⏲️555 Timer Calculator

About this tool

Choose astable mode to enter R1, R2 and C and get the frequency, period, high time, low time and duty cycle; monostable mode to get the one-shot pulse width from R and C; or design mode to find R2 for a target frequency. Type values with prefixes such as 1k, 4.7k, 100n or 10u.

How the 555 timer works

The 555, introduced in 1972, is one of the most popular integrated circuits ever made. Inside, two comparators watch a capacitor charge and discharge between one third and two thirds of the supply voltage. In astable mode the chip oscillates continuously: the capacitor charges through R1 and R2, so the output is high for 0.693 × (R1 + R2) × C, then discharges through R2 alone, so it's low for 0.693 × R2 × C. The frequency is about 1.44 ÷ ((R1 + 2R2) × C). With R1 = 1 kΩ, R2 = 10 kΩ and C = 10 µF that's about 6.87 Hz, with a duty cycle of 52.4%. Because the high time includes R1, the duty cycle in this standard circuit is always above 50%; adding a diode across R2 allows lower duty cycles. In monostable mode a trigger pulse produces a single output pulse lasting 1.1 × R × C, so 100 kΩ and 10 µF give 1.1 seconds, which is useful for delays, debouncing and timers. Design mode rearranges the formula to give R2 for a chosen frequency, R1 and C. Honest limits: these are the standard textbook formulas. Real timing varies with capacitor tolerance, as electrolytics can be ±20%, temperature and the chip version, so CMOS parts such as the 7555 differ slightly. Keep R1 above about 1 kΩ. The RC filter calculator, the LED resistor calculator and the Ohm's law calculator are related.

Frequently asked questions