555 Timer Calculator
The 555 timer calculator allows you to find the times of the output of a 555 astable mode timer circuit and more.
Frequency138.7207 Hzln(2)/((R₁+2R₂)C) — the familiar 1.44 constant is 1/ln(2) rounded
Period7.2087 ms
Output high time3.9509 mscharging through R₁ + R₂
Output low time3.2578 msdischarging through R₂ alone
Duty cycle54.808%always ABOVE 50% in this configuration, because the charge path includes R₁ and the discharge path does not
Minimum achievable duty cyclejust over 50%approached as R₁ becomes small against R₂ — going below 50% needs a diode across R₂ so the charge and discharge paths differ
Independent of supply voltageyesthe thresholds are fixed fractions of the supply, so the RC crosses them in the same time whatever the rail — which is what makes the 555 useful
For a 50% duty cycleadd a diode across R₂then t_high = ln(2)R₁C and t_low = ln(2)R₂C independently, giving 253.1044 Hz with equal resistors
At ten times the capacitance13.8721 Hzfrequency is inversely proportional to C, so a decade switch on the capacitor gives decade frequency ranges
Duty cycle if R₁ and R₂ were swapped85.075%
Practical limitsbelow about 1 MHza bipolar 555 becomes unreliable much above a few hundred kilohertz, and very large timing resistors make it sensitive to leakage
The formula
f = 1.44 ÷ ((R₁+2R₂)C); duty = (R₁+R₂) ÷ (R₁+2R₂)
Why the 555 ignores its supply voltage
The 555 compares its timing capacitor against two thresholds set at one third and two thirds of the supply rail. Because both are fractions of the same rail, an RC charging towards that rail crosses the gap between them in a time that depends only on R and C — never on the supply. That is the whole reason the chip has lasted since 1972.
The ln(2) in every 555 formula comes from that one-third-to-two-thirds span: an exponential covers it in exactly ln(2) time constants. The familiar 1.44 constant is just 1/ln(2) rounded to three figures.
In the standard astable the capacitor charges through R₁ and R₂ but discharges through R₂ alone, so the high time always exceeds the low time and the duty cycle is always above 50%. It approaches 50% as R₁ shrinks but never reaches it. Getting below 50% requires a diode across R₂ so that charge and discharge take genuinely separate paths.