Application note from Analog Devices to calculate for inclination angle from a single, dual or triple-axis accelerometers. Link here (PDF)
One common method for determining the tilt or inclination of a system is to integrate the output of a gyroscope. Although this method is straightforward, error associated with null bias stability can quickly compound as the integration period is increased, causing an apparent rotation even when the device is stationary.
Inclination sensing uses the gravity vector and its projection on the axes of the accelerometer to determine the tilt angle. Because gravity is a dc acceleration, any forces that result in an additional dc acceleration corrupt the output signal and result in an incorrect calculation. Sources of dc acceleration include the period of time when a vehicle is accelerating at a constant rate and rotating devices that induce a centripetal acceleration on the accelerometer. In addition, rotating an accelerometer through gravity causes an apparent ac acceleration as the projection of gravity on the axes of interest changes.