BYTETOOLS

555 Timer Calculator

Work out 555 astable frequency and duty cycle or monostable pulse width, and design backwards from a target frequency to E12 resistor values.

685.7143 Hz
Frequency
52.38%
Duty cycle
762.4619 µs
Output high
693.1472 µs
Output low

f = 1.44 ÷ ((R1 + 2R2) × C)

  • R1 + 2R2 = 21 kΩ, C = 100 nF → f = 685.7143 Hz, period 1.4583 ms
  • Duty cycle = (R1 + R2) ÷ (R1 + 2R2) = 11 kΩ ÷ 21 kΩ = 52.381%
  • t_high = ln2 × (R1 + R2) × C = 762.4619 µs — the capacitor charges through R1 and R2 together.
  • t_low = ln2 × R2 × C = 693.1472 µs — it discharges through R2 alone, which is why the duty cycle can never drop to 50% in this circuit.
  • Adding the two times gives 1.4556 ms, or 686.9976 Hz. That differs from the headline figure by 0.19% because the datasheet rounds 1/ln 2 = 1.4427 to 1.44. Both are far inside the tolerance of real components.

This circuit can never go below a 50% duty cycle, because R1 is in the charging path but not the discharging one. To get 50% or less, put a diode across R2 (cathode towards the discharge pin) so the capacitor charges through R1 only and discharges through R2 only. The duty cycle then becomes R1 ÷ (R1 + R2), and the diode's forward voltage shifts the timing slightly.

These equations assume an ideal 555 whose thresholds sit at exactly ⅓ and ⅔ of the supply, which is why the answers do not depend on supply voltage. A real bipolar NE555 has an initial timing accuracy of about ±5% before you add resistor and capacitor tolerance, and electrolytic capacitors can be −20/+80%, so measure rather than trust the third decimal place. Keep R1 above roughly 1 kΩ so the discharge transistor is not overloaded, and above a few hundred kilohms leakage starts to matter.

What is the 555 Timer Calculator?

For a 555 in astable mode, frequency is 1.44 divided by (R1 plus 2 times R2) times C, and duty cycle is (R1 plus R2) over (R1 plus 2 times R2). With 1 kilohm, 10 kilohms and 100 nanofarads that is 685.7 Hz at 52.4 percent.

  • Astable frequency, duty cycle, high time and low time
  • Monostable pulse width and maximum retrigger rate
  • Design mode that snaps to E12 preferred resistor values
  • Unit dropdowns from ohms to megohms and picofarads to millifarads
  • Explains the diode trick needed for duty cycles of 50 percent or less
  • States the datasheet rounding and real-world tolerance rather than hiding them

How to use the 555 Timer Calculator

  1. 1

    Pick astable, monostable or design mode.

  2. 2

    For astable, enter R1, R2 and C with their units to get frequency, duty cycle and the high and low times.

  3. 3

    For monostable, enter the timing resistor and capacitor to get the output pulse width.

  4. 4

    For design, type a target frequency and a duty cycle above 50 percent, and choose the capacitor you have.

  5. 5

    Read the suggested E12 resistors and the frequency and duty they actually achieve.

About the 555 Timer Calculator

The ByteTools 555 Timer Calculator covers all three things people need from the chip. In astable mode it gives frequency, duty cycle and the separate high and low times from R1, R2 and C. In monostable mode it gives the one-shot pulse width from the standard 1.1 times R times C. And in design mode you name a target frequency and duty cycle, and it works backwards to resistor values snapped to the E12 preferred series.

Resistors and capacitors each get their own unit dropdown, from ohms to megohms and picofarads to millifarads, so there is no need to convert anything by hand. Results are formatted into whichever unit reads best — nanoseconds through minutes, hertz through megahertz — and the working underneath spells out every substitution.

The page is honest about the datasheet's own rounding: 1.44 and 0.693 are rounded forms of 1 over ln 2 and ln 2, so the frequency and the sum of the high and low times differ by about 0.2 percent. It says so, and reminds you that a real NE555 is only accurate to a few percent anyway. Everything is computed in your browser with nothing uploaded.

Frequently asked questions

How do you calculate the frequency of a 555 in astable mode?

Frequency equals 1.44 divided by the quantity R1 plus twice R2, times C. With R1 at 1 kilohm, R2 at 10 kilohms and C at 100 nanofarads that gives about 686 hertz. The 1.44 is the datasheet's rounded form of 1 over the natural log of 2.

Why can a 555 not reach a 50 percent duty cycle?

The capacitor charges through R1 and R2 in series but discharges through R2 alone, so the high time is always longer than the low time. Putting a diode across R2 gives each phase its own resistor and lets the duty cycle drop to 50 percent or below.

What is the difference between astable and monostable mode?

Astable free-runs, producing a continuous square wave with no input needed. Monostable produces a single pulse of a fixed length each time it is triggered, then returns to its resting state. The timing formulas are different, which is why they are separate modes here.

Does the supply voltage change the timing?

Not in theory. The 555's comparator thresholds sit at one third and two thirds of the supply, so the ratios cancel and the timing depends only on R and C. In practice a real chip drifts slightly with supply voltage and temperature.

How accurate is a real 555 timer circuit?

Initial timing accuracy is around 1 percent for a monostable and a few percent for an astable, before component tolerance. Standard resistors are 1 to 10 percent and electrolytic capacitors can be minus 20 to plus 80 percent, so the capacitor usually dominates the error.

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