEX($B$2:$B$800,(ROW()-1)*12))/INDEX($B$2:$B$800,(ROW()-1)*12)

and copy down for required number of years

C2 value will be growth for 1959/60
C3 value will be growth for 1960-/61
etc

HTH