555 Timer Calculator
Calculate frequency and duty cycle for a 555 timer in astable mode.
About 555 Timer Calculator
The capacitor charges through R1 and R2 but discharges only through R2, so the high time always exceeds the low time. Duty cycle can therefore never reach 50 per cent in the basic astable configuration. Achieving a true 50 per cent duty cycle requires a diode across R2 so charge and discharge paths have equal resistance.
Formula
555 Timer
f = 1.44 ÷ ((R1 + 2R2) C); duty cycle = (R1 + R2) ÷ (R1 + 2R2)
- 0.693
- Natural logarithm of 2, from the one-third to two-thirds supply swing
Worked example
With the default values below, 555 Timer Calculator returns Output Frequency of 13.8750. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Resistor R1minimum 0.01 ohm
- 10,000 ohm
- Resistor R2minimum 0.01 ohm
- 47,000 ohm
- Timing capacitorminimum 0 F
- 0.000001 F
Results
- Output Frequency
- 13.8750 Hz
- Duty Cycle
- 54.808%
- Output High Time
- 39.50100 ms
- Output Low Time
- 32.57100 ms
- Period
- 72.07200 ms
Step by step
- 1Values used
- 2555 Timer
- 3Output Frequency= 13.8750 Hz
- 4Duty Cycle= 54.808
- 5Output High Time= 39.50100 ms
- 6Output Low Time= 32.57100 ms
- 7Period= 72.07200 ms
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is 555 Timer calculated?
f = 1.44 ÷ ((R1 + 2R2) C); duty cycle = (R1 + R2) ÷ (R1 + 2R2) The capacitor charges through R1 and R2 but discharges only through R2, so the high time always exceeds the low time. Duty cycle can therefore never reach 50 per cent in the basic astable configuration.
Why does 555 Timer matter?
Achieving a true 50 per cent duty cycle requires a diode across R2 so charge and discharge paths have equal resistance.
What values do I need to enter?
This calculator takes 3 inputs: Resistor R1, Resistor R2, Timing capacitor. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.