RC circuit formula tool
RC Time Constant Calculator
Calculate tau for a resistor-capacitor circuit and keep the timing assumptions with circuit worksheet notes.
Calculate RC Time Constant
Solve tau from resistance and capacitance, or back-calculate the resistance or capacitance needed for a target time constant.
Result
Time constant
1 s
Result notes
Keep the entered values, assumptions, and result together when adding this calculation to job notes or submittal records. Final installation choices should align with the applicable code edition, equipment listing, manufacturer instructions, local amendments, and AHJ requirements.
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Use this RC time constant chart: tau = R x C, 10 kOhm x 47 uF = 0.47 s, and 5 tau = 2.35 s for charge, discharge, or debounce checks.
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Use this series parallel resistance chart: 100 ohm + 220 ohm = 320 ohm series, 330 ohm || 680 ohm = 222 ohm, then mixed total = 442 ohm.
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Formula and field context
Calculate tau for a resistor-capacitor circuit and keep the timing assumptions with circuit worksheet notes.
Formula context
RC Time Constant Chart
The RC time constant is tau = R x C. This circuit worksheet reference shows that 10 kOhm x 47 uF gives tau = 0.47 seconds, and five tau is about 2.35 seconds. A charging capacitor reaches about 63% after one tau; use the chart for charge, discharge, debounce, or delay checks while still verifying the real threshold and voltage.
Formula
tau = R x C. Charging fraction = 1 - e^(-t / tau). Discharging fraction = e^(-t / tau).Variables to keep with the result
- tau is the RC time constant in seconds.
- R is resistance in ohms.
- C is capacitance in farads.
- t is elapsed time in seconds.
- Charge and discharge curves are exponential, not straight-line ramps.
Formula and variables
The RC time constant is tau = R x C. Resistance is in ohms and capacitance is converted from microfarads to farads before the calculation. One time constant is the basic timing unit for capacitor charge or discharge behavior. About five time constants is often used as a practical reference for a capacitor to approach its final value in a simple first-order circuit.
U.S. field context and example
RC timing appears in control circuits, signal filters, snubbers, delay networks, timing relays, and troubleshooting notes around capacitor behavior. For example, a 10,000 ohm resistor with a 100 uF capacitor has tau = 10,000 x 0.000100, or 1 second. If the worksheet target is a 1 second time constant and the resistor is fixed at 10,000 ohms, the needed capacitance is about 100 uF.
Assumptions and limits
This tool treats the circuit as a simple first-order RC network. Real timing can shift because of capacitor tolerance, leakage, dielectric behavior, source impedance, switch resistance, load resistance, and temperature. Use the full RC circuit calculator when charge percentage, discharge threshold, initial voltage, final voltage, or load interaction needs to be modeled rather than a single tau value.
Common mistakes
The most common mistakes are mixing uF and F, assuming one time constant means fully charged, ignoring the load connected to the capacitor, and using nameplate tolerance-free values as if they were measured parts. Keep the component tolerance and target threshold with the result when the timing note will be used for commissioning or troubleshooting records.