Inverse distance weighting (IDW) is a simple method for interpolation, a process of assigning values to unknown points by using values from known points. A simple IDW weighting factor is , where w(d) is the weighting factor applied to a known value, d is the distance from the known value to the unknown value, and p is a user-selected power factor. Here weight decreases as distance increases from the interpolated points. Greater values of p assign greater influence to values closest to the interpolated point.

A general form of interpolating a value using IDW is where Z is the value of the interpolated point, Z_i is a known value, and N is the total number of known points used in interpolation.

See also linear interpolation.