What is the 555 timer frequency formula?
f = 1.44 ÷ ((RA + 2 RB) × C). The exact constant is 1 ÷ ln 2 = 1.4427, because the capacitor charges and discharges between 1/3 and 2/3 of the supply.
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555 timer calculator
Set the timing parts in this 555 timer calculator and read the frequency, period and duty cycle in astable mode, or the pulse width in monostable mode, with the capacitor and output waveforms. Enter a target instead and it picks standard RA, RB and C inside the limits TI publishes for the NE555 and TLC555.
Mode
Examples
Ω
Ω
F
V
Sets the waveform levels and limit checks. Timing does not depend on it.
Timer
Frequency
138.7 Hz
- Period
- 7.209 ms
- Output high time
- 3.951 ms
- Output low time
- 3.258 ms
- Duty cycle (output high)
- 54.81 %
- Duty cycle (output low)
- 45.19 %
- First high pulse after power-up
- 6.262 ms C starts from 0 V
- Capacitor swing
- 3 V to 6 V
- Peak discharge pin current
- 1.028 mA
Uses ideal thresholds at 1/3 and 2/3 of VCC. TI lists a typical drift of 150 ppm/°C and 0.3 %/V for astable NE555 timing; the tolerance of R and C is usually larger.
Output (ideal)Capacitor
How to calculate 555 timer frequency
Use f = 1.44 ÷ ((RA + 2 RB) × C). With RA = 10 kΩ, RB = 47 kΩ and C = 100 nF the frequency is 138.7 Hz and the output is high for 54.8 % of each period. In monostable mode the pulse width is 1.1 × R × C.
555 timer formula: frequency, duty cycle and pulse width
In astable mode the capacitor charges through RA and RB, then discharges through RB alone, moving between 1/3 and 2/3 of the supply. Each half is the same exponential, so the supply voltage cancels out.
tH = ln 2 × (RA + RB) × C tL = ln 2 × RB × C
- tH
- output high time in s
- tL
- output low time in s
- RA
- resistor from VCC to DISCH, in Ω
- RB
- resistor from DISCH to THRES and TRIG, in Ω
- C
- timing capacitor from THRES to ground, in F
TI prints ln 2 as 0.693. The period, frequency and duty cycle follow:
f = 1 ÷ (tH + tL) ≈ 1.44 ÷ ((RA + 2 RB) × C)
Duty cycle (output high) = (RA + RB) ÷ (RA + 2 RB)
For a target frequency f and duty cycle D (above 0.5), solve for the two resistors:
RA = (2D − 1) ÷ (f × ln 2 × C) RB = (1 − D) ÷ (f × ln 2 × C)
In monostable mode the capacitor charges from about 0 V to 2/3 of the supply through a single resistor:
tw ≈ 1.1 × R × C
- tw
- output pulse width in s; the ideal factor is ln 3 = 1.0986
555 timer astable and monostable examples
Example 1: frequency of RA = 10 kΩ, RB = 47 kΩ, C = 100 nF
tH = ln 2 × 57 kΩ × 100 nF = 3.951 ms. tL = ln 2 × 47 kΩ × 100 nF = 3.258 ms. The period is 7.209 ms, so f = 138.7 Hz with a duty cycle of 54.8 %.
Example 2: 1 kHz at 60 % duty with a 10 nF capacitor
RA = 0.2 ÷ (1 kHz × ln 2 × 10 nF) = 28.9 kΩ and RB = 0.4 ÷ (1 kHz × ln 2 × 10 nF) = 57.7 kΩ. The nearest E24 values are 30 kΩ and 56 kΩ, which give 1.016 kHz at 60.6 %.
Example 3: a 5 second one-shot with a 47 µF capacitor
R = 5 s ÷ (1.1 × 47 µF) = 96.7 kΩ. The nearest E24 value, 100 kΩ, gives tw = 5.17 s. Electrolytic tolerance is usually wide, so trim R if the time matters.
555 timer RC values for common frequencies
Standard E6 capacitors and E24 resistors for a 60 % duty cycle on an NE555 at 9 V, chosen by the same search the calculator uses. Your duty cycle changes RA and RB, so use the 555 astable calculator at the top of the page for anything else.
| Target | C | RA | RB | Actual frequency | Duty |
|---|---|---|---|---|---|
| 1 Hz | 10 µF | 30 kΩ | 56 kΩ | 1.016 Hz | 60.6 % |
| 2 Hz | 4.7 µF | 30 kΩ | 62 kΩ | 1.993 Hz | 59.7 % |
| 5 Hz | 2.2 µF | 27 kΩ | 51 kΩ | 5.083 Hz | 60.5 % |
| 10 Hz | 1 µF | 30 kΩ | 56 kΩ | 10.16 Hz | 60.6 % |
| 50 Hz | 220 nF | 27 kΩ | 51 kΩ | 50.83 Hz | 60.5 % |
| 100 Hz | 100 nF | 30 kΩ | 56 kΩ | 101.6 Hz | 60.6 % |
| 440 Hz | 100 nF | 6.8 kΩ | 13 kΩ | 439.8 Hz | 60.4 % |
| 1 kHz | 10 nF | 30 kΩ | 56 kΩ | 1.016 kHz | 60.6 % |
| 5 kHz | 2.2 nF | 27 kΩ | 51 kΩ | 5.083 kHz | 60.5 % |
| 10 kHz | 1 nF | 30 kΩ | 56 kΩ | 10.16 kHz | 60.6 % |
| 20 kHz | 2.2 nF | 6.8 kΩ | 13 kΩ | 19.99 kHz | 60.4 % |
| 38 kHz | 2.2 nF | 3.6 kΩ | 6.8 kΩ | 38.13 kHz | 60.5 % |
| 50 kHz | 2.2 nF | 2.7 kΩ | 5.1 kΩ | 50.83 kHz | 60.5 % |
| 100 kHz | 1 nF | 3 kΩ | 5.6 kΩ | 101.6 kHz | 60.6 % |
555 monostable delay table: pulse width for common R and C
tw = 1.1 × R × C. Find the delay you need and read off a pair, or use the 555 timer monostable calculator above for other values.
