Formula Used
Time constant: tau = R × C
Voltage after time: V(t) = Vf + (V0 − Vf) × e−t/tau
Time to target: t = −tau × ln((Vtarget − Vf) / (V0 − Vf))
Current after time: I(t) = ((Vf − V0) / R) × e−t/tau
Charge: Q = C × V
Stored energy: E = 0.5 × C × V²
How to Use This Calculator
Enter resistance and select its unit. Enter capacitance and select its unit. Add the initial voltage and final voltage. Use the final voltage as the supply voltage for charging. Use zero, ground, or another reference for discharging. Enter the target voltage. Add an elapsed time to estimate voltage, current, charge, and energy at that point. Press Calculate. Download the result as CSV or PDF when needed.
Example Data Table
| Resistance |
Capacitance |
Final Voltage |
Initial Voltage |
Target Voltage |
Timing Point |
Expected Behavior |
| 10 kΩ |
100 µF |
12 V |
0 V |
7.58 V |
1 tau |
About 63.2% charged |
| 47 kΩ |
220 µF |
5 V |
0 V |
4.75 V |
3 tau |
About 95% charged |
| 2.2 kΩ |
470 µF |
0 V |
12 V |
1.2 V |
2.3 tau |
About 90% discharged |
| 100 kΩ |
10 µF |
9 V |
0 V |
8.94 V |
5 tau |
Nearly final voltage |
Understanding Capacitor Timing
Capacitors do not change voltage instantly. They move toward a final voltage through resistance. The speed depends on the RC time constant. One time constant equals resistance multiplied by capacitance. After one tau, charging reaches about 63.2 percent of the final move. Discharging loses the same percentage of the remaining difference. Engineers use this behavior in filters, delay circuits, soft starts, reset networks, snubbers, and pulse shaping.
Why the Time Constant Matters
A larger resistor slows current flow. A larger capacitor stores more charge. Both increase tau. A small tau gives fast action. A large tau gives slow action. The target voltage also matters. Reaching 50 percent needs less time than reaching 99 percent. This calculator solves both voltage at a selected time and time needed for a selected target. It also estimates charge, current, and energy at the calculated point.
Practical Design Notes
Real circuits can differ from ideal math. Capacitors have tolerance. Resistors have tolerance too. Leakage current can shift the final voltage. Equivalent series resistance may change peak current. The supply may have limits. High voltage capacitors can store dangerous energy. Always discharge them safely before handling. Use ratings above the expected voltage. Check polarity for electrolytic parts. Choose parts with proper ripple and temperature ratings.
Interpreting Results
The time to target is valid only when the target lies between the initial voltage and final voltage. During charging, the target usually rises toward the source. During discharging, it usually falls toward zero or another reference. The sign of current shows direction. Its magnitude falls exponentially as the capacitor nears the final value. The five tau rule is a useful shortcut. At five tau, the voltage is usually very close to the final value.
Better RC Planning
Use the table to compare typical values. Test several resistances and capacitors before selecting parts. Match the chosen tau to the device input threshold. Add margin for tolerance, temperature, and component aging. For precision delays, consider comparators or timer chips. For power circuits, check surge current and thermal stress. This tool gives a detailed first estimate for electrical planning.
Record each assumption with the result. Clear records help later maintenance and troubleshooting. They help technicians during audits too.
FAQs
What is capacitor charge time?
It is the time a capacitor needs to move from its initial voltage toward the source voltage through a resistance. The process follows an exponential curve.
What is capacitor discharge time?
It is the time a charged capacitor needs to move toward a lower final voltage. In many circuits, that final voltage is ground.
What does one time constant mean?
One time constant equals resistance multiplied by capacitance. After one tau, the voltage has completed about 63.2 percent of its total move.
Why does exact final voltage take infinite time?
The exponential curve keeps getting closer to the final voltage. In ideal math, it never reaches the exact final value in finite time.
Can I calculate current too?
Yes. The tool estimates initial current and current at the elapsed time. Current falls exponentially as the capacitor approaches the final voltage.
Which units are supported?
The calculator supports ohms, kilo-ohms, mega-ohms, picofarads, nanofarads, microfarads, millifarads, farads, microseconds, milliseconds, seconds, minutes, and hours.
Why is my target voltage invalid?
The target voltage must lie between the initial and final voltage. A value outside that path cannot be reached by the selected RC move.
Is this suitable for real circuit design?
It gives a strong first estimate. Add margin for tolerance, leakage, temperature, supply limits, and capacitor safety ratings before final design.