# 555 Timer Astable Calculator

URL: https://speedycalculator.com/calculators/physics/ne555-astable/
Last updated: 2026-10-30

> The 555 timer astable calculator finds the high time, low time, period, frequency and duty cycle of an NE555 oscillator from the timing capacitor and resistors R1 and R2.

## Inputs

- Capacitance C (µF, nF, pF, mF, F)
- Resistance R₁ (kΩ, Ω, MΩ)
- Resistance R₂ (kΩ, Ω, MΩ)

## Results

- High time t_H
- Low time t_L
- Period
- Frequency
- Duty cycle

## Formula

```
t_H = 0.693·(R₁ + R₂)·C,  t_L = 0.693·R₂·C,  f = 1.44/((R₁ + 2R₂)·C),  duty = t_H/(t_H + t_L)
```

## Explanation

The 555 timer, introduced in 1972, is still one of the most popular chips ever made. In astable mode it makes a continuous square wave for blinking LEDs, tone generators and clock signals.

### How to use the 555 Timer Astable Calculator

- **Capacitance C**
- **Resistance R₁**
- **Resistance R₂**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the capacitor and the two resistors.

### Inputs explained

FieldWhat to enterCapacitance CUnits: µF, nF, pF, mF, FResistance R₁Units: kΩ, Ω, MΩResistance R₂Units: kΩ, Ω, MΩ

### What each result means

- **High time t_H** = 0.693×([Resistance R₁] + [Resistance R₂])×[Capacitance C]
- **Low time t_L** = 0.693×[Resistance R₂]×[Capacitance C]
- **Period** = 0.693×([Resistance R₁] + 2×[Resistance R₂])×[Capacitance C]
- **Frequency** = 1÷(0.693×([Resistance R₁] + 2×[Resistance R₂])×[Capacitance C])
- **Duty cycle** = ([Resistance R₁] + [Resistance R₂])÷([Resistance R₁] + 2×[Resistance R₂])×100

### Example values

How the main result changes when **Capacitance C** changes and everything else stays at the example values above:

Capacitance C (µF)Frequency (Hz)513.74297.59.1619106.871412.55.4971154.581203.4357

## Worked example

Inputs — Capacitance C: 10 µF; Resistance R₁: 1 kΩ; Resistance R₂: 10 kΩ.

Results — High time t_H = 76.23 ms; Low time t_L = 69.3 ms; Period = 145.53 ms; **Frequency = 6.87144 Hz**; Duty cycle = 52.381 %.

## Key terms

- **Astable**: Free-running oscillator with no stable state.

## FAQ

**Why can't the duty cycle go below 50%?**
The capacitor charges through R₁ + R₂ but discharges only through R₂ – add a diode across R₂ to get below 50%.

**What is the frequency limit?**
Around 100–500 kHz for the bipolar NE555.

**Why 0.693?**
It is ln 2 – charging between 1/3 and 2/3 of V_CC.

**What is the minimum R₁?**
About 1 kΩ to protect the discharge transistor.


## Sources

- [OpenStax – University Physics Volume 2](https://openstax.org/details/books/university-physics-volume-2)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)
- [Texas Instruments – NE555 datasheet](https://www.ti.com/product/NE555)