555 Timer Calculator (Astable Mode)
By Santaji GadepatilinUpdated October 27, 2026
The 555 is one of the most popular chips ever made — a simple timer that can blink LEDs, generate square waves and create time delays with just a few resistors and a capacitor. See the 555 timer IC explained for how it works inside.
How to use the 555 timer calculator
Enter R1, R2 and the timing capacitor C. You get the output frequency, the high and low times and the duty cycle. To design for a frequency, enter it with two components and press × on the third.
The formulas (astable)
tH = 0.693 (R₁ + R₂) C · tL = 0.693 R₂ C
f = 1.44 ÷ ((R₁ + 2R₂) C) · D = (R₁ + R₂) ÷ (R₁ + 2R₂)
Example frequencies (R1 = 1 kΩ)
| R2 | C | Frequency | Duty cycle |
|---|---|---|---|
| 10 kΩ | 100 µF | 0.69 Hz | 52% |
| 10 kΩ | 10 µF | 6.86 Hz | 52% |
| 47 kΩ | 1 µF | 15.16 Hz | 51% |
| 10 kΩ | 100 nF | 685.71 Hz | 52% |
| 10 kΩ | 10 nF | 6.86 kHz | 52% |
| 1 kΩ | 10 nF | 48.00 kHz | 67% |
Monostable mode
In one-shot (monostable) mode, the output pulse lasts t = 1.1 × R × C after a trigger. 100 kΩ and 10 µF give about 1.1 s.
555 pin functions
| Pin | Name | Use |
|---|---|---|
| 1 | GND | 0 V |
| 2 | Trigger | Starts timing below ⅓ Vcc |
| 3 | Output | Up to about 200 mA |
| 4 | Reset | Tie to Vcc if unused |
| 5 | Control | 10 nF to ground for stability |
| 6 | Threshold | Ends timing above ⅔ Vcc |
| 7 | Discharge | Discharges the timing capacitor |
| 8 | Vcc | 4.5–16 V (bipolar NE555) |
The CMOS version (7555, TLC555) runs from 2 V and uses far less current.
Frequently asked questions
What is the 555 astable frequency formula?
f = 1.44 ÷ ((R1 + 2R2) × C). With R1 = 1 kΩ, R2 = 10 kΩ and C = 10 µF, the frequency is about 6.86 Hz.
Why can't the duty cycle go below 50%?
The capacitor charges through R1 + R2 but discharges through R2 only, so the high time is always longer. Add a diode across R2 to get duty cycles under 50%.
What is the minimum R1?
About 1 kΩ, to limit the discharge transistor current when it turns on.
How accurate is the 555?
Typically 1–2% plus component tolerances. Use film capacitors rather than electrolytics for stable timing.