NE555 Astable Calculator — Frequency, Period & Duty Cycle
The NE555 timer in astable mode produces a continuous square wave whose frequency and duty cycle depend on two resistors (Ra, Rb) and one capacitor (C). Enter the component values to find the output frequency, period, and the high and low times of each cycle.
kΩ
kΩ
µF
Free-running oscillation frequency of the astable circuit
- 1
High time tH
0.693 × (Ra + Rb) × C = 0.693 × (10 + 10) kΩ × 0.1 µF = 1.386 ms - 2
Low time tL
0.693 × Rb × C = 0.693 × 10 kΩ × 0.1 µF = 0.693 ms - 3
Period T
tH + tL = 1.386 + 0.693 ms = 2.079 ms - 4
Output frequency
1 ÷ T = 481f = 1/T; displayed in kHz when ≥ 1000 Hz.
How does this calculator work?
Wire Ra from VCC to pin 7, Rb from pin 7 to pins 2/6, and C from pins 2/6 to GND. The 555 oscillates at f = 1.44/((Ra+2Rb)·C). High time tH = 0.693(Ra+Rb)C, low time tL = 0.693·Rb·C. Duty cycle is always above 50 % — add a bypass diode to achieve 50 %.
Formula
How this is calculated
In astable mode the timing capacitor alternately charges through Ra+Rb and discharges through Rb alone. The internal comparators flip the output at two-thirds (charging threshold) and one-third (discharging trigger) of the supply voltage. Because these thresholds never require an external trigger pulse, the circuit oscillates continuously without any human input.
The charge time — during which the output is HIGH — is tH = 0.693 × (Ra + Rb) × C. The discharge time — output LOW — is tL = 0.693 × Rb × C. Adding them gives the total period T = tH + tL, and the oscillation frequency f = 1/T ≈ 1.44/((Ra + 2Rb)·C). The 0.693 factor is ln(2): the fraction of an RC time constant needed to charge from one-third to two-thirds of VCC (or to discharge in the opposite direction).
Because tH always includes the Ra path and tL does not, the duty cycle (Ra+Rb)/(Ra+2Rb) is always above 50 % in the standard configuration. To reach a 50 % duty cycle, add a steering diode in parallel with Rb (cathode toward pin 7) so the capacitor charges only through Ra, making tH ≈ 0.693·Ra·C. All values assume an ideal 555 and ignore output-current limits, chip propagation delay, and component tolerances — for precision above ~200 kHz, use a CMOS variant such as the TLC555.
Frequently asked questions
Not in the standard two-resistor wiring — tH always passes through Ra+Rb while tL only uses Rb, so the duty cycle is locked above 50 %. The common fix is to add a 1N4148 diode in parallel with Rb (cathode toward pin 7): the capacitor then charges only through Ra, making tH ≈ 0.693·Ra·C and tL ≈ 0.693·Rb·C, so duty ≈ Ra/(Ra+Rb). Set Ra = Rb for a 50 % output.
Bipolar NE555 chips are typically specified up to about 500 kHz – 1 MHz; keeping Ra ≥ 1 kΩ protects the internal discharge transistor. CMOS versions such as the TLC555 or LMC555 run at lower supply currents and can reach similar or slightly higher frequencies with better waveform symmetry at the upper end.
A capacitor charging through a resistor follows an exponential curve toward the supply voltage. The time to rise from one-third to two-thirds of VCC (or to fall from two-thirds back to one-third) equals ln(2) × R × C ≈ 0.693 × R × C — the same factor that appears in radioactive half-life and RC charging.
Also known as
TG we-Calculate Editorial Team. (2026). NE555 Astable Calculator — Frequency, Period & Duty Cycle [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/ne555-astable-calculator
TG we-Calculate Editorial Team. "NE555 Astable Calculator — Frequency, Period & Duty Cycle." TG we-Calculate. 2026. https://we-calculate.com/calculator/ne555-astable-calculator.
TG we-Calculate Editorial Team, "NE555 Astable Calculator — Frequency, Period & Duty Cycle," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/ne555-astable-calculator
@misc{wecalculate_ne555_astable_calculator, title = {NE555 Astable Calculator — Frequency, Period & Duty Cycle}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/ne555-astable-calculator}}, year = {2026}, note = {TG we-Calculate} }
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