Enter R1, R2, and capacitor C to find the ideal oscillation frequency and high and low timing for a standard 555 astable circuit.
Advanced options
The ideal number of complete output cycles per second.
The ideal time from the start of one cycle to the start of the next.
The capacitor charges through R1 and R2 during this interval.
The capacitor discharges through R2 during this interval.
This is the percentage of each ideal cycle that the output is high.
These are ideal estimates. Component tolerance, leakage, loading, supply conditions, and 555 timer variation can change measured timing.
Table of contents
How to use our 555 Astable Timer Calculator
- Find R1, R2, and timing capacitor C on the standard 555 astable schematic, parts list, component label, or meter reading.
- Enter R1, the resistor from the supply rail to the discharge pin. A bare number means kOhm; type a suffix for another unit, such as "470 ohm" or "1 MOhm".
- Enter R2, the resistor between the discharge pin and timing capacitor. A bare number also means kOhm.
- Enter C. A bare number means uF; type a suffix for another unit, such as "100 nF" or "0.0001 F".
- Select "Calculate." Read "Oscillation frequency" first for ideal cycles per second, then use the period and high/low times for the waveform timing.
- Open "Advanced options" only if you need a different number of visible decimal places. This does not change the calculation.
- For a hardware check, compare the result with a scope or frequency counter using the actual installed component values.
Definitions
Astable: A circuit mode that repeatedly switches its output between high and low without an external trigger.
Frequency (Hz): The number of complete cycles per second. One hertz is one cycle per second. [2]
Period (ms): The time for one complete high-and-low cycle.
High time: The part of each cycle when the output is high. In the standard circuit, C charges through R1 and R2.
Low time: The part of each cycle when the output is low. In the standard circuit, C discharges through R2.
Duty cycle: The percentage of a complete cycle spent at a high output.
Common mistakes and quick fixes
Mistake: Entering 10 for a 10 ohm resistor.
Fix: A bare resistor number means kOhm. Enter "10 ohm" or enter "0.01" for 0.01 kOhm.
Mistake: Entering 100 for a 100 nF capacitor without a suffix.
Fix: A bare capacitor number means uF. Enter "100 nF" or "0.1" for the same capacitance.
Mistake: Putting a resistance unit in the capacitor field, or a capacitance unit in a resistor field.
Fix: R1 and R2 accept ohm, kOhm, and MOhm. C accepts pF, nF, uF, mF, and F.
Mistake: Treating the ideal result as a guaranteed bench measurement.
Fix: Check installed resistor and capacitor values, circuit loading, supply conditions, and the timer datasheet when measured timing matters.
Limitations & Key Assumptions / Boundary Conditions
- This calculator uses ideal equations for the standard two-resistor 555 astable circuit. It does not model diode steering, modified timing paths, or other nonstandard designs.
- Measured timing can differ because of resistor and capacitor tolerance, capacitor leakage, timer threshold variation, temperature, supply conditions, and output loading.
- R1, R2, and C must all be greater than zero. With positive R1 and R2 in this standard circuit, the ideal high-time share is greater than 50%.
- Use measured component values and suitable test equipment when timing accuracy is important.
Methodology
Ideal timing equations
The calculator converts resistance entries to ohms and capacitance entries to farads before calculating. The standard astable relationships use ln(2), approximately 0.693147. [1]
high_time_ms = ln(2) * (R1 + R2) * C * 1000
low_time_ms = ln(2) * R2 * C * 1000
period_ms = high_time_ms + low_time_ms
frequency_hz = 1000 / period_ms
duty_cycle_percent = 100 * high_time_ms / period_ms
Worked example
For R1 = 10 kOhm, R2 = 100 kOhm, and C = 0.1 uF, the ideal high time is 7.625 ms and low time is 6.931 ms. The period is 14.556 ms, the frequency is 68.700 Hz, and the high-time share is 52.381%.
Visible rounding
The calculator keeps unrounded values for its equations. "Result decimal places" changes only how many decimal places appear in the result cards.