555 Timer Calculator (Astable and Monostable)

What this tool does

In astable mode the capacitor charges through R1 and R2 and discharges through R2 alone, between one third and two thirds of the supply, which is where the ln 2 ≈ 0.693 comes from. Because charging always goes through more resistance, the duty cycle is above 50 % — a diode across R2 is the usual trick to get below it. Real frequencies drift a few per cent with capacitor tolerance and the chip's own thresholds, so for anything precise use a trimmer. The chart shows how the frequency moves as R2 changes.

Formula

f = 1 ÷ (ln 2 × (R1 + 2R2) × C) ≈ 1.44 ÷ ((R1 + 2R2) C) · duty = (R1 + R2) ÷ (R1 + 2R2) · monostable: t = ln 3 × R C ≈ 1.1 R C

Variables

Worked example

Limitations

• Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
• The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.