This calculator determines the RMS value of a trapezoidal waveform. Figure 1 shows the waveform and its parameters. All you need to do is to input the time values t1, t2, t3, the signal period T and the signal amplitude Vp. Each time has to have the same units. If you set t1 in microseconds, as an example, all the other time values, t2, t3 and T have to have the same units, microseconds. Vp is a voltage level, in Volts, but the calculator can also be used for currents.
If you would like to know the math behind this calculator read my post MasteringElectronicsDesign.com:The RMS Value of a Trapezoidal Waveform – Part 1.