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555 Timer Calculator

Calculate timing for a conventional 555 timer circuit. Switch between an astable oscillator and a monostable one-shot, enter resistor and capacitor values, and see the timing results.

Ideal first-order estimates. Real timing varies with the selected 555 IC, component tolerance, capacitor leakage, temperature, parasitics and output load. Texas Instruments advises using a typical NE555 at 100 kHz or below to reduce distortion; check the datasheet for your exact part.

Astable oscillator: tHIGH = 0.693 × (RA + RB) × C; tLOW = 0.693 × RB × C; T = tHIGH + tLOW; f = 1/T; Output waveform duty cycle = tHIGH/T. Monostable one-shot: tPULSE ≈ 1.1 × R × C. Texas Instruments NE555 datasheet

How does the 555 timer calculator work?

Choose astable mode for repeating high and low intervals, or monostable mode for one pulse. Enter the timing components and select their units. The equations are ideal first-order estimates.

Frequently asked questions

How does the 555 timer calculator work?

Choose astable mode for repeating high and low intervals, or monostable mode for one pulse. Enter the timing components and select their units. The equations are ideal first-order estimates.

Are my circuit values uploaded?

No. The calculation runs in this browser. Values are not uploaded or saved by this tool.

What is the difference between astable and monostable mode?

Astable mode repeats high and low intervals as an oscillator. Monostable mode produces one output pulse after a trigger; this calculator estimates its ideal width.

Why does the conventional astable waveform duty cycle stay at or above 50%?

In the standard RA/RB circuit, the capacitor charges through RA and RB, then discharges through RB only. That makes the high interval at least as long as the low interval. Other topologies, such as adding a diode, can change this.

Will a built circuit match this result exactly?

No. This uses ideal equations and does not model a particular part or component tolerance. Check the selected timer datasheet and measure the assembled circuit.