555 timer calculator

The 555 has been in production since 1973 and is still the first thing most people reach for when they need a blink, a beep or a delay. Two resistors and a capacitor set everything it does. This works out what your values give you — and, going the other way, tells you when the part is being pushed past where it behaves.

How it is calculated

f = 1.44 ÷ ((R₁ + 2R₂) × C) · tₕ = 0.693 × (R₁ + R₂) × C · t_w = 1.1 × R₁ × C

In astable mode the capacitor charges through both resistors and discharges through the second one alone. That is why the output is high longer than it is low, and why a 50 % duty cycle is out of reach with this arrangement: the charge path always contains one resistor more than the discharge path.

Source: Texas Instruments — NA555, NE555, SA555, SE555 datasheet (SLFS022K, revised March 2026): equations 1 to 6 under "Astable Operation"

Questions people ask

Why can I not get a 50 % duty cycle?

Because charging goes through R₁ and R₂ while discharging goes through R₂ alone, so the high time is always the longer of the two. Making R₂ much larger than R₁ pushes the duty cycle towards 50 % without ever reaching it. Getting exactly half needs a different arrangement, not different values.

How fast can a 555 go?

The datasheet gives the range as under 1 mHz to 100 kHz and says plainly: to avoid distortion, use at a maximum frequency of 100 kHz or below. This page flags anything above that. If you need more, the datasheet points at the CMOS version rather than at cleverer component values.

Does the supply voltage change the timing?

No, and that is the useful part. The thresholds sit at two thirds and one third of the supply, and the capacitor charges towards the same supply, so the two effects cancel. The timing holds as long as the supply is steady during the interval — it is drift during the cycle that hurts, not the level.

Why does the frequency differ slightly from one over the period?

Because the datasheet rounds. The period is built from the constant 0.693, but the frequency equation prints 1.44 — and one divided by 0.693 is 1.443. The gap is about two parts in a thousand, far inside the tolerance of any capacitor you will use. This page keeps both figures as the source gives them rather than quietly reconciling them.

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