555 Timer Calculator

About this calculator

In astable mode, a 555 timer IC charges and discharges a timing capacitor C through two resistors, R1 and R2, producing a continuous square wave whose frequency follows f = 1.44 / ((R1 + 2·R2) × C), with the output high while the capacitor charges through R1+R2 and low while it discharges through R2 alone — which is also why the duty cycle in this basic configuration is always above 50%.

This exact circuit topology, introduced with the original 555 chip in 1971, is a staple of hobby and educational electronics: it drives LED flashers, tone generators, PWM motor controllers, and clock signals for simple digital logic, and its frequency can be tuned just by swapping resistor or capacitor values.

Hobbyists building their first oscillator circuit, electronics students learning RC timing, and makers prototyping blinking-LED or alarm circuits all use this formula to pick R1, R2 and C for a target frequency before soldering anything down.