555 timer frequency calculator
Astable (free-running) mode: f = 1.44 / ((R1 + 2×R2) × C). Pick resistor and capacitor units with the dropdowns — results update instantly as you type.
Inputs (astable mode)
Results
Frequency (f)
f = 1.44 / ((R1 + 2R2) × C)
Period (T)
T = 1 / f
Duty cycle (D)
D = (R1 + R2) / (R1 + 2R2)
High time (t_high)
t_high = 0.693 × (R1 + R2) × C
Low time (t_low)
t_low = 0.693 × R2 × C
R1 and R2 are the current values (10 kΩ and 10 kΩ), C is 100 nF. The astable model above assumes the classic 555 timing equations.
About the 555 astable circuit
In astable mode the 555 charges and discharges C through R1 and R2, producing a square wave. Charge goes through R1 + R2, discharge only through R2, so the output is high for longer than it is low — duty cycle always exceeds 50%.
Need parts for the build? Find NE555 and timer ICs or timing capacitors in our catalog.
Frequently asked questions
How do I calculate the frequency of a 555 timer in astable mode?
The tool uses f = 1.44 / ((R1 + 2 × R2) × C), the standard 555 astable timing equation. With the defaults R1 = R2 = 10 kΩ and C = 100 nF the frequency is 1.44 / (30 kΩ × 100 nF) = 480 Hz. Period, duty cycle and the separate high and low times are shown alongside.
Why is the 555 timer duty cycle always above 50%?
In astable mode the output charges the timing capacitor through R1 + R2 but discharges it only through R2, so the high time is always longer than the low time. The calculator uses D = (R1 + R2) / (R1 + 2 × R2), which is always greater than 50%.
Can I enter R1 and R2 in kΩ and C in nF directly?
Yes. The unit dropdowns cover Ω, kΩ and MΩ for the resistors and pF, nF, µF and mF for the capacitor, and engineering suffixes such as 1k or 100n are also parsed straight from the input.
Values are reference estimates using the standard 555 timing equations. Real part tolerances and supply voltage affect the actual frequency. ICBOMS provides this tool for reference only.