| R \ C | 10 nF | 100 nF | 1 µF | 10 µF | 100 µF |
|---|---|---|---|---|---|
| 10 kΩ | 110 µs | 1.1 ms | 11 ms | 110 ms | 1.1 s |
| 47 kΩ | 517 µs | 5.17 ms | 51.7 ms | 517 ms | 5.17 s |
| 100 kΩ | 1.1 ms | 11 ms | 110 ms | 1.1 s | 11 s |
| 470 kΩ | 5.17 ms | 51.7 ms | 517 ms | 5.17 s | 51.7 s |
| 1 MΩ | 11 ms | 110 ms | 1.1 s | 11 s | 110 s |
What goes wrong in real circuits
The timing is only as good as R and C. TI lists a typical drift of 150 ppm/°C and 0.3 %/V for astable NE555 timing and 50 ppm/°C and 0.1 %/V for monostable, which is small next to a 10 % capacitor. Electrolytics add leakage that lengthens long intervals, so use film or C0G parts where you can. The first high pulse after power-up is longer than the rest because the capacitor starts from 0 V. TI highly recommends a 0.1 µF ceramic bypass capacitor from VCC to ground and shows 10 nF from CONT to ground as an option to evaluate. In monostable mode the trigger must return high before the interval ends, and TI gives 10 µs as the shortest NE555 pulse. The output does not reach the rails on a bipolar part: TI lists a typical high level of 13.3 V from a 15 V supply at 100 mA. The chart draws the output as an ideal 0 V to VCC swing.
Glossary
- Astable
- A free-running mode: the output oscillates continuously between high and low with no trigger.
- Monostable
- A one-shot mode: a trigger starts a single output pulse of fixed width.
- Duty cycle
- The fraction of each period that the output is high.
- Threshold (THRES)
- The pin that ends the timing interval when it passes 2/3 of the supply.
- Trigger (TRIG)
- The pin that starts the timing interval when it falls below 1/3 of the supply.
- Discharge (DISCH)
- The open-collector pin that empties the timing capacitor while the output is low.
Related guides
- RC time constant calculator: the charge curve behind every 555 timing interval.
- Standard resistor values: E12, E24 and E96 lists for RA and RB.
- LED resistor calculator: size the resistor for an LED on the output.
Frequently asked questions
What frequency do RA = 1 kΩ, RB = 10 kΩ and 100 nF give?
687 Hz, with a high time of 762.5 µs, a low time of 693.1 µs and a duty cycle of 52.4 %.
How do you calculate the duty cycle of a 555 timer?
Duty cycle (output high) = (RA + RB) ÷ (RA + 2 RB). RA = 10 kΩ and RB = 47 kΩ give 54.8 %. The 555 timer astable calculator at the top of this page also works backward from a target frequency and duty cycle to RA, RB and C.
How do you calculate the 555 monostable pulse width?
t = 1.1 × R × C. With 100 kΩ and 10 µF the output stays high for 1.1 s. The ideal factor is ln 3 = 1.0986, which TI rounds to 1.1.
Can a 555 astable give a 50 % duty cycle?
Not with the basic circuit, because the capacitor charges through RA + RB and discharges through RB alone, so the high time is always longer. Making RA small gets close but raises the discharge pin current. A steering diode across RB or a divide-by-two flip-flop gets to 50 %.
Does the 555 frequency depend on the supply voltage?
Nominally no, because the thresholds and the charging current both scale with the supply. TI still lists a typical sensitivity of 0.3 % per volt for astable NE555 timing, and a changing supply during a cycle disturbs it.
What is the maximum frequency of a 555 timer?
TI recommends 100 kHz or below for the bipolar NE555 and suggests the CMOS TLC555 above that. The TLC555 is rated at 1.2 MHz minimum and 2.1 MHz typical in astable mode.
Does this NE555 calculator also cover the TLC555?
Yes. It starts as an NE555 timer calculator, and the Timer option switches it to the TLC555, which changes the limit checks and the parts search, such as the 100 kHz limit TI recommends for the NE555 and the 1.2 MHz minimum rating of the TLC555. The timing formulas stay the same.
What are the largest timing resistors you can use?
TI gives RA + RB of about 3.4 MΩ at 5 V and 10 MΩ at 15 V for the NE555, set by the threshold input current. The TLC555 has very low input current and can use larger values, but leakage in the capacitor and on the board becomes the limit.
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