555 Monostable Timer Calculator
Calculate one-shot pulse duration, or find a timing resistor or capacitor.
t = ln(3) × R × C ≈ 1.099 × R × CCalculations stay in your browser. Share links contain the input values you choose to share.
How to use this calculator
- Choose pulse duration, timing resistor, or timing capacitor as the unknown.
- Enter the two known values and select their units.
- Read the ideal timing result and the corresponding component values.
- Select available parts and recalculate with their nominal values.
A quick worked example
A 100 kΩ timing resistor with a 10 µF capacitor produces an ideal pulse of 1.0986 seconds, often rounded to 1.1 seconds. For a 1 second target with 10 µF, the ideal resistor is approximately 91.024 kΩ.
A few useful answers
What starts a monostable pulse?
A short negative-going pulse at trigger pin 2 starts the timing interval. RESET must remain HIGH and the trigger must return HIGH before the interval ends.
Why use 1.099 instead of 1.1?
The ideal capacitor charges from zero to two-thirds of the supply. Solving the charging equation gives ln(3) × RC; 1.1 is the customary rounded coefficient.
Can it produce repeated pulses by itself?
A monostable produces one timed pulse per valid trigger. For a continuous output waveform, use the 555 astable calculator.
Model & assumptions: Ideal 555 monostable with initially discharged timing capacitor and threshold at ⅔ VCC. Trigger duration, reset behavior, tolerances, capacitor leakage, and device 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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