555 Timer Calculator
Astable frequency, period, high/low times and duty from R1, R2 and C, monostable pulse width from R and C, or the resistors for a target frequency or pulse.
The timing a 555 circuit will produce with the parts you have — or the parts to fit for the timing you want — with the exact ln 2 and ln 3 formulas rather than the rounded rules.
Example: R1 = 1 kΩ, R2 = 10 kΩ, C = 10 nF: high 76.2 µs, low 69.3 µs, 6.87 kHz at 52.4 % duty. Monostable with 100 kΩ and 10 µF gives a 1.10 s pulse.
A capacitor between ⅓ and ⅔ of the supply.
Everything follows from that.
Where the astable and monostable formulas come from, why duty cannot go below 50 % in the basic circuit, and what the datasheet still owns.
Astable
The capacitor charges through R1 + R2 from ⅓ to ⅔ of the supply and discharges through R2 alone back to ⅓. Charging between those two thresholds of an RC network takes exactly ln 2 × RC, so t_high = 0.693 (R1 + R2) C and t_low = 0.693 R2 C; the frequency is 1 over their sum, and the familiar 1.44 ÷ ((R1 + 2R2) C) is the same expression with ln 2 rounded. Because R2 appears in both intervals and R1 only in the high one, the high time is always the longer, so duty is above 50 % — a diode across R2 or a different topology is needed for less, and the solver refuses targets it cannot meet.
Monostable
Triggered, the output goes high and the capacitor charges from zero to ⅔ of the supply through R, which takes ln 3 × RC ≈ 1.1 RC; the output then drops. The trigger pulse must end before the timeout. Solving for R at a chosen C is a single division.
Choosing R and C, and what the datasheet owns
The formulas are ideal. Real timers have threshold and bias currents, so very large resistors drift; very small R1 lets the discharge pin sink too much current; the capacitor's tolerance and leakage move the timing by their own percentage; and the supply range, output drive and temperature behaviour differ between the bipolar NE555 and CMOS variants. The page flags the common traps but takes the limits from no datasheet — check the one for the part you use. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Texas Instruments, NE555/SA555/NA555 Precision Timers datasheet (SLFS022) — astable and monostable timing equations and operating notes
Last reviewed 19 September 2026. How results are checked: How we verify.