Calculate capacitor size, voltage, or startup energy from any two values in volts, kilovolts, joules, kilojoules, farads, microfarads, and nanofarads.
Capacitor Size Formula
The calculator uses the stored energy formula for a capacitor. Enter any two values and leave one blank to solve for the missing value.
E = 0.5*C*V^2
C = 2*E/V^2
V = sqrt(2*E/C)
- E = energy stored or startup energy requirement, in joules (J)
- C = capacitance or capacitor size, in farads (F)
- V = voltage across the capacitor, in volts (V)
If you leave Energy Startup Requirement blank, the calculator uses voltage and capacitance to find the stored energy.
If you leave Capacitor Size blank, the calculator uses energy and voltage to find the required capacitance.
If you leave Voltage blank, the calculator uses energy and capacitance to find the required voltage.
The calculation is done in base units: volts, joules, and farads. If you choose kV, kJ, μF, nF, or pF, the calculator converts the value before applying the formula and then converts the result back to your selected unit.
Capacitance Unit Conversions
Capacitor values are often written in microfarads, nanofarads, or picofarads instead of farads. These conversions are useful when checking the result.
| Unit | Symbol | Equivalent in farads |
|---|---|---|
| Farad | F | 1 F |
| Microfarad | μF | 0.000001 F |
| Nanofarad | nF | 0.000000001 F |
| Picofarad | pF | 0.000000000001 F |
Common Capacitor Size Ranges
| Application type | Typical capacitance range | Typical note |
|---|---|---|
| Small signal or RF circuits | pF to nF | Used for coupling, tuning, and filtering small signals. |
| General electronics filtering | nF to μF | Common in decoupling and noise reduction. |
| Power supply smoothing | μF to mF | Larger values help reduce voltage ripple. |
| Startup or pulse energy storage | mF to F+ | Used when a circuit needs a short burst of stored energy. |
Example Calculations
Example 1: Find capacitor size
You need 50 J of startup energy at 100 V. Find the required capacitance.
C = 2*E/V^2
C = 2*50/100^2 = 0.01 F
The required capacitor size is 0.01 F, which is the same as 10,000 μF.
Example 2: Find stored energy
A 4700 μF capacitor is charged to 24 V. First convert capacitance to farads:
4700 μF = 0.0047 F
Then calculate the stored energy:
E = 0.5*0.0047*24^2 = 1.3536 J
The stored energy is 1.3536 J.
FAQ
How do I choose the right capacitor size from the result?
The result gives the ideal capacitance from the energy equation. In a real circuit, you usually choose the next standard capacitor value above the calculated value. You should also check the capacitor voltage rating, ripple current rating, tolerance, ESR, and the circuit’s safety requirements.
Why does voltage affect capacitor size so much?
Stored energy depends on voltage squared. If voltage doubles, the same capacitor can store four times as much energy. This means a higher voltage can greatly reduce the required capacitance, but only if the circuit and capacitor rating safely allow that voltage.
Can I use this for AC motor start capacitors?
This calculator is based on stored energy in a capacitor, using E = 0.5CV². AC motor start and run capacitors are usually selected based on motor design, supply frequency, phase shift, current, and manufacturer specifications. Do not use stored energy alone as the final sizing method for an AC motor capacitor.
