The 555 timer calculator above covers both of the chip's classic configurations. In astable mode it turns two resistors and a capacitor into a frequency, a period, a high time, a low time and a duty cycle. In monostable mode it turns one resistor and a capacitor into a single output pulse width. Both modes also run backwards, so you can start from the timing you want and get the component values that produce it.
Arb Digital builds free tools that pick one job and finish it properly. The 555 has been in production since 1972 and remains one of the most widely used integrated circuits ever made, largely because its behaviour is completely described by three passive components and two simple equations. This page implements those equations exactly as the manufacturer's datasheet states them, rather than the rounded versions that circulate on hobby forums.
What This 555 Timer Calculator Does
Astable mode is the oscillator. The capacitor charges through RA and RB in series until it reaches two thirds of the supply, then discharges through RB alone until it falls to one third, and the cycle repeats indefinitely with no external trigger. Because the charge path has more resistance than the discharge path, the output high time is always longer than the low time and the duty cycle is always above fifty per cent.
Monostable mode is the one-shot. A negative-going trigger starts a single timing cycle; the capacitor charges through RA until it hits the two-thirds threshold, at which point the output goes low and the capacitor is discharged. The chip then sits idle until triggered again. This is the configuration behind push-button delays, missing-pulse detectors and debounce circuits.
The two design modes reverse the arithmetic. Give the astable designer a target frequency, a target duty cycle and a capacitor value, and it returns the two resistors. Give the monostable designer a target pulse width and a capacitor, and it returns the single resistor. Both suggest the nearest standard E24 value, because resistors are not sold in arbitrary values.
How to Use It
- Pick the mode first. Everything else follows from whether you want a free-running oscillator or a single triggered pulse.
- Choose the capacitor before the resistors. Capacitors come in far fewer values than resistors and have much worse tolerance, so it is easier to fix C and solve for R than the other way round.
- Keep resistances in a sensible window. Below about 1 kΩ the discharge transistor has to sink too much current; above about 1 MΩ the chip's own input bias currents start to matter and timing drifts.
- Check the duty cycle you asked for is reachable. The basic two-resistor astable cannot produce fifty per cent or less. If you need that, the circuit needs a diode across RB or a different topology.
- Snap to standard values and re-check. Enter the nearest E24 resistors back into the forward mode to see what frequency you will actually get, which is rarely exactly what you asked for.
The Formula: How 555 Timing Is Calculated
Both modes come from the same exponential charging curve. A capacitor charging through a resistor toward a fixed supply follows a curve with time constant τ = RC; OpenStax University Physics Volume 2, section 10.5 on RC circuits, states that the units of RC are seconds and that this quantity is the time constant. Moving between the one-third and two-thirds supply points takes exactly ln 2 time constants, which is 0.693, and that constant is where every 555 formula comes from.
The Texas Instruments xx555 Precision Timers datasheet gives the astable relations directly: the high-level duration is tH = 0.693 × (RA + RB) × C, the low-level duration is tL = 0.693 × RB × C, the period is their sum, T = 0.693 × (RA + 2RB) × C, and the frequency is f = 1.44 ÷ ((RA + 2RB) × C). For monostable operation the same document gives the output pulse duration as approximately tW = 1.1 × RA × C.
Work the default values. With RA = 10 kΩ, RB = 47 kΩ and C = 1 µF, the sum RA + 2RB is 104 kΩ. The period is 0.693 × 104,000 × 0.000001 = 0.0721 seconds, so the frequency is about 13.87 hertz. The high time is 0.693 × 57,000 × 0.000001 = 39.5 milliseconds and the low time is 0.693 × 47,000 × 0.000001 = 32.6 milliseconds. The duty cycle is 57 ÷ 104, which is 54.8 per cent.
Why the Duty Cycle Cannot Go Below Fifty Per Cent
This is the single most common frustration with the chip, and it falls straight out of the circuit topology. The capacitor charges through RA and RB in series, but discharges through RB only, because the discharge pin sits between the two resistors. The charging resistance is therefore always larger than the discharging resistance, so charging always takes longer, so the output is high for longer than it is low.
The duty cycle is (RA + RB) divided by (RA + 2RB). Making RA very small pushes this toward one half but never reaches it, and RA cannot be reduced to zero because the discharge transistor would then be shorted straight to the supply. Making RA large pushes the duty toward one hundred per cent.
The standard fix is a diode in parallel with RB, arranged so that charging current bypasses RB entirely and discharging current does not. Charging then happens through RA alone and discharging through RB alone, which makes any duty cycle reachable. The cost is a diode forward voltage in the charging path, so the timing shifts by a few per cent from the ideal formula and usually needs trimming.
Why Supply Voltage Does Not Change the Timing
A 555 running from 5 volts and the same 555 running from 15 volts produce the same frequency, which surprises people who expect a higher supply to charge the capacitor faster. It does charge faster — and the thresholds it has to reach are proportionally higher, by exactly the same factor.
Inside the chip an internal resistor divider sets the trigger comparator at one third of the supply and the threshold comparator at two thirds. Those are fractions, not fixed voltages. The capacitor charges exponentially toward the supply, and the time to travel between two fixed fractions of the target is independent of the target. The datasheet states this directly, noting that charge and discharge times, and therefore the frequency and duty cycle, are independent of the supply voltage.
