555 Astable Timer Calculator
Find frequency, duty cycle, and high and low times for a standard 555 oscillator.
f = 1 / [ln(2) × (R₁ + 2R₂) × C]Calculations stay in your browser. Share links contain the input values you choose to share.
How to use this calculator
- Enter the timing resistor R1 between VCC and discharge pin 7.
- Enter R2 between pin 7 and the joined threshold/trigger node.
- Enter the timing capacitor from the joined pins 2/6 to ground.
- Read the frequency, period, duty cycle, and output HIGH/LOW times.
A quick worked example
With R1 = 10 kΩ, R2 = 100 kΩ, and C = 10 µF, the frequency is about 0.687 Hz. HIGH time is 762.46 ms, LOW time is 693.15 ms, and output HIGH duty cycle is 52.38%.
A few useful answers
Can this circuit produce a duty cycle below 50%?
Not in the standard two-resistor configuration modeled here. Its HIGH duty cycle is (R1 + R2) / (R1 + 2R2), which is above 50% for positive resistors. A diode-assisted or different circuit needs different equations.
Why do some calculators use 1.44 or 0.693?
Those are rounded constants. This tool uses ln(2) = 0.693147… and its reciprocal 1.442695… to keep the timing and frequency outputs consistent.
Will the physical circuit match exactly?
Real timing depends on component tolerances, capacitor leakage, discharge behavior, and the specific 555 variant. The waveform is a normalized timing illustration; it does not predict output voltage or edge speed.
Model & assumptions: Standard 555 astable connection; ideal thresholds of ⅓ and ⅔ VCC. No diode modifications. Supply voltage, output loading, leakage, and device speed limits are not simulated.
Reference: TI xx555 datasheet, application information ↗. Diagram shows timing connections; pin 5 bypass and supply decoupling omitted.
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