Calculator Tools
555 Timer Calculator
Calculate a 555 timer's frequency, period, duty cycle, and pulse width in astable or monostable mode, or solve for the R1, R2, or capacitor value you need.
555 timer calculator
Timer mode
Free-running square-wave output set by R1, R2, and C. Use it for blinking LEDs, clock pulses, tone generation, and PWM-style signals. The output high time is always longer than the low time, so the duty cycle is above 50% in this standard circuit.
Solve for
Enter the other values and the highlighted quantity is calculated. The greyed-out field is the one being solved.
Result
Frequency
The quantity you chose to solve for.
988.1 Hz
Frequency
Output cycles per second.
988.1 Hz
Period (T)
One full cycle, T = 1 / f.
1.012 ms
Time high (t1)
0.693 × (R1 + R2) × C.
540.7 µs
Time low (t2)
0.693 × R2 × C.
471.3 µs
Duty cycle
(R1 + R2) / (R1 + 2 × R2).
53.42%
R1 + R2
Total charge-path resistance.
7.8 kΩ
Output waveform split
53.42% high
In the standard two-resistor astable the high time always exceeds the low time, so the duty cycle is above 50%.
How the math works
Astable frequency: f = 1.44 / ((R1 + 2 × R2) × C). The constant 1.44 is 1 / ln(2). The capacitor charges toward the supply through R1 + R2 and discharges through R2 alone, which sets the high and low times.
High and low times: thigh = 0.693 × (R1 + R2) × C and tlow = 0.693 × R2 × C, where 0.693 is ln(2). The period is the sum, and the duty cycle is thigh / T = (R1 + R2) / (R1 + 2 × R2). Since R1 is always in the charge path, the standard astable duty cycle is always above 50%. To reach 50% or below, add a diode across R2 or use a different topology.
Monostable pulse: t = 1.1 × R × C, where 1.1 is ln(3). One trigger produces one output pulse of this width regardless of how long the trigger is held (the trigger must return high before the pulse ends).
These are the standard textbook equations and assume an ideal 555 with comparator thresholds at 1/3 and 2/3 of the supply voltage. Real timing shifts slightly with supply voltage, temperature, and capacitor tolerance, and electrolytic capacitors in particular can drift the result by 10 to 20 percent.
555 pinout (8-pin DIP)
| Pin | Name | Function |
|---|---|---|
| 1 | GND | Ground (0 V) reference |
| 2 | TRIG | Trigger: starts the timing when pulled below 1/3 V+ |
| 3 | OUT | Output: drives the load, swings near 0 V and V+ |
| 4 | RESET | Active-low reset; tie to V+ when not used |
| 5 | CTRL | Control voltage; bypass to ground with 10 nF if unused |
| 6 | THRES | Threshold: ends charge when it reaches 2/3 V+ |
| 7 | DIS | Discharge: open-collector that drains the capacitor |
| 8 | V+ | Positive supply, typically 4.5 V to 15 V |
Resistor and capacitor values come from standard E-series ranges, so the nearest stock part may shift the real frequency or pulse width slightly from the calculated value. The CMOS variants (7555, TLC555) follow the same equations but draw far less current and run at lower supply voltages.
How to use
- Pick a mode: Astable for a free-running oscillator (R1, R2, C) or Monostable for a single timed pulse (R, C).
- Choose what to solve for. Leave that field blank-equivalent (it greys out) and fill in the others.
- Enter each value and select its unit from the dropdown (pF to F for capacitors, ohms to MΩ for resistors).
- Read the highlighted answer plus the full breakdown: frequency, period, high and low times, and duty cycle, or the pulse width.
- Click Copy summary to save the component values and results, or Reset to start over.
About this tool
555 Timer Calculator works out the timing for the two circuits the classic 555 timer chip (and its NE555, 7555, and TLC555 relatives) is built around, and it can run the math in either direction so you can start from the components you have or from the result you want. In astable mode the 555 free-runs as a square-wave oscillator: a timing capacitor charges toward the supply through R1 plus R2 and discharges through R2 alone, which gives a frequency of 1.44 divided by ((R1 + 2 times R2) times C). The tool reports the frequency, the period, the separate high and low times, and the duty cycle, and it draws a small bar showing how the output splits between high and low. Because R1 is always in the charge path, the standard two-resistor astable can never reach a 50 percent duty cycle on its own; its high time always exceeds its low time, and the page explains that you need a diode across R2 or a different topology to get to or below 50 percent. In monostable mode the chip fires a single output pulse after each trigger, with a width of 1.1 times R times C, useful for button debounce, timed relays, turn-on delays, and missing-pulse detectors. The constants come straight from the textbook equations (1.44 is 1 over the natural log of 2, 0.693 is the natural log of 2, and 1.1 is the natural log of 3) and assume an ideal 555 whose comparator thresholds sit at one third and two thirds of the supply. The solve-for control lets you fix the quantities you know and compute the one you do not, so you can target a blink rate or a pulse length and read off the resistor or capacitor value to fit, with clear warnings when no positive component value can hit a target (for example when R2 alone already caps the maximum frequency). Values are entered with proper engineering unit suffixes, from picofarads to farads and from ohms to megohms, and results auto-scale to a readable prefix. A reference table lays out the eight-pin DIP pinout (trigger, threshold, discharge, control, reset, output, and the supply and ground pins) so the math connects to a real schematic. Everything runs in your browser with no uploads, and it pairs naturally with the RC filter calculator, the resistor and capacitor code tools, the Ohm's law calculator, and the voltage divider calculator. Remember that real parts have tolerance: electrolytic timing capacitors in particular can shift the result by 10 to 20 percent, so treat the calculated figure as a starting point and trim with the nearest standard value.
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