About this calculator
The 555 has been in production since 1972 and still turns up everywhere, because two resistors and a capacitor are enough to make it oscillate, blink, delay or generate a pulse. Its timing comes entirely from an external RC network charging between two internal thresholds set at ⅓ and ⅔ of the supply.
This calculator covers the three things people actually need from a 555: the frequency and duty cycle of an astable oscillator, the pulse width of a monostable one-shot, and the reverse problem of picking resistors to hit a frequency you have in mind.
How it works
In astable mode the capacitor charges through Ra + Rb and discharges through Rb alone, because the discharge pin sits between the two resistors. The output is high while charging and low while discharging, so the two halves of the cycle have different lengths.
Each phase is an RC curve crossing from ⅓ to ⅔ of the supply, which takes ln(2) ≈ 0.693 time constants. That is where the 0.693 in every 555 formula comes from. The full period is the two phases added together, and frequency is its reciprocal.
Because the charge path always includes Rb plus Ra while the discharge path is Rb alone, the duty cycle of a basic 555 astable is always above 50%. Getting below that needs a diode across Rb so charging bypasses it.
In monostable mode a trigger pulse starts a single charge cycle that runs from 0 all the way to ⅔ of the supply. That is ln(3) ≈ 1.1 time constants, giving the familiar T = 1.1 × R × C.
All of this is set by ratios of the supply voltage rather than absolute voltages, so 555 timing is independent of supply voltage — the same circuit runs at the same frequency on 5 V or 12 V.
Worked example
A 1 Hz LED blinker — the classic first 555 project.
- Pick C = 10 µF, a convenient electrolytic
- Aim for roughly 50% duty, so make Rb much larger than Ra
- Try Ra = 6.8 kΩ, Rb = 68 kΩ
- f = 1.44 / ((6800 + 136000) × 10e-6) = 1.44 / 1.428 = 1.008 Hz
- duty = (6800 + 68000) / 142800 = 52.4%
About 1.01 Hz at 52% duty — a visibly even blink. Making Rb larger relative to Ra pushes the duty closer to 50% but never reaches it.
Practical notes
- Keep Ra at 1 kΩ or above. The discharge transistor on pin 7 has to sink
Vcc / Raevery cycle, and a small Ra will cook it. - Bypass pin 5 (control voltage) to ground with 10 nF. Without it, supply noise modulates the internal thresholds and the output jitters.
- The classic bipolar NE555 produces a large current spike on every output transition. Put 100 nF across the supply pins, right at the chip.
- For battery work use a CMOS version — the TLC555 or ICM7555 draw a fraction of the supply current, work down to about 2 V, and need no Ra minimum.
- Electrolytic capacitors have wide tolerance (often ±20%) and drift with temperature and age. For timing that has to be repeatable, use film or C0G ceramic and accept the smaller values.
- Above roughly 100 kHz the internal propagation delays start to matter and the real frequency falls below the formula.
Frequently asked questions
How do I calculate 555 timer frequency?
Use f = 1.44 / ((Ra + 2Rb) × C) with resistance in ohms and capacitance in farads. The calculator handles the unit conversions for you.
Why can't I get a 50% duty cycle from a 555?
The capacitor charges through Ra and Rb but discharges through Rb alone, so the high time is always longer than the low time. Put a diode across Rb (cathode toward the discharge pin) so charging bypasses Rb, and the two phases become independent.
What is the difference between astable and monostable?
Astable free-runs, producing a continuous square wave with no input. Monostable sits idle until triggered, then emits exactly one pulse of a fixed width and returns to rest. Astable is for oscillators and blinkers, monostable for timers and debouncers.
Does supply voltage change the frequency?
No. The thresholds are fixed fractions of the supply, so a higher supply charges the capacitor faster but also raises the target voltage by the same proportion. The two cancel, which is a large part of why the 555 is so useful.
What is the maximum 555 frequency?
The datasheet claims 500 kHz for a standard NE555, but propagation delay makes the output increasingly inaccurate above about 100 kHz. CMOS versions do better. Above a megahertz, use a dedicated oscillator or a microcontroller timer.