555 timer calculator
Enter R and C for the frequency and pulse length, or a target for the resistors.
Must be above 50% — the plain astable cannot go lower.
What are the 555 timer formulas?
In astable mode the output is high for tH ≈ 0.693 × (R1 + R2) × C and low for tL ≈ 0.693 × R2 × C, so the frequency is f ≈ 1.44 ÷ ((R1 + 2 × R2) × C). In monostable mode one trigger gives a pulse of tw ≈ 1.1 × R × C. These are the equations in section 6.3 of Texas Instruments' NE555 datasheet (SLFS022K). Example: R1 = 5 kΩ, R2 = 3 kΩ and C = 0.15 µF — the values in the datasheet's typical astable waveform — give 1.44 ÷ (11,000 × 0.00000015) = 873 Hz, high for 0.83 ms and low for 0.31 ms.
Why can't the duty cycle go below 50%?
The capacitor charges through R1 + R2 but discharges only through R2, so the output is high for 0.693 × (R1 + R2) × C and low for 0.693 × R2 × C. The high share, (R1 + R2) ÷ (R1 + 2 × R2), is therefore always above 50% and gets closer to it as R1 shrinks next to R2. Below 50% needs a different circuit; the TI datasheet gives no formula for one, so this calculator does not cover it.
How do I pick R and C for a target frequency?
Choose a capacitor you have, then find the resistors: R1 + 2 × R2 = 1.44 ÷ (f × C). For a duty cycle D (above 50%), R2 = (1 − D) × that sum and R1 = (2D − 1) × that sum. Example: 1 kHz at 60% with C = 0.1 µF gives a sum of 14.4 kΩ, R2 = 5.76 kΩ and R1 = 2.88 kΩ. The calculator rounds each to the nearest standard value and shows the frequency and duty cycle you actually get.
What are E12 and E24 resistor values?
They are the preferred values in IEC 60063. E12 has 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82, and E24 adds 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75 and 91 — each times a power of ten, so 5.6 kΩ and 5.6 MΩ are both E12 values.
What limits does the datasheet give?
TI advises keeping astable operation at 100 kHz or below to reduce distortion, and the shortest monostable pulse is about 10 µs because of the comparator's storage time. R1 + R2 should stay under 3.4 MΩ at a 5 V supply and 10 MΩ at 15 V. The constants are rounded — 0.693 is ln 2 and 1.44 is 1 ÷ ln 2 — so the high and low times add up to within about 0.2% of 1 ÷ f. As long as the supply voltage is steady, the timing does not depend on it; the tolerances of your resistors and capacitor are what limit accuracy.
Sources: Texas Instruments, “xx555 Precision Timers” (NA555, NE555, SA555, SE555), SLFS022K, March 2026, sections 5.5 and 6.3; IEC 60063 preferred number series as tabulated by Vishay (document 28372). Need the charge time of a capacitor through a resistor? Use the RC time constant calculator; to combine parts, the series and parallel calculator. How we check our calculators.