555 Timer Astable Calculator
Enter R1, R2 and the timing capacitor to size a 555 timer running as a free-running (astable) oscillator. The calculator returns the frequency, the period, the high and low output times and the duty cycle, each shown in a readable unit. A classic NE555 astable always sits above a 50 percent duty cycle because R1 is in the charge path but not the discharge path.
How to use the 555 timer calculator
- Type resistor R1 in the R1 field and pick its unit: ohm, k ohm or M ohm.
- Type resistor R2 and choose its unit the same way.
- Type the timing capacitor C and pick F, uF, nF or pF.
- Read the frequency, period, duty cycle and high and low times in the result cards.
- Click Copy to grab all five values as text for your notes or schematic.
Examples
1 k, 10 k and 1 uF (the classic values)
R1 = 1 k ohm, R2 = 10 k ohm, C = 1 uF
Frequency 68.5714 Hz, period 14.5833 ms, high 7.623 ms, low 6.93 ms, duty 52.381%
Slow LED blinker near 1 Hz
R1 = 1 k ohm, R2 = 68 k ohm, C = 10 uF
Frequency 1.0511 Hz, duty 50.365%, high 478.17 ms, low 471.24 ms
Audio tone at a few kilohertz
R1 = 1 k ohm, R2 = 1 k ohm, C = 100 nF
Frequency 4.8 kHz, period 208.3333 us, duty 66.6667%
Frequently asked questions
What is the 555 astable frequency formula?
Frequency equals 1.44 / ((R1 + 2 R2) C), with R in ohms and C in farads. For R1 = 1 k, R2 = 10 k and C = 1 uF the calculator returns 68.5714 Hz. The 1.44 constant is 1 divided by the natural log of 2, which governs how the capacitor charges and discharges.
How is the duty cycle calculated?
Duty cycle is (R1 + R2) / (R1 + 2 R2) times 100. The output is high while the capacitor charges through R1 and R2 in series, and low while it discharges through R2 alone, so the two resistors weight the ratio.
Why is the duty cycle always above 50 percent?
In the standard circuit the charge path runs through R1 and R2 while the discharge path uses only R2, so the high time is always longer than the low time. That pins the duty cycle above 50 percent. Adding a diode across R2 lets the charge bypass R2 and brings the duty near 50 percent.
What are the high and low times?
High time is 0.693 (R1 + R2) C and low time is 0.693 R2 C, where 0.693 is the natural log of 2. They sum to the full period. In the classic example the high time is 7.623 ms and the low time is 6.93 ms, adding to 14.5833 ms.
What units can I enter for R and C?
Resistors accept ohm, k ohm or M ohm, and the capacitor accepts F, uF, nF or pF. The tool converts each value to base ohms and farads before applying the formula, so you can mix scales freely, for example a k ohm resistor with a nF capacitor.
How do I pick R and C for a target frequency?
Start by choosing a capacitor: larger caps suit slow blinkers and smaller caps suit audio and higher frequencies. Then solve R1 + 2 R2 = 1.44 / (frequency times C) for the resistor pair. Keep R1 and R2 between roughly 1 k and 1 M to stay within the chip's comfortable range.
How close will a real circuit be to these numbers?
Treat the result as a design target, not a guarantee. Standard resistors carry 1 to 5 percent tolerance and electrolytic capacitors can drift 10 to 20 percent, so the built frequency may differ noticeably. The formula also ignores the small internal timing offsets of the chip.
Does this cover monostable (one-shot) mode?
No. This tool models astable oscillator mode only. A monostable pulse width uses a different formula, 1.1 R C with a single timing resistor, and produces one pulse per trigger rather than a continuous waveform.
Is anything sent to a server?
No. Every calculation runs in your browser, so the values you type stay on your device and nothing is uploaded or stored.
Learn more
- Designing a 555 astable oscillator
How the 555 charges and discharges its capacitor, how to choose R1, R2 and C, and how to reach a 50 percent duty cycle.
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