C# - Calculating amplitude from logged acceleration data [Using Accelerometer]

Question

I'm developing a small Windows Phone application in C#. This app is recording acceleration data, GPS-location and compass data.

Here my code for fetching acceleration data:

void accelerometer_ReadingChanged(object sender, AccelerometerReadingEventArgs e)
{
    Deployment.Current.Dispatcher.BeginInvoke(() => MyReadingChanged(e));
}

void MyReadingChanged(AccelerometerReadingEventArgs e)
{
    if (accelerometer != null)
    {                        
        xdata = e.X.ToString("0.000");
        ydata = e.Y.ToString("0.000");           
        zdata = e.Z.ToString("0.000");
    }
}

After that another code will save the data to MySQL-Database...

My app is taking 8 values per second. So I want to calculate the amplitude (height) in centimeters [cm] using the data. Normally the formula for calculating the amplitude using acceleration data is:

s = 0,5 * a * t^2 (+ s0)

s: distance in meters - t: time in seconds - a: acceleration im m/s^2

I tried to calculate the amplitude with it, but it doesn't work. There were only wrong values.

So my question:

Which unit or which format has the given output from accelerometer? Is it really meters/second^2 [m/s^2] ? If it isn't, I'd be delighted if you'll send me the right unit, at most the complete formula / way how to calculate. The saved data is completely correct, so I think my formula has some mistakes... I'm using Nokia Lumia 1520.

I hope you'll understand my problem, because of my english-skills. ;)

Thank you in advance!