Capacitor Charge Calculator
Stored charge and energy of a capacitor from capacitance and voltage.
What it does
A capacitor parks charge on two plates and energy in the field between them. Charge grows linearly with both size and voltage while energy grows linearly with size but squares with voltage, so voltage moves the energy needle fastest.
The tool above takes capacitance in microfarads with voltage in volts. It returns charge in millicoulombs with energy in joules and millijoules. The default 470 uF at 25 V holds 11.75 mC and 0.1469 J.
Physics texts state both rules plainly: Q = CV with 1 farad as 1 coulomb per volt, and energy as half C V squared, equal to half Q V. Worked checks agree: a 12 uF part at 4 V holds 48 uC with 96 uJ, and the classic defibrillator case of 8 uF at 10,000 V stores 400 J.
Real parts add ratings that math alone misses. Working voltage must clear the circuit peak with margin, dielectrics set the ceiling per thickness, and leakage plus tolerance shift stored values a few percent.
How to use
- Type capacitance in microfarads.
- Type the voltage across the part.
- Read charge first, energy beside it.
- Scale: 1000 uF is 0.001 F, 1 mC is 0.001 C.
- Check ratings before trusting big joule numbers.
- Discharge high energy parts through a resistor, never a short.
Formula
- C
- Capacitance in farads; microfarads divide by 1,000,000.
- V
- Voltage across the plates in volts.
- Q, E
- Charge in coulombs, energy in joules.
Unit ladder: 1 F is 1 C per V. Energy forms agree: half C V squared equals Q squared over 2C equals half QV.
Worked example
Example: 470 uF at 25 V
Defaults: 470 microfarads, 25 volts.
- Farads: 470 / 1,000,000 = 0.00047 F.
- Charge: 0.00047 x 25 = 0.01175 C, shown as 11.75 mC.
- Square: 25 x 25 = 625.
- Energy: 0.5 x 0.00047 x 625 = 0.146875 J, shown as 0.1469 J.
- Millijoules: 0.146875 x 1000 = 146.88 mJ.
- Tool shows 11.75 mC, 0.1469 J, 146.88 mJ.
Reference table
| Case | Charge | Energy |
|---|---|---|
| 470 uF at 25 V | 11.75 mC | 0.1469 J |
| 1000 uF at 12 V | 12 mC | 0.072 J |
| 8 uF at 10000 V | 80 mC | 400 J |
| Air dielectric strength | Constant 1.00059 | 3,000,000 V/m |
| Teflon dielectric strength | Constant 2.1 | 60,000,000 V/m |
| Vacuum permittivity | 8.85e-12 F/m | Sets plate math |
Limits
- Ideal part assumed; tolerance and leakage ignored.
- Ratings ignored; overvoltage parts fail short.
- DC steady state; AC needs frequency math.
- Temperature drift excluded.
Common mistakes
- Forgetting micro means divide by a million.
- Scaling energy linearly with voltage instead of squared.
- Reading mC as uC, off by a thousand.
- Trusting nominal capacitance at working voltage.
- Shorting big capacitors to discharge them.
Try it yourself
Quick checks. Answers sit tucked under each question.
1000 uF at 12 V?
0.001 x 12 is 12 mC; energy 0.072 J.
8 uF at 10000 V?
80 mC and 400 J, the defibrillator scale.
Double the voltage?
Charge doubles, energy quadruples.
Questions
What does 470 uF at 25 V hold?
11.75 mC of charge and 0.1469 J of energy.
Charge or energy first?
Charge answers how much; energy answers how hard it can push back.
Why does voltage matter more?
Energy squares voltage but only scales capacitance linearly.
uF to F?
Divide by 1,000,000: 470 uF is 0.00047 F.
Series or parallel?
Parallel adds C directly; series adds reciprocally. This page rates one part.
Is stored energy dangerous?
Small electronics parts sting; joule scale parts bite. Discharge through resistance.
Sources
- OpenStax College Physics: Capacitors and Dielectrics
- OpenStax University Physics: Energy Stored in a Capacitor