Capacitor Charge Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: August 19, 2026Two Numbers Every Capacitor Spec Sheet Implies
A capacitor's rating — say, 100µF at a 12V working voltage — doesn't directly state how much electrical charge it can hold or how much energy it stores when fully charged, even though both follow immediately from the capacitance and voltage. This calculator computes both: the charge in coulombs, which determines how much current the capacitor can briefly supply, and the energy in joules, which determines how much work it can do.
The Formula
Charge and energy stored in a capacitor are standard results from its capacitance and voltage:
Energy: E = ½ × C × V²
Note that energy scales with the square of voltage, not linearly — doubling the voltage across a capacitor quadruples the energy it stores, even though the charge only doubles.
Where This Calculation Matters
- Power supply and filter design — sizing a smoothing or decoupling capacitor to hold enough charge to ride through brief voltage dips.
- Flash and pulse circuits — calculating the energy a capacitor can deliver in a single discharge, relevant to camera flashes, defibrillators, and pulsed lasers.
- Supercapacitor and energy storage applications — estimating usable energy storage for backup power or regenerative braking systems that use large-value capacitors instead of batteries.
- Safety assessment — a charged capacitor can retain a dangerous stored energy or charge even after power is removed, which is why some equipment specifies a discharge time before servicing.
Charge and Energy for Common Capacitor Values
| Capacitance | Voltage | Charge (Q) | Energy (E) |
|---|---|---|---|
| 10 µF | 9 V | 90.0 µC | 405.0 µJ |
| 100 µF | 12 V | 1.2 mC | 7.2 mJ |
| 1000 µF | 5 V | 5.0 mC | 12.5 mJ |
| 1 F (supercapacitor) | 5 V | 5.0 C | 12.5 J |
A 1-farad supercapacitor stores orders of magnitude more charge and energy than a typical microfarad-range electrolytic capacitor — which is why supercapacitors are used for short-term energy backup rather than filtering.
How to Use This Calculator
- Enter the Capacitance value and choose its unit (pF, nF, µF, mF, or F).
- Enter the Voltage (V) across the capacitor.
- Select Calculate to see the stored charge and energy, automatically scaled to a readable unit.
Related Calculations
Decoding the resistor sitting next to that capacitor on a circuit? Use the Resistor Color Code Calculator. Estimating runtime for a battery-powered circuit instead? See the Battery Life Calculator.
Principles of Electrostatic Capacitance and RC Circuit Transient Dynamics
A capacitor charge calculator computes fundamental electrostatic parameters — stored charge (Q, in Coulombs), electric potential difference (V, in Volts), capacitance (C, in Farads), stored electrostatic potential energy (E, in Joules), and time-dependent exponential charging/discharging voltage curves in resistor-capacitor (RC) circuits. In electrical engineering, capacitors provide power supply ripple smoothing, timing circuits, and pulse-power discharge.
The Fundamental Electrostatic Capacitance Equations
Stored Electrostatic Energy: E = 0.5 × C × V² = 0.5 × Q × V = Q² / ( 2 × C )
RC Circuit Time Constant (τ) and Transient Charging Equation
When a capacitor charges through a series resistor R from a DC voltage source V0, voltage across the capacitor Vc(t) follows an exponential growth curve governed by the Circuit Time Constant (τ = R × C, in seconds):
Discharging Voltage: Vc(t) = V0 × exp( -t / τ )
Charging Current: I(t) = ( V0 / R ) × exp( -t / τ )
Voltage Charge Milestones in RC Circuits
| Elapsed Time Constant Multiple | Capacitor Charge % of V0 | Remaining Charging Current % |
|---|---|---|
| t = 1τ (1.0 Time Constant) | 63.21% Charged | 36.79% Current |
| t = 2τ | 86.47% Charged | 13.53% Current |
| t = 3τ | 95.02% Charged | 4.98% Current |
| t = 5τ (Steady State) | 99.33% (≈ 100% Fully Charged) | < 0.67% Current (Zero DC Current) |
Step-by-Step Worked Calculation Example
Example: Analyzing an RC Timing Circuit
Problem: A 100 μF capacitor (C = 100 × 10-6 F) connects in series with a 10.0 kΩ resistor (R = 10,000 Ω) to a 12.0 V DC power supply. Calculate: (1) The RC time constant τ; (2) The total electric charge stored when fully charged; (3) The total stored electrostatic energy; and (4) The exact voltage across the capacitor after t = 1.50 seconds of charging.
