Coefficient of determination is a measure of the proportion of variance for a predicted outcome. It decides how well the regression model fits the data.

Here is a online tool to determine this. You just have to enter the X series and Y series in the block provided and get the coefficient of determination value instantly.
Lets see how to determine the coefficient of determination:
Step 1 : Read the problem and note down the X and Y series. Find the value of XY, X2 and Y2 and list out the values of $\sum$ X, $\sum$ Y, $\sum$ XY, $\sum {X^2}$, $\sum{Y^2}$.

Step 2 : The correlation is given by 
Correlation (r) = $\frac{n \sum{XY} - \sum{X} \sum{Y}}{\sqrt{(n \sum{X^2} - (\sum X)^{2}) (n \sum {Y^2} - (\sum Y)^2)}}$
Substitute the values in above formula and get the correlation r.

Step 3 : Now determine the coefficient of correlation (r2) by squaring the value of r.