hello the calculation of makes the following FORM = (DS-DI) * H / 2 ie (14cm-6.5cm) * 4.5 / 2 = 21,375 THIS QUANTITY must be squared and will give approximately the amount of power of the toroid = (21,375) * (21,375) = 456,89 ES TO SAY 457 watts so that toroid if it is used for 400w, the rest will be to calculate the output voltage and the gauge of the AWG wire in both the primary and secondary windings for the established application. pd = this TOROID IS FOR 457 APROX WATTS