About the 555 Timer Calculator
This 555 timer calculator works out the timing of the classic NE555 (and CMOS versions such as the TLC555 and LMC555) from its external resistors and capacitor. In astable mode it gives the output frequency, period, high time, low time and duty cycle of the free-running square wave; in monostable (one-shot) mode it gives the width of the pulse produced each time the chip is triggered.
It is for hobbyists building LED flashers, tone generators, PWM controllers, debounce and delay circuits, and for students checking lab results. Resistances are entered in kilohms and capacitance in microfarads, so times come out directly in milliseconds, and a chart shows the expected output waveform.
In the standard astable circuit the capacitor charges through R1 + R2 and discharges through R2 alone, so the duty cycle is always above 50%; add a diode across R2 if you need less. Results are nominal: capacitor tolerance (often ±10–20%), leakage in electrolytics and the chip’s own threshold accuracy shift real timing by a few percent. Keep R1 above about 1 kΩ to protect the discharge transistor.
With the default inputs, the output frequency is 6.87 Hz. Change any value above to recalculate instantly.
How to use the 555 timer calculator
- 1Choose astable (free-running oscillator) or monostable (one-shot) mode.
- 2Enter R1 and R2 for astable, or R for monostable, in kilohms.
- 3Enter the timing capacitor in microfarads (0.01 µF = 10 nF).
- 4Read the frequency, high/low times and duty cycle, or the pulse width.
- 5Adjust R or C until the timing matches your target, then pick standard values.
Formula and method
The 555 compares the capacitor voltage with internal thresholds at 1/3 and 2/3 of the supply. In astable mode the capacitor charges from 1/3 to 2/3 Vcc through R1 + R2, taking ln 2 ≈ 0.693 time constants (the output is high), then discharges from 2/3 to 1/3 Vcc through R2 alone (the output is low). Adding the two gives the period; frequency is its reciprocal, which the datasheet writes as 1.44/((R1 + 2R2)C).
In monostable mode a trigger starts the capacitor charging from 0 V through R, and the output stays high until it reaches 2/3 Vcc, which takes ln 3 ≈ 1.0986 time constants — the datasheet rounds this to 1.1RC. Because both thresholds are fractions of Vcc, the timing does not depend on supply voltage. With R in kΩ and C in µF, R × C is in milliseconds.
- R1, R2
- Astable timing resistors RA and RB (kΩ)
- R
- Monostable timing resistor (kΩ)
- C
- Timing capacitor (µF)
- tH, tL
- Output high and low times (ms)
- f
- Output frequency
- tw
- Monostable pulse width
Worked examples
LED flasher: R1 1 kΩ, R2 10 kΩ, C 10 µF
tH = 0.693 × (1 + 10) kΩ × 10 µF ≈ 76.2 ms and tL = 0.693 × 10 × 10 ≈ 69.3 ms, so T ≈ 145.6 ms and f ≈ 6.87 Hz with a 52.4% duty cycle — a fast, nearly even blink.
Audio tone: R1 1 kΩ, R2 6.8 kΩ, C 10 nF
f = 1.44 ÷ ((1 + 2 × 6.8) kΩ × 10 nF) ≈ 9.88 kHz with a 53.4% duty cycle, a high-pitched tone you can feed to a small speaker through a capacitor.
Monostable delay: 100 kΩ and 10 µF
tw = ln 3 × 100 kΩ × 10 µF ≈ 1,098.6 ms, about 1.1 seconds — the datasheet’s 1.1RC gives 1,100 ms. Triggers need to be spaced more than about 1.1 s apart, so it can fire at most about 0.91 times per second.
Frequently asked questions
How do I calculate 555 timer frequency?+
In astable mode f ≈ 1.44 / ((R1 + 2R2) × C), with R in ohms and C in farads. For R1 = 1 kΩ, R2 = 10 kΩ and C = 10 µF that gives about 6.87 Hz.
How do I get a 50% duty cycle from a 555?+
In the standard circuit duty is (R1 + R2)/(R1 + 2R2), always above 50%. Make R2 much larger than R1 to approach 50%, or put a diode across R2 so the capacitor charges through R1 only and discharges through R2, then set R1 = R2.
What is the 555 monostable pulse width formula?+
tw = 1.1 × R × C (more precisely ln 3 × RC ≈ 1.0986 RC). For example 100 kΩ and 10 µF give a pulse of about 1.1 seconds after each trigger.
Does supply voltage change 555 timing?+
Almost not at all. The capacitor is compared against fixed fractions (1/3 and 2/3) of the supply, so the timing is largely independent of Vcc. Temperature and capacitor tolerance have a bigger effect.
What are the limits on R and C values?+
Keep R1 at least about 1 kΩ so the discharge pin is not overloaded, and total resistance below a few megohms (bipolar 555 threshold current becomes significant). Capacitors from about 100 pF to a few hundred µF work; large electrolytics leak and lengthen timing.