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555 timer calculator (astable & monostable)

Design a 555 timer circuit in either mode. In astable mode, R1, R2 and C set the output frequency, high and low times and duty cycle; in monostable mode, R1 and C set the one-shot pulse width. The classic hobbyist chip, sized in seconds.

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The 555 timer calculator (astable & monostable) turns Mode, R1, R1 unit, R2 (astable only), R2 unit, Timing capacitor C, C unit into Output frequency (astable), Output HIGH time, Output LOW time, Duty cycle, Pulse width (monostable), instantly and for free.

How to use it

  1. Enter your values: Mode, R1, R1 unit, R2 (astable only), R2 unit, Timing capacitor C, C unit.
  2. Read the result instantly: Output frequency (astable), Output HIGH time, Output LOW time, Duty cycle, Pulse width (monostable).

Frequently asked questions

How does the 555 timer calculator (astable & monostable) work?

It takes Mode, R1, R1 unit, R2 (astable only), R2 unit, Timing capacitor C and C unit and derives Output frequency (astable), Output HIGH time, Output LOW time, Duty cycle and Pulse width (monostable) from them. The calculation is live as you type, so the result updates on every change.

Which values does the calculator ask for?

7 values: Mode, R1, R1 unit, R2 (astable only), R2 unit, Timing capacitor C and C unit. Nothing else is required — no account, no file upload.

Which “Mode” option should I choose?

You can pick between « Astable (oscillator) » and « Monostable (one-shot) ». Each one changes what the calculator works out, so switch and compare — the default is « Astable (oscillator) ».

When would I actually use this?

At the bench: sizing a resistor for an LED, reading a colour band, working out a divider ratio, and checking a part will not cook.

What is the most common mistake?

Choosing a resistance and forgetting the power rating. A value that is electrically right will still burn if the package cannot dissipate what passes through it — check the watts as well as the ohms.

How accurate is it, and what are the limits?

Standard bipolar 555 formulas: astable f = 1.44/((R1+2·R2)·C), tHIGH = 0.693·(R1+R2)·C, tLOW = 0.693·R2·C; monostable T = 1.1·R1·C. A plain astable duty cycle is always above 50 % — add a diode across R2 for 50 % or less.

What is the difference between the 555 timer calculator (astable & monostable) and the Impedance calculator (RLC)?

This one returns Output frequency (astable) and Output HIGH time; the Impedance calculator (RLC) returns Impedance magnitude |Z| and Phase angle. That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Inductive reactance calculator is the closest one after this: Solve the inductive reactance relation XL = 2πfL for any of its three variables. Choose whether to find the reactance, the frequency or the inductance, enter the other two, and it returns the answer — plus, if you supply an RMS voltage, the resulting current and reactive power.

What else is worth having open alongside it?

PCB trace width calculator (IPC-2221) and Capacitor code calculator (decoder) — they come up in the same task often enough to be worth a second tab.

Where do the figures come from, and how current are they?

Ohm's law and the divider formulas are exact. Colour bands follow the IEC standard; real components carry a tolerance, so a nominal value and a measured one will not match exactly.

Further reading

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ExplainerResistors in Series and in Parallel: The Formulas, and the Three Things They Do Not Tell YouSeries adds, parallel adds reciprocals — that part is easy. The parts that catch people: which member dominates the answer, why two 5 % resistors in parallel are still 5 %, and why the smallest resistor in a parallel pair takes the largest share of the power.How-toHow to Calculate an LED Series ResistorSize the series resistor for any LED with one formula: R = (Vs − Vf) / I. Follow the steps, see a worked 5 V example, and pick the right resistor.ExplainerWhat Is Ohm's Law? V = I × R Explained SimplyOhm's law links voltage, current, and resistance with one equation: V = I × R. Learn the triangle trick and see worked examples.ExplainerThe Voltage Divider: The Formula Is Trivial, the Assumption Behind It Is NotVout = Vin·R2/(R1+R2) is only true when nothing is connected to the output. Draw current and the voltage collapses, by an amount that follows one exact law. Here is that law, the ten-times rule checked against it, and why a lower-resistance divider costs you power forever.ComparisonSolar Panels: the Self-Consumption Rate Decides the PaybackSince June 2026 a French rooftop is paid 1.1 cents for an exported kilowatt-hour and avoids 25.61 cents by consuming the same one. That ratio of 23 to 1 makes the self-consumption rate — the variable almost every quote omits — worth more than the amount of sun falling on the roof.ComparisonHeat Pump or Gas Boiler: the SCOP Your Local Prices DemandBefore any grant, the price of electricity divided by the price of gas sets a seasonal efficiency the heat pump must beat just to break even on running cost. In late 2025 that number was 1.53 in Portugal and 2.79 in Germany — and on old radiators a German heat pump can cost more to run than the boiler it replaced.