⏱️

555 Timer Calculator

Calculate frequency, period, duty cycle and pulse width from R and C for an NE555 astable/monostable circuit.

Mode

kΩ

µF

Frequency

6.857 Hz

Period

145.8 ms

Duty cycle

52.38 %

High time

76.23 ms

Low time

69.3 ms

Disclaimer: This calculator is provided for general informational and educational purposes only, on an “as is” basis and without any warranty of accuracy or fitness for a particular purpose. Results may contain errors — always verify independently before relying on them in real designs. PartAndStock accepts no liability for any loss or damage arising from use of this tool, including when embedded on third-party sites.

Find NE555 suppliers

Learn the theory: What Is a 555 Timer?

How to use

1Select the mode: astable (continuous oscillation) or monostable (one-shot).
2For astable, enter R1, R2 (kΩ) and C (µF).
3Frequency, period and duty cycle are calculated instantly.
4In monostable mode, enter R and C to get the pulse width (T).

How it works

Find the timing parameters of the 555 timer IC in astable (oscillator) and monostable (one-shot) modes from your R and C values.

Astable: f = 1.44 / ((R1 + 2·R2)·C)  ·  duty = (R1+R2)/(R1+2·R2)  ·  Monostable: T = 1.1·R·C

Astable vs monostable

In astable mode the 555 oscillates continuously and produces a square wave at the output; its frequency is set by R1, R2 and C (LED blinking, tone generation, clock pulses). In monostable mode the circuit rests until a trigger pulse arrives, then the output goes high once for T = 1.1·R·C and returns low (button delay, one-shot timer).

Duty cycle and component choice

In a standard astable circuit the charge path is R1+R2 while the discharge path is only R2, so the duty cycle is always above 50%. To reach 50% or below, a diode is added in parallel with R2. Use a low-tolerance film/ceramic capacitor for timing; electrolytic capacitors' tolerance and leakage shift the frequency.

Worked examples

R1=10 kΩ, R2=100 kΩ, C=1 µF → f ≈ 6.86 Hz, duty ≈ 52%
R1=1 kΩ, R2=1 kΩ, C=0.1 µF → f ≈ 4.8 kHz
Monostable: R=100 kΩ, C=1 µF → pulse ≈ 110 ms

555 Formulas

Quantity	Formula
Frequency (astable)	f = 1.44 / ((R1 + 2·R2)·C)
Period	T = 0.693 · (R1 + 2·R2) · C
High time	t_H = 0.693 · (R1 + R2) · C
Low time	t_L = 0.693 · R2 · C
Duty cycle	(R1 + R2) / (R1 + 2·R2)
Monostable pulse	T = 1.1 · R · C

Frequently Asked Questions

How is the 555 astable frequency calculated?+

f = 1.44 / ((R1 + 2·R2) × C), with R in ohms and C in farads. For example R1=10 kΩ, R2=100 kΩ, C=1 µF gives f ≈ 6.86 Hz.

What is the difference between astable and monostable?+

Astable oscillates continuously and produces a square wave; monostable gives a single pulse of T = 1.1·R·C for each trigger.

Why can't the duty cycle go below 50%?+

In the standard circuit the charge path is R1+R2 and the discharge path only R2, so duty is always above 50%. Adding a diode across R2 lets you reach 50% or below.

Which capacitor should I use?+

Use a low-tolerance film or ceramic capacitor for timing. Electrolytic capacitors' high tolerance and leakage can shift the frequency.

What supply voltage does the NE555 need?+

The classic bipolar NE555 typically runs on 4.5–15 V; the CMOS 7555 version works at lower voltage and draws less current.

Explore more articles

What Is a Capacitor?What Is Ohm's Law?AC vs DC Difference

All tools Find NE555 suppliers