What the supply does change is output drive, power consumption and the maximum frequency at which the output edges stay clean. The classic bipolar 555 draws several milliamps quiescent and produces a substantial current spike at each transition, which is why a decoupling capacitor across the supply pins is not optional. CMOS variants of the same pinout draw a fraction of that and run from lower supplies, at the cost of much less output current.
Where the Real Accuracy Limits Are
The equations are exact; the components are not. A standard aluminium electrolytic capacitor may be specified at plus or minus twenty per cent, and its actual value drifts with temperature and age. Carbon film resistors are typically five per cent. Stack those together and a nominal 1 kHz oscillator can easily land anywhere between 800 and 1,300 hertz, with the capacitor contributing most of the error.
If timing accuracy matters, the capacitor is where to spend money. A film capacitor at two or five per cent tolerance, with a low temperature coefficient, transforms the repeatability of the circuit. Metal film resistors at one per cent are cheap and remove most of the remaining error. Beyond that, the 555 itself contributes a small initial error in its internal divider and a temperature drift of a few tens of parts per million per degree.
There is also a practical frequency ceiling. The datasheet recommends operating at a maximum of 100 kHz or below to avoid distortion, and suggests a CMOS version of the timer where higher frequencies are needed. Above that the internal propagation delays become a significant fraction of the period and the output waveform stops matching the calculation. If you need a stable high frequency, an oscillator built around an inductor and capacitor is a better fit, and the LC resonant frequency calculator covers that case.
Choosing Components That Actually Exist
Solving the equations gives an exact resistance that no manufacturer sells. Resistors come in preferred series — E12 with twelve values per decade, E24 with twenty-four, E96 with ninety-six — each spaced so that the tolerance bands just about touch. This calculator reports the nearest E24 value, which is the finest series commonly stocked in five and one per cent parts.
The practical approach is to fix the capacitor at a standard value first, solve for the resistors, snap them to E24, then run the forward calculation again to see the frequency you will actually get. If that is not close enough, change the capacitor by a decade and repeat, or use a trimmer for one of the resistors. The resistor colour code calculator decodes the bands on parts you already have, the resistor combination calculator finds series and parallel pairs that hit awkward values, and the resistance converter and capacitance converter handle unit changes between ohms, kilohms, farads and microfarads.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Counting the control pin capacitor as the timing capacitor — the 0.01 µF part on pin 5 decouples the internal reference and has no effect on frequency.
- Asking for a duty cycle of fifty per cent or less — the basic astable cannot reach it, and the circuit needs a diode across RB before the arithmetic becomes possible.
- Leaving the reset pin floating — the datasheet says to tie reset to the supply when it is not being used, or the chip will trigger on noise.
- Using resistors below about 1 kΩ — the discharge transistor has to sink the current through them at every cycle, and small values overheat the chip or destroy it.
- Expecting an electrolytic capacitor to hold its value — twenty per cent tolerance and a strong temperature dependence mean the frequency you measure will not be the frequency you calculated.
Related Free Tools From Arb Digital
Handle component values with the resistor colour code calculator, the resistor combination calculator, the resistance converter and the capacitance converter. For the rest of the circuit, the Ohm's law calculator covers currents and voltages, the voltage divider calculator covers reference levels, and the LED resistor calculator sizes the series resistor if the output is driving an indicator. Frequency questions carry on with the frequency converter and the LC resonant frequency calculator. The full free online tools hub lists everything Arb Digital publishes.
Frequently Asked Questions
The manufacturer's datasheet gives f = 1.44 divided by the product of (RA + 2RB) and C, with resistance in ohms and capacitance in farads. The period is 0.693 times (RA + 2RB) times C, which is the same relationship expressed the other way round.
Because the capacitor charges through RA and RB in series but discharges through RB alone, so charging always takes longer than discharging. Putting a diode across RB lets charging current bypass it, which makes any duty cycle reachable at the cost of a small timing shift.
No. The comparator thresholds are set at one third and two thirds of the supply by an internal divider, and the capacitor charges toward that same supply, so the two effects cancel. The datasheet states that frequency and duty cycle are independent of supply voltage.
The output pulse duration is approximately 1.1 times RA times C. The factor of 1.1 comes from the time taken to charge from zero to two thirds of the supply, which is the natural logarithm of three rather than the logarithm of two used in astable mode.
The datasheet recommends operating at 100 kHz or below to avoid distortion, and suggests a CMOS version of the timer for higher frequencies. Above that ceiling the chip's internal propagation delays become a significant fraction of the period.
Almost always the capacitor. Aluminium electrolytics are commonly specified at plus or minus twenty per cent and drift with temperature and age, while the resistors are typically within five per cent. A film capacitor removes most of the discrepancy.
Keep both above about 1 kilohm so the discharge transistor is not overloaded, and below about 1 megohm so the chip's own input currents stay negligible. Fix the capacitor at a standard value first, then solve for the resistors and snap them to the nearest E24 value.
This tool is provided for educational and design-estimating use. Component tolerances mean a built circuit will differ from the calculated figures, and nothing on this page is electrical safety guidance.
