555 Astable Frequency Calculator
The 555 timer is one of the most popular integrated circuits ever made, and in its astable configuration it free-runs as an oscillator, producing a continuous square wave. The frequency of that wave is set by two resistors and one capacitor: the capacitor charges through both resistors and discharges through only the second, which is why the formula weights the second resistor twice. The oscillation frequency equals 1.44 divided by the quantity R1 plus twice R2, all multiplied by the capacitance. This calculator takes R1 and R2 in ohms and the capacitor C in farads, then returns the frequency in hertz to two decimal places. Enter the capacitance in farads, so a 10 nanofarad timing capacitor is 0.00000001 farads. Raising any of the components lowers the frequency, because larger resistances and capacitances slow the charge and discharge cycle. The constant 1.44 comes from the natural-logarithm thresholds built into the 555's internal comparators. The definitions of the ohm, the farad and the hertz are maintained by the National Institute of Standards and Technology. Every figure here is computed deterministically from the standard astable formula, shown in full below, with a worked example that reconciles exactly to the calculator so you can verify each step yourself.
The 555 astable frequency is 1.44 / ((R1 + 2 R2) C). With R1 = 1,000, R2 = 10,000 ohms and C = 10 nF, the frequency is 6,857.14 Hz. Larger components lower the frequency.
555 astable frequency formula
f = 1.44 / ((R1 + 2 R2) C)
f = oscillation frequency (Hz)
R1, R2 = timing resistors (ohms)
C = timing capacitor (farads)
Add R1 to twice R2, multiply by the capacitance, then divide that into 1.44. R2 is counted twice because the capacitor charges through both resistors but discharges only through R2. The constant 1.44 comes from the 555's one-third and two-thirds supply thresholds.
Worked example
A 555 astable circuit uses R1 = 1,000 ohms, R2 = 10,000 ohms and a 10 nF capacitor (0.00000001 farads).
- Compute R1 + 2 R2: 1,000 + 2 x 10,000 = 21,000
- Multiply by C: 21,000 x 0.00000001 = 0.00021
- Divide 1.44 by that: 1.44 / 0.00021 = 6,857.14
- The oscillation frequency is 6,857.14 Hz
So the 555 oscillates at about 6,857.14 Hz. These are the calculator's default inputs, so the result above matches the widget exactly.
555 Astable Frequency Calculator: frequently asked questions
How do you calculate a 555 astable frequency?
Use f = 1.44 / ((R1 + 2 R2) C), where R1 and R2 are the timing resistors in ohms and C the capacitor in farads. With R1 = 1,000, R2 = 10,000 and C = 10 nF, the frequency is 1.44 / (21,000 x 0.00000001) = 6,857.14 Hz.
Why is R2 multiplied by two?
In the standard astable circuit the capacitor charges through R1 and R2 in series but discharges through R2 alone. The full period combines a charge time set by R1 plus R2 and a discharge time set by R2, and summing them gives the R1 + 2 R2 term.
Where does the constant 1.44 come from?
The 555 switches at one-third and two-thirds of the supply voltage. The charge and discharge times each involve the natural logarithm of 2 (about 0.693), and combining them across the period yields the factor 1.44 = 1 / (0.693 x ... ) in the frequency formula.
How do I change the duty cycle?
The duty cycle is (R1 + R2) / (R1 + 2 R2), always above 50 percent in the basic circuit. Making R1 small relative to R2 brings it closer to 50 percent. Frequency and duty cycle are set separately by the resistor choices.
What is the 555 astable frequency formula?
The oscillation frequency is f = 1.44 / ((R1 + 2 R2) C).
Official sources
- Electrical units and measurement standards: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.