Capacitors store energy in an electric field between two plates, and they appear in nearly every circuit — smoothing power supplies, blocking DC, timing and filtering. This calculator covers the three jobs people most often need help with: combining several capacitors into one equivalent value, working out the charge and energy held at a given voltage, and decoding the cryptic numbers printed on small ceramic and film capacitors.
How to use the capacitor calculator
- Choose a task under What do you want to do?
- Combine: pick Series or Parallel, list the values (prefixes such as 100n, 4.7u, 22p and codes like 4n7 work) and choose the unit for plain numbers. Add a voltage to see how charge, voltage and energy divide among the parts.
- Charge and energy: enter the capacitance and voltage to get Q = CV and E = ½CV², including charge in mAh and energy in watt-hours.
- Read a code: type the marking, such as 104, 472J, 22 or 2u2.
Capacitor formulas
Parallel capacitors act like one capacitor with a larger plate area, so their values add. In series, every capacitor holds the same charge, and the voltages add up to the total — which makes the combination smaller than any single part, exactly the reverse of resistors.
Worked examples
10 µF, 22 µF and 47 µF in series across 12 V (the default)
1/C = 1/10 + 1/22 + 1/47 µF⁻¹, so C = 5.998 µF. In parallel the same parts would total 79 µF.
The string holds Q = 5.998 µF × 12 V = 71.97 µC. Each capacitor carries that same charge, so the voltages are 71.97 ÷ 10 = 7.197 V, 3.271 V and 1.531 V — the smallest capacitor takes the largest share.
Energy in a filter capacitor. A 470 µF capacitor at 25 V holds Q = 11.75 mC and E = ½ × 470 µF × 25² = 146.9 mJ. That is tiny compared with a battery — about 0.00004 Wh — but it can be released in microseconds, which is why a shorted capacitor sparks.
Reading 104K. 10 × 10⁴ pF = 100,000 pF = 100 nF (0.1 µF), with K meaning ±10%, so the part may measure anywhere from 90 nF to 110 nF.
Capacitor codes at a glance
| Marking | Value | Common use |
|---|---|---|
| 22 | 22 pF | Crystal oscillator load |
| 101 | 100 pF | RF and timing |
| 102 | 1 nF | Filtering, snubbers |
| 103 | 10 nF | Debounce, filtering |
| 104 | 100 nF (0.1 µF) | Power-supply decoupling |
| 105 | 1 µF | Bulk decoupling |
| 106 | 10 µF | Tantalum and ceramic bulk |
A third digit of 8 or 9 means ×0.01 or ×0.1, used for values below 10 pF. Tantalum capacitors carry the same code up to 7 (107 = 100 µF). Electrolytic capacitors are usually large enough to print the value in µF and the voltage directly.
Choosing and using capacitors
Pick a voltage rating comfortably above the highest voltage the part will see — 1.5 to 2 times is common for electrolytics. Ceramic capacitors of the X7R and especially Y5V types lose much of their capacitance as DC voltage rises, so a 10 µF part on a 5 V rail may deliver far less than its label. Electrolytic and tantalum capacitors are polarized and must be installed the right way round.
To see how quickly a capacitor charges through a resistor, use the RC time constant calculator, and for its impedance at a frequency, the reactance and resonance calculator. The metric prefix converter helps when switching between pF, nF and µF.
Large or high-voltage capacitors can deliver a dangerous shock even when unplugged. These calculations are for estimation; follow your local electrical code and use a licensed electrician for installations.
Frequently asked questions
How do capacitors add in series and in parallel?
The opposite of resistors. In parallel, capacitances simply add: 10 µF + 22 µF + 47 µF = 79 µF. In series, the reciprocals add: 1/C = 1/10 + 1/22 + 1/47, giving about 6.0 µF, which is always less than the smallest capacitor.
How much energy does a capacitor store?
E = ½CV². A 470 µF capacitor charged to 25 V stores 0.5 × 0.00047 × 625 = 0.147 J. Energy grows with the square of voltage, so the same capacitor at 50 V would hold four times as much.
What does 104 mean on a capacitor?
The first two digits are the value and the third is the number of zeros, in picofarads: 10 followed by four zeros is 100,000 pF, which is 100 nF or 0.1 µF. A letter after the number gives the tolerance, such as J for ±5%, K for ±10% and M for ±20%.
Why put capacitors in series?
Mostly to raise the voltage rating: two identical 400 V capacitors in series can handle about 800 V, at half the capacitance. Because leakage currents differ, high-voltage series strings use equal balancing resistors across each capacitor so the voltage splits evenly.
Is a large capacitor dangerous after the power is off?
It can be. Capacitors in power supplies, microwave ovens and camera flashes can hold a lethal charge long after unplugging. Discharge them through a suitable resistor and verify with a meter before touching the terminals.