Physics
Instant, private, and free
Capacitor Size Calculator.
Calculate the capacitance required for an RC target time constant and its corresponding cutoff.
Set your values
Results update as you type.
Results update automatically as you type.
Educational idealized model with every parameter shown; this tool does not size wiring, components, or safety systems.
Use Cases
Design RC timing circuits
Determine the capacitor value needed to achieve a specific time constant for timing applications, such as in oscillators, delays, or pulse generation.
Example: For a 10 kΩ resistor and a 0.1 s time constant, the required capacitance is 10 µF.
Set filter cutoff frequency
Calculate the capacitor size to set the desired cutoff frequency in RC low-pass or high-pass filters for signal processing.
Example: To get a 1 kHz cutoff with a 1 kΩ resistor, you need about 159 nF.
Frequently Asked Questions
- How do I use the capacitor size calculator?
- Enter the resistance in ohms (Ω) and the target time constant in seconds (s). The calculator will compute the required capacitance in farads (F) using the formula C = τ / R, and also display the corresponding cutoff frequency (fc = 1 / (2πRC)).
- What is the relationship between capacitance, resistance, and time constant?
- The RC time constant (τ) is the product of resistance (R) and capacitance (C): τ = R × C. For a given resistance and desired time constant, the required capacitance is C = τ / R. This calculator uses that relationship to find the capacitor size.
- What is the cutoff frequency and how is it calculated?
- The cutoff frequency (fc) is the frequency at which the output power of an RC circuit drops to half its input power (-3 dB point). It is calculated as fc = 1 / (2πRC). This calculator provides it based on the resistance and computed capacitance.
Tips & Common Mistakes
Tips
- Use standard capacitor values close to the calculated result; you may need to combine capacitors in series or parallel to achieve the exact value.
- Ensure the resistance value is in ohms and the time constant in seconds to get capacitance in farads. Convert units if necessary.
- For high-frequency applications, consider parasitic capacitance and inductance that can affect the actual time constant.
- Double-check your target time constant: a larger time constant requires a larger capacitor or higher resistance.
Common Mistakes to Avoid
- Forgetting to convert units, e.g., using milliseconds for time constant without converting to seconds, leading to incorrect capacitance.
- Using the cutoff frequency formula incorrectly: remember it is fc = 1/(2πRC), not 1/(RC).
- Assuming the calculated capacitance is a standard value; always check against available capacitor values.
Last updated: August 13, 2026