555 Timer Calculator: Astable Frequency, Duty Cycle and Monostable Pulse Width

Find the frequency or the pulse length of a 555 timer circuit.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose astable or monostable and enter the resistors and the timing capacitor.

For an astable circuit it gives the high time 0.693 (R1 + R2) C, the low time 0.693 R2 C, the frequency and the duty cycle.

For a monostable it gives the pulse width 1.1 R C.

Formulas used

  • f = \frac{1.44}{(R_1 + 2R_2)C}
  • t_H = 0.693(R_1 + R_2)C
  • t_L = 0.693R_2C
  • T = 1.1RC

Worked example

With the default numbers in the calculator above:

  1. Output high time: 0.40 ms
  2. Output low time: 0.33 ms
  3. Period and frequency: 1387.50 Hz (period 0.72 ms)
  4. Duty cycle (the time the output is high): 54.81 %

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Mode Monostable (one pulse when triggered).

  1. Output pulse width: 0.11 ms

Common questions

Why is the duty cycle always above 50 percent?

The capacitor charges through R1 and R2 but discharges only through R2, so the high time is longer than the low time.

Does the supply voltage change the frequency?

No. The thresholds are fixed fractions of the supply, so the supply drops out of the time.

How do I get a duty cycle of 50 percent or less?

Put a diode across R2 so the capacitor charges through R1 only, or use a flip-flop to halve the frequency.

What range of resistor can I use?

About 1 kilohm to 10 megohm for R and a capacitor with low leakage.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: 555 timer data sheet (Texas Instruments NE555); Horowitz and Hill, The Art of Electronics.

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