This tool calculates output parameters of a transformerless power supply where a capacitor is used to drop the voltage. The formulas and information come from Microchip application note AN954, written by Reston Condit.

The capacitor C1 must be X2 type capacitor.

Warning: transformerless power supplies provide no isolation from mains power, and are very dangerous. You must use isolation transformer whenever you want to connect anything, such as scope, DVM, or any other external equipment to the circuit. Even with isolation transformer high voltages are present, be very careful.