555 Timer Calculator
Frequency4.800 Hz
Duty Cycle66.67 %
Period0.208333 s
High Time0.138600 s
Low Time0.069300 s
This 555 Timer Calculator sizes the output of the classic NE555 chip wired as an astable (free-running) oscillator. Enter the two timing resistors R1 and R2 and the timing capacitor C, and it returns the oscillation frequency, period, duty cycle, and the separate high and low pulse durations. It is the fastest way to pick parts for a square-wave clock, LED blinker, or tone generator.
Formula
f = 1.44 / ((R1 + 2·R2)·C); duty = (R1 + R2) / (R1 + 2·R2); tHigh = 0.693·(R1 + R2)·C; tLow = 0.693·R2·C
- R1
- Resistor between Vcc and the discharge/threshold node, in ohms
- R2
- Resistor between discharge and threshold pins, in ohms
- C
- Timing capacitor, in farads
- f
- Output frequency in hertz
How it works
- Enter R1, R2, and C using the unit selectors (ohms/kΩ/MΩ for resistors, F down to pF for the capacitor).
- The calculator applies the standard astable equations: frequency f = 1.44 / ((R1 + 2·R2)·C) and duty cycle = (R1 + R2) / (R1 + 2·R2).
- High time (capacitor charging through R1 + R2) and low time (discharging through R2) are reported separately so you can verify the on/off ratio.
Worked example
A blinker with R1 = 10 kΩ, R2 = 10 kΩ, and C = 10 µF.
- R1 + 2·R2 = 10000 + 20000 = 30000 Ω.
- f = 1.44 / (30000 × 0.00001) = 1.44 / 0.3 = 4.8 Hz.
- Duty = (10000 + 10000) / 30000 = 66.67%.
- tHigh = 0.693 × 20000 × 0.00001 = 0.1386 s; tLow = 0.693 × 10000 × 0.00001 = 0.0693 s.
About 4.8 Hz at a 66.67% duty cycle (0.1386 s high, 0.0693 s low).
Frequently asked questions
- Why is the duty cycle always above 50% in standard astable mode?
- The capacitor charges through R1 + R2 but discharges only through R2, so the high time is always longer than the low time. A duty cycle of exactly 50% requires a diode across R2 or a different topology.
- What capacitor and resistor values give a clean output?
- Keep R1 and R2 between about 1 kΩ and 1 MΩ and the capacitor above roughly 100 pF. Very small resistors draw excess current, while very large ones make timing sensitive to leakage.
- Does the supply voltage change the frequency?
- No. The astable frequency depends only on R1, R2, and C because the comparator thresholds track the supply ratio. Supply voltage affects output drive and current, not timing.