Ultra wideband pulse, which is the essential component of UWB impulse radio, is widely adopted as the derivative of Gaussian pulse due to the relatively low generation complexity. This paper makes an overall analysis on the capacity of M-ary PAM impulse radio employing derivatives of Gaussian pulse up to tenth order with pulse width from 0.1 ns to 1 ns subject to FCC indoor and outdoor spectral masks respectively. For each pulse, the maximum transmission power subject to FCC spectral masks is first calculated, then link budget is made to estimate the capacity in terms of the maximum transmission range corresponding to prescribed signal to noise ratio, data rate and receiver bandwidth, based on which the influence of modulation scheme, receiver bandwidth, and pulse parameters on the capacity is analyzed. The presented results can facilitate system designers to enhance the capacity of M-ary PAM impulse radio subject to FCC spectral masks.