555 Timer Calculator (Astable Mode)

Enter RA, RB, and C to compute frequency, period, duty cycle, and timing. Runs fully in your browser.

How to Use

  1. Enter resistor values RA and RB, and capacitor C.
  2. Choose the correct units (kΩ, µF, etc.).
  3. Results update instantly; open Show Work to see the formulas and steps.
  4. Use Share Link to generate a URL that restores your values.
Astable Summary
Based on the standard 555 astable equations (idealized).
Frequency
—
Period
—
Duty
—
tHIGH
—
tLOW
—
VCC (optional)
—
Tip: If you need ~50% duty cycle, the classic 555 astable usually needs a diode or alternative configuration.
Inputs
Provide RA, RB, and C. VCC is optional (for reference notes only).
Typical: 1kΩ – 1MΩ (avoid extremely low values)
Typical: 1kΩ – 10MΩ
Example: 0.1µF = 100nF
Used for reference notes only (timing is mainly RA/RB/C)
Show Work (step-by-step)
—
Work is shown using base units (Ω and F, seconds, Hz) for consistency.

Astable Formulas (Reference)

High time
tHIGH = 0.693 × (RA + RB) × C
Low time
tLOW = 0.693 × RB × C
Period
T = tHIGH + tLOW = 0.693 × (RA + 2RB) × C
Frequency
f = 1 / T ≈ 1.44 / ((RA + 2RB) × C)
Duty cycle
D = tHIGH / T = (RA + RB) / (RA + 2RB)

These are the common “textbook” equations for the classic 555 astable configuration (no diode, ideal behavior).

FAQ

Why can’t I get exactly 50% duty cycle?

In the classic astable, the charge path uses RA + RB but the discharge path uses RB, so duty cycle is usually > 50%. A diode or alternative configuration is commonly used for near-50%.

Does VCC affect frequency?

In the ideal equations here, timing is mainly set by RA, RB, and C. Real parts and thresholds can introduce small variations.

What if RA or RB is extremely small?

Very low resistance can over-stress the discharge transistor and increase current. Use practical resistor ranges for safe operation.

Tool Info

Last updated:

Updates may include edge-case handling, unit expansion, and additional reference helpers.