Capacitor Charge Calculator
Calculate capacitor charge instantly using Q = C × V. Optionally include resistance and time to estimate partial charging in an RC circuit.
What this calculator does
This capacitor charge calculator helps you find how much electrical charge a capacitor stores at a given voltage. The base relationship is simple:
where Q is charge in coulombs, C is capacitance in farads, and V is voltage in volts. If you enable the RC option, the tool also estimates charge, capacitor voltage, and current at time t during charging.
Core equations used
1) Final capacitor charge (steady state)
This value is the maximum stored charge after enough time has passed in a DC circuit. If voltage is negative, charge is negative as well, representing polarity.
2) Charging in an RC circuit
Q(t) = C × V × (1 − e−t/τ)
VC(t) = V × (1 − e−t/τ)
I(t) = (V/R) × e−t/τ
The time constant τ tells you how quickly charging happens. As a rule of thumb:
- At 1τ, the capacitor reaches about 63.2% of final charge.
- At 3τ, it reaches about 95%.
- At 5τ, it is effectively fully charged (~99.3%).
How to use the capacitor charge calculator
- Enter capacitance and select its unit (F, mF, µF, nF, or pF).
- Enter voltage and select its unit (mV, V, or kV).
- Click Calculate for immediate charge and energy results.
- For transient analysis, check Include RC charging and provide resistance and time.
Unit conversions included
The calculator automatically converts all values to SI base units before computing:
- Capacitance: mF, µF, nF, pF → F
- Voltage: mV, kV → V
- Resistance: kΩ, MΩ → Ω
- Time: ms, µs, min → s
Practical examples
Example A: Basic charge
A 220 µF capacitor at 12 V stores: Q = 220×10−6 × 12 = 2.64×10−3 C (2.64 mC).
Example B: RC charging snapshot
For C = 100 µF, R = 10 kΩ, and V = 5 V:
- τ = RC = 1 second
- At t = 1 s, charge is ~63.2% of final
- At t = 5 s, capacitor is ~99.3% charged
Why capacitor charge matters
Charge calculations are useful in timing circuits, filtering, power smoothing, pulse circuits, and sensor interfaces. Knowing charge and energy helps with part selection, safety margins, and performance tuning.
Common mistakes to avoid
- Mixing units (for example using µF as if it were F).
- Forgetting that energy scales with the square of voltage.
- Ignoring resistor value when estimating charging time.
- Assuming instant charging in real circuits.
Final thoughts
This tool gives a quick and practical way to compute capacitor charge and RC behavior without manual conversion steps. If you work with electronics, it is a fast sanity check before simulation or prototyping.