# [Capacitor Discharge Calculator](https://blog.hirnschall.net/tools/capacitor-discharge/)

author: [Sebastian Hirnschall](https://blog.hirnschall.net/about/)

meta description: Calculate capacitor discharge time, voltage, current, and charge over time. Supports RC exponential discharge and constant current discharge. Solve for time or voltage.

meta title: Capacitor Discharge Calculator — RC and Constant Current

date published: 01.04.2025 (DD.MM.YYYY format)
date last modified: 22.04.2025 (DD.MM.YYYY format)

---

Calculator
----------

Fill in the known variables. Leave exactly one of \( t \) or \( V(t) \) empty — the calculator solves for it.
Provide \( R \) and/or \( \tau \) for RC exponential discharge, or \( I \) for constant current discharge.

* Initial voltage (V0):
* V
  mV
  kV

* Capacitance (C):
* F
  mF
  µF
  nF
  pF

* Resistance (R) — RC mode:
* Ω
  kΩ
  MΩ

* Time constant (τ) — RC mode:
* s
  ms
  µs
  min

---

* Time (t):
* s
  ms
  µs
  min

* Target voltage V(t) — leave empty to solve for V(t):
* V
  mV
  kV

---

* Constant current (I) — constant current mode:
* A
  mA
  µA

Fill in the known variables to calculate.

Calculate



Capacitor Discharge — Explanation
---------------------------------

This calculator covers two physically distinct discharge scenarios: exponential RC discharge and constant current discharge.

RC Exponential Discharge
------------------------

When a charged capacitor discharges through a resistor, the voltage, current, and charge all decay exponentially:
\[
V(t) = V\_0 \cdot e^{-t/\tau}
\]
\[
I(t) = \frac{V\_0}{R} \cdot e^{-t/\tau}
\]
\[
Q(t) = C \cdot V\_0 \cdot e^{-t/\tau}
\]
where \( \tau = R \cdot C \) is the time constant. All three quantities share the same exponential envelope — only the scaling factor differs.

To find the time at which the voltage reaches a target value \( V(t) \), rearrange:
\[
t = -\tau \cdot \ln\!\left(\frac{V(t)}{V\_0}\right)
\]

The Time Constant τ
-------------------

\( \tau = R \cdot C \) sets the speed of the discharge. After one time constant the voltage has dropped to \( 1/e \approx 36.8\% \) of \( V\_0 \). After five time constants the capacitor is considered fully discharged for most practical purposes:
\[
\begin{align}
t = 1\tau &\Rightarrow V = 36.8\%\ V\_0 \\
t = 2\tau &\Rightarrow V = 13.5\%\ V\_0 \\
t = 3\tau &\Rightarrow V = 5.0\%\ V\_0 \\
t = 4\tau &\Rightarrow V = 1.8\%\ V\_0 \\
t = 5\tau &\Rightarrow V = 0.7\%\ V\_0
\end{align}
\]

Constant Current Discharge
--------------------------

When a capacitor is discharged by a constant current source — such as a current-regulated load or a charge pump — the voltage decreases linearly rather than exponentially:
\[
V(t) = V\_0 - \frac{I \cdot t}{C}
\]
Rearranged to solve for the time to reach a target voltage:
\[
t = \frac{C \cdot (V\_0 - V(t))}{I}
\]
The capacitor fully discharges at \( t\_{\text{max}} = C \cdot V\_0 / I \).

More info
---------

Looking for more helpful tools and calculators? Explore a wide range of resources to simplify your engineering projects and calculations. Head over to our [tools section](https://blog.hirnschall.net/tools/) to find our free online calculators.  
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