Astable 555 Timer Calculator Formula
Understand the math behind the astable 555 timer calculator. Each variable explained with a worked example.
Formulas Used
Frequency
frequency_hz = 1.44 / ((r1_ohm + 2 * r2_ohm) * c_farads)Period
period_ms = (r1_ohm + 2 * r2_ohm) * c_farads * 1000 / 1.44 * 1000Duty Cycle
duty_cycle_pct = (r1_kohm + r2_kohm) / (r1_kohm + 2 * r2_kohm) * 100Variables
| Variable | Description | Default |
|---|---|---|
r1_kohm | R1(kΩ) | 10 |
r2_kohm | R2(kΩ) | 10 |
c_uf | Capacitor(µF) | 0.1 |
r1_ohm | Derived value= r1_kohm * 1000 | calculated |
r2_ohm | Derived value= r2_kohm * 1000 | calculated |
c_farads | Derived value= c_uf / 1000000 | calculated |
How It Works
555 Timer Astable Mode
Formulas
f = 1.44 / ((R1 + 2R2) x C)
Duty Cycle = (R1 + R2) / (R1 + 2R2) x 100%
Duty cycle is always above 50% in standard configuration. Use a diode across R2 for below 50%.
Worked Example
R1 = 10k, R2 = 10k, C = 0.1 uF.
r1_kohm = 10r2_kohm = 10c_uf = 0.1
- 01f = 1.44 / ((10000 + 20000) x 0.0000001) = 1.44 / 0.003 = 480 Hz
- 02Duty cycle = (10 + 10) / (10 + 20) x 100 = 66.7%
Frequently Asked Questions
Can duty cycle be below 50%?
Not in standard configuration. Place a diode across R2 so it is bypassed during discharge to achieve sub-50% duty cycle.
How accurate is a 555 timer?
Typically within a few percent. Temperature and component tolerance cause drift.
What is the maximum frequency?
Standard 555 works reliably up to about 500 kHz. CMOS versions can reach several MHz.
Ready to run the numbers?
Open Astable 555 Timer Calculator