555 Timer Astable Calculator
Frequency, period and duty cycle of a 555 timer free-running oscillator. With the formula, a worked example and typical values.
Try your own numbers
Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
Open in the calculator with these values
About this calculator
This page calculates frequency, period, output high time and 2 more results from R1, R2 and timing capacitor C. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Duty cycle is always above 50% in this circuit. A diode across R2 or a different topology is needed for a lower duty cycle. Capacitor tolerance sets the frequency accuracy.
Cross-check: 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed.
What this calculator checks
- The arithmetic of the published formula for the values you entered, in the units shown.
What remains unverified
- That the inputs suit your application. Take ratings, resistances and constants from the manufacturer data sheet or project specification.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | Not applicable: The astable 555 timing equations are textbook approximations. Actual timing depends on IC variant, tolerances, threshold levels and manufacturer data. |
| Standard review state | Not applicable |
| Scope-screening note | not recorded |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| R1 (VCC to discharge) | 10 kΩ |
| R2 (discharge to threshold) | 47 kΩ |
| Timing capacitor C | 10 µF |
- Th = ln2 × (10 k + 47 k) × 10 µF = 0.3951 s
- Tl = ln2 × 47 k × 10 µF = 0.3258 s
- f = 1 / (0.3951 + 0.3258) = 1.387 Hz, duty = 54.8 %
Assumptions
- Ideal components, supply well within the 555 range
- The common 1.44 / ((R1 + 2R2) C) is this formula with ln2 rounded
Duty cycle is always above 50% in this circuit. A diode across R2 or a different topology is needed for a lower duty cycle. Capacitor tolerance sets the frequency accuracy.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| R1 (VCC to discharge) | kΩ | greater than 0 kΩ | |
| R2 (discharge to threshold) | kΩ | greater than 0 kΩ | |
| Timing capacitor C | µF | greater than 0 µF |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Frequency | Hz | Shown to 3 decimal places | |
| Period | s | Shown to 4 decimal places | |
| Output high time | s | Shown to 4 decimal places | |
| Output low time | s | Shown to 4 decimal places | |
| Duty cycle (%) | (number) | Shown to 1 decimal place |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with r1 (vcc to discharge)
| R1 (VCC to discharge) (kΩ) | Frequency (Hz) | Period (s) | Output high time (s) |
|---|---|---|---|
| 1 | 1.519 | 0.6585 | 0.3327 |
| 2.5 | 1.495 | 0.6689 | 0.3431 |
| 5 | 1.457 | 0.6862 | 0.3604 |
| 10 | 1.387 | 0.7209 | 0.3951 |
| 20 | 1.266 | 0.7902 | 0.4644 |
| 40 | 1.077 | 0.9288 | 0.603 |
| 100 | 0.744 | 1.3447 | 1.0189 |
Other inputs are the same as in the worked example above.
Standards and further reading
These are textbook circuit relationships. The component data sheet sets the real limits, tolerances and ratings, and takes priority over the calculated value.
Frequently asked questions
What do I need to enter?
Enter r1 (VCC to discharge), r2 (discharge to threshold) and timing capacitor C. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Ideal components, supply well within the 555 range. The common 1.44 / ((R1 + 2R2) C) is this formula with ln2 rounded.
Which calculators should I use next?
Common next steps are RC Time Constant and Cutoff and Frequency and Period. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- 555 Timer Formulas: Frequency and Duty Cycle: 555 timer formulas for astable and monostable mode: frequency, high and low time, duty cycle and pulse length
More Electronics and Instrumentation calculatorsAll engineering calculators555 Timer Astable in Elecmetric