555 Timer Calculator
Enter your R1, R2, and C values for a 555 timer in astable mode to find the output frequency and duty cycle.
What This Calculator Does
The 555 timer IC in astable mode acts as a free-running oscillator, continuously switching its output high and low to generate a square-wave pulse train. This calculator finds the output frequency, duty cycle, and high/low pulse times from the two timing resistors and the timing capacitor in a standard astable configuration.
The Formula
The standard astable 555 formulas are f = 1.44 ÷ ((R1 + 2R2) × C) for frequency, thigh = 0.693 × (R1 + R2) × C for the time the output stays high, and tlow = 0.693 × R2 × C for the time it stays low, where R1 and R2 are in ohms and C is in farads. Duty cycle is thigh ÷ (thigh + tlow) × 100%.
- Duty cycle in this classic configuration is always above 50%, since thigh includes both R1 and R2 while tlow only includes R2.
- To get closer to 50%, many designs add a diode across R2 or use a modified circuit topology.
These are the standard datasheet formulas for the classic bipolar 555 timer in astable mode, used for general electronics education and design planning; actual behavior can vary slightly with supply voltage and specific IC variant (CMOS vs bipolar).