555 Timer Calculator (Astable and Monostable)
Pick astable (free-running oscillator), monostable (one pulse) or design (find C for a target frequency). Enter the resistors and capacitor. In monostable mode R1 is the timing resistor and R2 is not used.
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Formula
- Astable high time:
t_high = ln2 × (R1 + R2) × C - Astable low time:
t_low = ln2 × R2 × C - Period and frequency:
T = ln2 × (R1 + 2 R2) × C,f = 1 / T(about1.44 / ((R1 + 2 R2) C)) - Duty cycle:
(R1 + R2) / (R1 + 2 R2) - Monostable pulse:
t = 1.1 × R × C - Design:
C = 1 / (ln2 × (R1 + 2 R2) × f)
Worked example
R1 = 1 kΩ, R2 = 10 kΩ, C = 1 µF: t_high = 0.6931 × 11,000 × 1e-6 = 7.62462 ms, t_low = 6.93147 ms, T = 14.5561 ms, f = 68.6998 Hz, duty 52.381 %.
Monostable with R = 10 kΩ, C = 100 µF: t = 1.1 × 10,000 × 0.0001 = 1.1 s.
Design for 1 kHz with R1 = 1 kΩ, R2 = 10 kΩ: C = 1 / (0.6931 × 21,000 × 1000) = 68.6998 nF.
Frequently asked questions
What is the 555 astable frequency formula?
f = 1 / (ln2 × (R1 + 2 R2) × C), about 1.44 / ((R1 + 2 R2) C). With R1 = 1 kΩ, R2 = 10 kΩ and C = 1 µF the period is 0.693 × 21,000 × 0.000001 = 14.56 ms, so f = 68.7 Hz.
How do I get the duty cycle?
Duty cycle = (R1 + R2) / (R1 + 2 R2). For 1 kΩ and 10 kΩ that is 11 / 21 = 52.4 %. The output is always above 50 % in this basic circuit, because the capacitor charges through R1 + R2 but discharges through R2 only.
How long is the monostable pulse?
t = 1.1 × R × C. With R = 10 kΩ and C = 100 µF that is 1.1 × 10,000 × 0.0001 = 1.1 s. The exact constant is ln 3 = 1.0986, which the datasheet rounds to 1.1.
How do I choose C for a target frequency?
Pick R1 and R2 first, then C = 1 / (ln2 × (R1 + 2 R2) × f). For 1 kHz with 1 kΩ and 10 kΩ, C = 1 / (0.693 × 21,000 × 1000) = 68.7 nF; use the nearest standard value, such as 68 nF or 47 nF plus 22 nF.
Can I use this for safety or final design sign-off?
No. This calculator is for learning and design checks, not safety certification. Real timing is shifted by resistor and capacitor tolerance, temperature, supply voltage and the 555 variant (CMOS or bipolar), so verify the circuit on the bench.