555 Timer Calculator

This free 555 timer calculator works out the frequency, period, duty cycle and high and low times of a 555 astable oscillator from R1, R2 and C, and the pulse width of a 555 monostable (one-shot). In design mode, enter a target frequency and duty cycle and it picks standard E24 resistor values for you. It works for the NE555, LM555 and CMOS TLC555 / 7555, and for the dual 556.

555 timer formulas

Astable: f = 1 ÷ (0.693 × (R1 + 2 R2) × C) ≈ 1.44 ÷ ((R1 + 2 R2) × C)
t_high = 0.693 × (R1 + R2) × C · t_low = 0.693 × R2 × C
Duty cycle = (R1 + R2) ÷ (R1 + 2 R2)
Design: R2 = (1 − D) ÷ (0.693 f C) · R1 = (2D − 1) ÷ (0.693 f C)
Monostable: T = 1.1 × R × C

Worked example: LED flasher

With R1 = 1 kΩ, R2 = 68 kΩ and C = 10 µF, f = 1.44 ÷ ((1k + 136k) × 10 µF) = 1.05 Hz — about one flash per second — with a duty cycle of (1k + 68k) ÷ 137k = 50.4%. For an audible 1 kHz tone at 60% duty with a 100 nF capacitor, design mode gives R1 = 3 kΩ and R2 = 5.6 kΩ (1,016 Hz).

555 astable frequency chart (R1 = 1 kΩ)

Capacitor	R2 = 1 kΩ	R2 = 10 kΩ	R2 = 100 kΩ
10 nF	48.1 kHz	6.87 kHz	718 Hz
100 nF	4.81 kHz	687 Hz	71.8 Hz
1 µF	481 Hz	68.7 Hz	7.18 Hz
10 µF	48.1 Hz	6.87 Hz	718 mHz

555 monostable pulse width chart

Capacitor	R = 10 kΩ	R = 100 kΩ	R = 1 MΩ
1 µF	11 ms	110 ms	1.1 s
10 µF	110 ms	1.1 s	11 s
100 µF	1.1 s	11 s	110 s

555 pinout and tips

Pins: 1 GND, 2 Trigger, 3 Output, 4 Reset (tie to Vcc), 5 Control (10 nF to GND), 6 Threshold, 7 Discharge, 8 Vcc.
Astable wiring: R1 from Vcc to pin 7, R2 from pin 7 to pins 6 and 2, C from pins 6/2 to ground.
Timing capacitors: use film or C0G ceramic for stable timing; electrolytics drift and leak. Decode capacitor markings with the capacitor code calculator.
Decouple the supply with 100 nF close to pin 8 — the bipolar 555 draws a current spike every time it switches.
Need a filter rather than an oscillator? Use the RC filter calculator.

Frequently asked questions

What is the 555 timer frequency formula?

In astable mode, f = 1.44 ÷ ((R1 + 2 × R2) × C). The output is high for 0.693 × (R1 + R2) × C and low for 0.693 × R2 × C, so the duty cycle is (R1 + R2) ÷ (R1 + 2 × R2). The constant 0.693 is ln 2, because the capacitor charges and discharges between 1/3 and 2/3 of the supply.

How do I make a 555 blink an LED once per second?

For 1 Hz with about 60% duty and a 10 µF capacitor, use R1 = 30 kΩ and R2 = 56 kΩ. That gives 1.02 Hz at 61% duty. Use the design mode above for any other frequency.

How do I get a 50% duty cycle from a 555?

In the standard astable circuit the duty cycle is always above 50%, because the capacitor charges through R1 + R2 but discharges through R2 only. For 50%, add a diode across R2 (anode at pin 7) and make R1 = R2, or use the CMOS 7555 with a single resistor from the output to the capacitor.

What is the 555 monostable formula?

In monostable (one-shot) mode the output pulse lasts T = 1.1 × R × C after a trigger. For example, 100 kΩ and 10 µF give 1.1 s. The 1.1 is ln 3, because the capacitor charges from 0 to 2/3 of the supply.

What is the maximum frequency of a 555 timer?

The bipolar NE555 is reliable up to roughly 100 kHz; CMOS versions such as the TLC555 or ICM7555 reach 1–2 MHz. Keep R1 at 1 kΩ or more so the discharge transistor is not overloaded, and avoid very large resistors (above about 10 MΩ), where leakage makes timing unpredictable.