555 Timer Monostable Pulse Width Calculator

In monostable ("one-shot") mode, a trigger pulse starts the 555's timing capacitor charging from 0V toward the supply rail through a single resistor R. The output stays high until the capacitor crosses 2/3 of the supply voltage, at which point the internal comparator flips the output back low and discharges the capacitor, ready for the next trigger. The time to charge from 0V to exactly 2/3 of the rail works out to ln(3) × R × C — the standard NE555 monostable derivation found in the TI/Fairchild datasheets.

Datasheets commonly round ln(3) ≈ 1.0986 up to the quick rule of thumb "T = 1.1RC"; this calculator uses the exact ln(3) rather than that rounded constant, so the result is slightly more precise than the rule-of-thumb figure most references quote.

This is the direct one-shot counterpart to this site's 555 Timer Astable calculator, which covers the free-running oscillator mode (continuous frequency and duty cycle) instead of a single timed pulse.