MasterTool

555 Timer Calculator

Calculate 555 (NE555) astable frequency, period and duty cycle, or monostable pulse width, from R and C.

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What this tool does

Calculate 555 (NE555) astable frequency, period and duty cycle, or monostable pulse width, from R and C. Uses the standard datasheet formulas with ideal components. Real timing shifts with capacitor tolerance, leakage and supply voltage.

How to use the 555 Timer Calculator

  1. Enter or select 555 mode.
  2. Enter or select resistance r1 (ohms).
  3. Enter or select resistance r2 (ohms, astable).
  4. Enter or select capacitance c (µf).
  5. Read the calculated result; change any measurement to compare alternatives.

Formula

astable: f = 1 ÷ (ln2 × (R1 + 2R2) × C); high = ln2(R1 + R2)C; low = ln2·R2·C; monostable: pulse = ln3 × R1 × C ≈ 1.1·R1·C
mode
555 mode
r1
Resistance R1 (ohms)
r2
Resistance R2 (ohms, astable)
c
Capacitance C (µF)

Uses the standard datasheet formulas with ideal components. Real timing shifts with capacitor tolerance, leakage and supply voltage.

Worked example

For 555 timer calculator, the following measurements illustrate the exact method: 555 mode: astable; Resistance R1 (ohms): 1000; Resistance R2 (ohms, astable): 10000; Capacitance C (µF): 10.

Inputs

  • 555 modeAstable (oscillator)
  • Resistance R1 (ohms)1000
  • Resistance R2 (ohms, astable)10000
  • Capacitance C (µF)10

Result

  • Frequency (Hz)6.87
  • Period (s)0.15
  • High time (s)0.08
  • Low time (s)0.07
  • Duty cycle (% high)52.38

Results explained

Frequency (Hz)
Frequency (Hz) from the formula above. Uses the standard datasheet formulas with ideal components. Real timing shifts with capacitor tolerance, leakage and supply voltage.
Period (s)
Period (s) from the formula above. Uses the standard datasheet formulas with ideal components. Real timing shifts with capacitor tolerance, leakage and supply voltage.
High time (s)
High time (s) from the formula above. Uses the standard datasheet formulas with ideal components. Real timing shifts with capacitor tolerance, leakage and supply voltage.
Low time (s)
Low time (s) from the formula above. Uses the standard datasheet formulas with ideal components. Real timing shifts with capacitor tolerance, leakage and supply voltage.
Duty cycle (% high)
Duty cycle (% high) from the formula above. Uses the standard datasheet formulas with ideal components. Real timing shifts with capacitor tolerance, leakage and supply voltage.

Frequently asked questions

f = 1.44 ÷ ((R1 + 2·R2) × C), with R in ohms and C in farads. The output is high for 0.693·(R1 + R2)·C and low for 0.693·R2·C each cycle.

The period is ln2 × (1,000 + 2×10,000) × 10 µF ≈ 0.1455 s, so f ≈ 6.87 Hz, high ≈52% of the cycle.

A trigger pulse starts one output pulse of width T = 1.1 × R × C (using R1 here). With R1 = 10 kΩ and C = 10 µF the pulse lasts about 0.11 s; use 100 µF for about 1.1 s.

The capacitor charges through R1 + R2 but discharges only through R2, so the high time is always longer than the low time and duty cycle stays above 50% unless a diode bypasses R2 during charging.

Common practice keeps R between about 1 kΩ and 10 MΩ and C from hundreds of pF up to hundreds of µF; very large electrolytics leak enough to make long timings inaccurate.