Electronics · Timers
555 timer calculator.
Enter RA, RB and C for the blink rate and duty cycle of an astable 555, or find the capacitor for a frequency, or the pulse width of a monostable.
Inputs
Result
Enter the timing parts.
555 timer formulas
Astable frequency is f = 1.44 ÷ ((RA + 2×RB) × C). The Texas Instruments datasheet gives the high time as 0.693 × (RA + RB) × C and the low time as 0.693 × RB × C, so the period is 0.693 × (RA + 2RB) × C. The output-high duty cycle is (RA + RB) ÷ (RA + 2RB), which is always above 50% with this basic circuit.
Worked example. RA = 1 kΩ, RB = 10 kΩ, C = 10 µF: f = 1.44 ÷ (21,000 × 0.00001) = 6.857 Hz. High time 76.23 ms, low time 69.3 ms, period 145.5 ms, duty 52.4%. About seven blinks a second for an LED.
Finding C. C = 1.44 ÷ (f × (RA + 2RB)). For 1 kHz with RA = 1 kΩ and RB = 10 kΩ: 68.57 nF. The nearest E12 value, 68 nF, gives 1008 Hz.
Monostable. One trigger pulse lasts tw = 1.1 × R × C. With R = 100 kΩ and C = 10 µF that is 1.1 s.
Limits. The datasheet lists a 4.5 V to 16 V supply for the NE555 family and says to keep astable operation at 100 kHz or below to limit distortion. The charge and discharge times are independent of the supply voltage, but real parts and capacitor tolerance shift the result, and CMOS versions (such as the TLC555) have their own datasheets. These are the datasheet's approximate formulas.
Frequently asked questions
How do I calculate 555 timer frequency?
f = 1.44 ÷ ((RA + 2RB) × C). With RA = 1 kΩ, RB = 10 kΩ and C = 10 µF that is 6.857 Hz.
Can a 555 astable give 50% duty cycle?
Not with the basic circuit: duty is (RA + RB) ÷ (RA + 2RB), which approaches 50% only when RA is tiny compared with RB. Adding a diode across RB is a common fix, but that changes the formulas, so check a reference for it.
How long is the pulse of a 555 monostable?
tw = 1.1 × R × C. For 100 kΩ and 10 µF the pulse is 1.1 s.
What supply voltage does a 555 need?
The TI datasheet lists 4.5 V to 16 V for the NA555, NE555 and SA555 (the SE555 goes to 18 V). Frequency and duty do not depend on the supply voltage.
Why is my real frequency different from the calculator?
Capacitor tolerance (often 10 to 20%), resistor tolerance and leakage in large electrolytics all move it. The formulas are marked approximate in the datasheet.
Sources
Checked
- Texas Instruments: NA555, NE555, SA555, SE555 precision timers datasheet (SLFS022K, revised March 2026)
- Wikipedia: E series of preferred numbers
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Scope and safety
Low-voltage hobby use only. These results are arithmetic from the formulas shown, not an engineering design or professional advice. Real parts vary with tolerance, temperature and age, so check the datasheet for every part. Never work on mains-powered circuits from a calculator result, and treat lithium and other rechargeable batteries with care.