Step 1: Calculate RC Time Constant (τ = R × C):
τ = 10,000 Ω × ( 100 × 10-6 F ) = 1.00 Second
Step 2: Calculate maximum stored charge (Q = C × V):
Q = ( 100 × 10-6 F ) × 12.0 V = 1.20 × 10-3 Coulombs = 1.20 mC
Step 3: Calculate stored electrostatic energy (E = 0.5 × C × V²):
E = 0.5 × ( 100 × 10-6 F ) × ( 12.0 V )² = 0.5 × 10-4 × 144 = 0.0072 Joules = 7.20 mJ
Step 4: Calculate capacitor voltage at t = 1.50 seconds (t / τ = 1.5):
Vc(1.5s) = 12.0 × [ 1 - exp(-1.50) ] = 12.0 × [ 1 - 0.22313 ] = 12.0 × 0.77687 = 9.322 Volts
Conclusion: After 1.5 seconds, the capacitor reaches 9.32V, holding 1.20 mC and 7.20 mJ of energy at steady state.
Equivalent Series Resistance (ESR) and Dielectric Power Losses
In practical circuit design, real-world capacitors possess non-ideal parasitic elements modeled as Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL):
- Ripple Current I²·ESR Heating: In high-frequency switching switch-mode power supplies (SMPS), high AC ripple current flowing through the capacitor's internal ESR generates internal Joule heating:
Internal Power Dissipation (Ploss) = IRMS, ripple² × ESRExcessive ESR heating dries out liquid electrolyte in aluminum electrolytic capacitors, leading to bulging caps and premature motherboard failures.
Dielectric Material Comparison
| Capacitor Dielectric Type | Capacitance Range | Primary Application & Stability |
|---|---|---|
| Ceramic MLCC (C0G / NP0) | 1 pF to 100 nF | Ultra-high stability, zero voltage coefficient; RF filters and precision oscillators |
| Ceramic MLCC (X7R / X5R) | 100 nF to 100 μF | High volumetric density; power supply IC decoupling bypass |
| Aluminum Electrolytic | 1 μF to 100,000 μF | Bulk DC power supply energy storage and low-frequency ripple smoothing |
| Electric Double-Layer (Supercapacitors) | 1 Farad to 5,000+ Farads | Rapid regenerative braking energy capture and backup SRAM memory retention |
Dielectric Breakdown Voltage and Safety Derating
Every capacitor possesses a maximum Dielectric Breakdown Working Voltage (VDC). In high-reliability aerospace and industrial power supplies, electrical engineers apply a mandatory 50% Voltage Derating Rule (e.g., specifying a 50V-rated capacitor for a 24V DC circuit bus), preventing dielectric puncture from inductive transient voltage spikes.
Supercapacitor Ragone Plot Dynamics
On an engineering Ragone Plot (Specific Power vs. Specific Energy), supercapacitors occupy the high-power density domain (up to 10,000 W/kg), delivering near-instantaneous charging/discharging in milliseconds across hundreds of thousands of duty cycles, complementing lower-power lithium chemical batteries.
Capacitor Inrush Current and Pre-Charge Circuits
When connecting large capacitor banks (such as in EV motor inverters) to a DC voltage source, initial uncharged capacitance presents a momentary short circuit, drawing massive Inrush Currents (hundreds of amps). Engineers incorporate series NTC thermistors or pre-charge relay resistors to limit inrush current.
Capacitive Bleeder Resistor Safety
High-voltage power supply filter capacitors incorporate parallel bleeder resistors to discharge stored electrostatic potential within seconds after AC power disconnect, protecting maintenance technicians from lethal electric shocks.